-Only marginally, and in restricted areas of view. Voyager, Juno and Galileo all carried good cameras, intended for use at Jupiter.
More below.
To Grasp the Heavens
- Thread starter sts-200
- Start date
More below.
-I don’t think I’ve been to specific on the Mars science suite so far, other than it includes a 40cm telescope. It’s reasonable to expect that to be capable of both visible and IR imaging, and to be capable of programmable exposure and tracking (as it is intended for photography from low Mars orbit), so it should be a capable imaging instrument at Jupiter. By the standards of spacecraft of the day, the ship has significant recording abilities, so large numbers of images could be taken.
In addition, it’s not unreasonable to expect the crew to have at least one film camera (likely 70mm), so if there’s any unexposed film that could be used too – those images might be limited, but would be of very high quality.
Other Mars instruments are likely to focus on things that can be measured quickly (they’re only in orbit for a few weeks), so long-term stuff like atmospheric monitoring or gravity mapping would be a impossible.
Let’s say the other dedicated instruments include UV & IR spectrometers and a laser altimeter.
The Mars mission was put together in a hurry, so that might be about as far as they go – I’m sure there would be lots of other things wanted, but not available in time.
On a deep space mission, they would undoubtedly have various charged and neutral particle detectors, and X-Ray and Gamma ray spectrometers to take measurements while in flight. These might not be directional, so are not really planetary science. Inside the Hab, there would be various radiation detectors, for both scientific and crew monitoring purposes (after all, it’s the first time humans have gone deep into space).
As you say, in addition to that, there will be basic ship’s equipment that can be leveraged (or might have been slightly adapted) to provide science. Most obviously, this will centre on radio systems, where likely experiments include:
-Radio occultation measurements using the ship’s comms channels to Earth.
-Use of the rendezvous radar as a rangefinder/primitive atmospheric sounder.
-Synthetic aperture radar
-Bi-static radar
-Some of this could be done in multiple frequencies – if only due to the various comms and radar channels that the ship can transmit/receive.
There is also some science that can be (maybe retrospectively) from basic operations, such as using navigation camera images or horizon sensor data.
Not all of this will be of use at Jupiter (e.g. the laser altimeter and bi-static radar are probably useless), but it doesn’t sound like a bad science suite.
-In terms of size, mass, power and comms limits they are far better off than a traditional probe, however nothing is being used for what it was designed for, and it is only a flyby.
-I shall join those who pooh-pooh the idea, but not just on budget grounds. The bigger the spacecraft are, the more complex they are and the more everything interferes with everything else, making some types of experiment more difficult.
There might be economies of scale with large and/or large numbers of comsats etc…, but there won’t be with science missions, where, by definition, they are trying to do something new.
Given the trends of the last decades, I want to see more frequent (probably smaller) missions, not just a few huge ones.
That being said, mass/launcher power is usually a restriction, and lifting it would reduce the need for the redesigns that bedevil so many missions, and might offer flexibility in relatively “dumb” areas, e.g. carrying more fuel. A Cassini that launched straight to Jupiter instead of messing about with gravity assists could have got there a few years earlier, meaning we could be a few years closer to building the follow-on.
There are also missions that will need a more powerful rocket – e.g. landing on Europa.
- True, and a large ship with huge fuel tanks and a variety of liquid loops will induce yet more motion. It rules out some types of science, such as gravity mapping (derived from ship’s navigation and two-way ultra-stable oscillator signals with Earth), because the whole thing is bouncing around too much.
-In terms of image quality and the ability to acquire large numbers of images they’re ahead. In terms of flexibility of data gathering, three or four flybys by purpose-designed probes which are not constrained by the orbital mechanics of returning home would probably be of more value, as they could all observe in different ways. A single orbiter would undoubtedly be of far greater value.
-The ship was designed in the mid-80s, the Mars mission in about ’91, based on studies from a year or two before, so the early monochrome LCD screens are not entirely out of the question for text-based status displays. However, the rewiring needed to give them any control would be time-consuming and possibly risk damage to vital systems.
For the flyby, the ship has the best autopilot in the Universe – gravity. They don’t need any navigational control during the flyby, and other systems can be switched off or setup in such a way as to require no intervention. There is still the option of some, very limited, remote control from Earth. More important for them to focus on setting up these systems, improving their radiation shielding and protecting their plants.
As an “add on” to the ship, the Mars science suite is a separate system with its own controls, so perhaps they could rig up a basic link to that, if only to be able to monitor what the control system is doing and reset it if it fails.
I would expect a control program for the science kit to be developed on the ground (with the crew’s input) and sent up for the system to run. Some sensors are just going to be “on” recording data, obviously others such as the ‘scope will require an active control plan. They would then playback the data tapes to Earth after the flyby. They have a unique opportunity to return film, but I would expect most if not all of that would have been used up on either Mars or The Comet.
As a scientific expedition, it’s a long way from ideal, but it isn’t a science trip – It’s a way of surviving and going home, with a few observations tacked on where possible.
Flying on Vapour
The ship is falling towards the sun, and it would keep falling for another eighteen months. At the end of that time, the Earth would get in the way.
The lack of news, and then the ongoing stream of miserable news from Earth is past, and for most, life back home is now starting to improve. The summer of 1999 is less awful than that of 1998, while even affluent Westerners have grown used to the rationing and shortages that will be an unavoidable part of life for some years to come. Global GDP fell by at least 30% in 1998, but the effects of the inflationary spike, the market collapse and the near-total disruption of world trade are starting to subside. Both before and after the impact, there were emergency measures in place to try to dampen the effects, and legal changes to try to ensure the courts didn’t get in the way of potentially valuable businesses. Most banks are technically bankrupt, but still trading. Most pension funds are insolvent, but are still paying out. Almost all insurance companies are bust, but that doesn’t mean they are rejecting all claims. The financial and social meltdown that the doom-mongers had predicted didn’t quite happen, or at least, it didn’t happen in the way anyone thought it would. Everyone was in the same boat, and there were no “safe havens”; ultimately, Gold is just a shiny metal, the Dollar is just a number in a machine, while government bonds were either meaningless or not in short supply.
It wasn’t long before the markets and investors realised that panic would get them nowhere, and that the underlying value is often still there. Half the world still stands, and broadly the wealthier half at that. Even in the impact zone, most people have survived, most factories still stand, most land is habitable. The world would still need electronics and banks and iron ore and … everything, and in greater quantities than ever before.
Across the impact hemisphere, the repair of the numerous pinpricks that had disrupted almost every road and railway are largely complete. That didn’t mean life was back to normal, indeed in some ways 1999 would be worse than the previous year. Life would never be the same again, but by the end of that summer, there is a sense that the raw struggle for survival is over, and that the future now beckons. The sense of optimism was uneven, and the start faltered and varied, but 1999 would see the first hints of the longest and largest period of economic expansion ever seen; what economists would later call the “Comet Boom”.
Meanwhile, aboard the Victorious, preparations are underway for the final act of the mission, even though there are still hundreds of millions of miles left to travel. At the end of their original Mars mission, the ship would have used its engine to slow into an elliptical orbit about the Earth, and they would then have re-entered on board their Ares CSM “Odyssey”. The V-ship would then have been recovered and refitted by a new crew.
However, they have used almost all the fuel simply to reach Jupiter, and there is no possibility of the ship entering Earth orbit. Instead, the crew will have to board the Odyssey, and re-enter Earth's atmosphere directly from their approach orbit. This type of direct entry was one of the backup plans for the original Mars mission; if something had gone dramatically wrong with Victorious, the Odyssey had been designed to resist the heat and forces imposed by the Earth's atmosphere at the entry speed of 12.5km/s that would have resulted from her Mars-Venus-Earth return trajectory.
However, their arrival from Jupiter will be much faster than it would have been from Mars, and will result in a hyperbolic excess speed at Earth of 9,679m/s. The effects of Earth’s gravity will then accelerate the capsule to produce an entry speed of 14.7km/s. At this speed, both the heating rate and total heat load will be too high for Odyssey’s heatshield to survive the descent through the atmosphere.
When NASA were designing the Ares Command Module, they made good, conservative engineering assumptions. The heatshield is a modernised version of the design used by the extremely successful Apollo CMs, which had been developed to return from the Moon (although early versions were only capable of operating in Earth orbit). Odyssey's shield could certainly withstand 13km/s, and there is better than 99% confidence in her coping with 13.5km/s, subject to close control of the trajectory. 14km/s would be extremely risky; even if the CM survived, the crew would be subjected to extreme G-loads. Anything significantly above 14 would be virtually suicidal.
To allow the crew to re-enter at a safe speed, they would have to slow down. This was one of the key reasons why they had chosen the Jupiter route, rather than attempting to return to Earth directly. A direct return might have been just barely possible, but the manoeuvre that was needed would have used up all the ship’s available fuel, exposing them to higher radiation from the reactor for the remainder of the flight and giving them little margin for error. Once they reached Earth, they would have been left with no choice but to attempt a direct re-entry at 13.9km/s.
Superficially, therefore, the 14.7km/s re-entry speed of their trajectory from Jupiter is a much worse option; at that speed, their chances of survival are effectively zero. However, the shorter engine burn needed to send them around the giant planet had left them with a propellant reserve available to slow down once they reached Earth.
Making reasonable allowances for course corrections and RCS requirements during the flight out to Jupiter and back, it had been estimated that Victorious would have 6,200kg of liquid propellant remaining by the time she reaches home. Precise measurements of the level of fuel remaining are not possible, and there would be a couple of hundred kilos left in sumps and lines, so that figure is a low estimate, but when dealing with their lives, it would be unwise to always assume the best.
However, even when they are empty of liquid, each of the ship’s four propellant tanks still contains 1.6 tons of gaseous Hydrogen, an unavoidable consequence of the temperature of the tanks and the need to use gas pressure to force the liquid propellant down lines and into the engine’s pumps. On a normal V-Ship flight, or the Mars mission, this gas would be unusable, however the peculiarities of their long flight out to Jupiter allow them to trick the ship’s systems into giving them a little more fuel.
While the ship is far from the heat of the sun, the excess capacity of the cryogenic fuel coolers can be used to liquify some of the ullage gas in the three outer tanks. By diverting all excess cooling capability into the central tank only, it would always be cooler than the three outboard tanks, allowing the gas to migrate into the cold trap with the tiny pressure differential that developed.
Over the months and years, the pressurant gas therefore slowly accumulates in the central tank.
As Hydrogen gas hits the cooling baffles and the liquid in the tank, it condenses. Molecule by molecule, drop by drop, the store of usable liquid fuel increases, and with it, the crew’s chances of survival.
At such extremely low temperatures, the surplus cooling capacity is just a few Watts and it is therefore a very slow process. The crew also have to remember to shut the isolation valves between tanks before making any RCS or course correction burns. They were unable to do so on several occasions (usually when an automatic firing happened before they could inhibit it), and the worst failure set the process back by several months, as cold, dense gas and vapour was allowed to slosh back into the outer tanks during a prolonged thruster firing.
A second problem would be caused by the extreme cold. The cryocooler was designed to keep the propellant tanks at between 20-22K, but to achieve a meaningful rate of conversion from gas to liquid, the central tank had to be cooled to below 17K. This stretched the performance of the cooling system to the limit; the cooling loops must be colder than the tanks, and it was easy to allow them to become too cold. The cryocooler mechanism relies on Helium as a working fluid, and operating the whole system a few degrees closer to zero could cause this gas to liquify during the later stages of the heat-exchange process. Although this wouldn’t damage it, the system is designed to use gaseous Helium, and the presence of liquid in the cooling loops dramatically reduces their effectiveness. Such overcooling happened on numerous occasions in the early days, but by the time they reached Jupiter they had become adept at juggling heat loads, tank pressures and cooling power to ensure that gas recovery could proceed relatively uninterrupted.
Despite these setbacks, by the time they were clear of Jupiter’s sphere of influence, they had converted over a ton of gas into LH2 using this method. By the time they reach Earth, they expect to recover a further 700-800kg. On the latter part of the flight, they have another trick to maximise their propellant reserves. Even after they have recovered pressurant gas using the cooling system, there will still be about 800kg remaining in each of the outer tanks.
By isolating all but one of these tanks from the engine, they can use some of that remaining gas to feed their RCS thrusters. These little engines produce just a few pounds of thrust, using gas or liquid tapped off from the main tanks and warmed using waste heat from the reactor. Using blowdown pressure from the tanks means that they won’t perform quite as well as normal, as due to the varying gas pressure, their thrust will be reduced. However, so far out in space, they have plenty of time for manoeuvres. Theoretically, they could drain the tank all the way to vacuum, but there is a practical limit due to the inertial platform’s ability to sense the thrust provided by very low thruster pressures, and the degree to which the pressure can be sustained by allowing the tanks to warm up in the sun’s feeble rays.
During the fall back towards Earth, they expect to eek out a further 500-600kg of fuel, by using two of the outer tanks in this way. One of the three will be reserved as a backup in case they need to transfer liquid fuel from the central tank, for instance if a leak develops.
By misusing and adapting their systems in these ways, they will have at least 8.6t of liquid fuel available as they reach Earth, enough to slow down by 820m/s. Dumping their stocks of water and gases just before the burn could raise that to about 920, but it would still result in a re-entry at close to 14km/s.
Even with an additional couple of hundred metres-per-second that can be performed by Odyssey’s Service Module after they undock from the ship, that is still cutting it very close.
Fortunately, there is one final trick for them to use.
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Considering that I suggested that (with the intention of putting the capsule in LEO instead of entering later, which would have required some delta-V after the skip which the capsule does not have) and it was first misunderstood as many many skips for a months long aerocapture of very small gradual speed reductions, rather than as intended one pass to skip to orbit, and then dismissed, I doubt it.very nice. first time commenting here, but is the final trick skipping of the atmosphere? Works wonders in Kerbal for high speed reentrys.
Remember a skip must reduce the speed below escape speed, and if it does, reduce it well below that, almost all the way down to LEO orbital speed, or else the craft will be in one of those slow highly elliptical orbits where the crew starves to death before the damn capsule comes down to perigee again. We have to go from 14 km/sec to 8000, losing 6 km per sec in one pass, in energy terms go from 100 MJ/kg to just 32, which is to say lose twice as much energy on the first pass as on the second. If the heat shield can't take losing all 100 in one shot, I don't think spacing it out would help.
This is because it is an ablative heat shield. If it were a Space Shuttle like reusable shield, of any design, metal heat sink or tile or whatever, it would have a critical heat flux above which it would break down, but if kept below that by balancing higher speed with lower force, then reentering in several passes with each one pushing the TPS near the limit can buy more total energy shedding, if one can cleverly figure out how to get more than one pass.
But with an ablative, you have so much heat dose you can take, and whether you take it fast or take it slow, a given reduction in kinetic energy will cost a given amount of erosion of the ablative, at whatever rate you can set up to accomplish it. It would mean for a spacecraft such as a Shuttle fitted with replaceable ablative instead of ceramic, or Musk's upcoming BFS, that entry can start out being more aggressive because the ablative is much more flexible about how fast you burn it off--I think, up to an upper limit anyway. If the heat flux is too intense then I suppose the mode of erosion goes through a sort of phase transition and might be guessed to be much more destructive--fewer Joules shed per millimeter of erosion. It might even start cracking and spalling and just plain disintegrating long before a more carefully husbanded approach would have eroded it away. But below that critical limit you have a certain amount of delta V available after which you've burned it off.
So it is unclear to me whether the author is saying the rate of heat production is so high at 14 km/sec entry speed that the undesired phase change happens and destroys the shield without having ablated efficiently, or simply that 14 km/sec is over the optimistic heat flux budget.
Either way, the only advantage a skip reentry would deliver is if a more tentative first pass, at higher altitude for lower air density, could put the heat flux below the critical limit. Trouble is, while less deceleration in terms of meters/sec is needed to get down to orbital speed than to land, since it is happening at higher speed it counts for more--heat flux is going to relate to speed times force. And unfortunately during a skip one first dives into exponentially thickening air, passes a perigee, and then rises again through exponentially declining air densities--the absolute path length is low and cumulative air drag is lower due to the exponential tails versus peak, so there is a premium on time that does not apply to something coming in from orbital speed; the short effective time means going with high acceleration while you've got it, so despite the slightly more modest delta-V goal the intensity of drag while you've got it must be increased, not decreased, and this multiplies with the higher speed to make the deceleration very sharp and the energy flux rate pretty ginormous--just the thing for burning through the shield fast, or even faster if it peaks above critical flux levels. Ablative might be better able to handle it than reusable ones but it still seems to me drawing it out is all that could be accomplished; if speed is still excessive and total heat dose is exceeded, then doom is inevitable.
You're welcome. Glad you are enjoying it.
It isn't, although there will be some complexity to come with the re-entry.
Unfortunately real world heatshields are limited by total heat flux, or peak heating rates, or time leading to a maximum permissible temperature behind the shield.
In this case, the limit is likely to be be due to backshield temperatures. The structure of the phenolic honeycomb shield (the story's Ares is loosely an '80s version of the real NASA's Orion) could undoubtedly tolerate higher speeds, but it would let through too much heat to the structure of the rest of the capsule.
The holy grail of aerospace engineering - a reset button and an infinite fuel option...Works wonders in Kerbal for high speed reentrys.
Unfortunately on 6th/7th March 2001, the Moon is in the wrong place.
-When entering from deep space, it’s the combination of the requirement for single-pass aerocapture and maximum heat flux that usually does for tile/shingle designs; they can’t take the peak heating rates on that crucial first pass.
As you say, once in orbit you can then make a series of passes, each within the limit, so there are often options for re-entries from as far as the Moon.
That isn’t quite the whole story, as there are still heat loads on the structure of the vehicle. Solid ablative and reusable tile shields don’t stop the heat reaching the vehicle, they just reduce the rate a lot. For instance, theoretically, something not dissimilar to a Shuttle heat tile could survive a lunar re-entry (if you took it slowly enough), however the vehicle behind might not, as its structure would be warmed up by the heat leaking though the shield. The Shuttle itself had this problem, and dealt with it by allowing the aluminium sub-frame to warm up, then cooled it using air once it had decelerated. Without the ability to do that, the structure would have been severely compromised.
As an entertaining aside, in the very early days of real-world Shuttle development (long before any major contracts were awarded), Rockwell wanted to use BAC as a contractor on the Shuttle, because they had the knowledge and the test rigs to deal with issues surrounding Aluminium hot structures. Needless to say, that plan never really stood a chance, but from a development point of view it wasn’t a bad idea.
-Yes, ablatives do have limits, both due to heating rate and dynamic pressure (usually spallation limits), but they can be very high. The actual record is above 40km/s (the Galileo sub-probe), and they could be built to go higher than that. There are also “low limits”, where the heating rate is too low to cause ablation. That can cause problems too, as the thing is merely acting as an insulator
-Essentially the latter. It’s probably a little more subtle, as the heat shield itself is highly unlikely to “burn through” with such a small overspeed. However, the addition heat load will cause more ablation, thinning the shield and causing more heat to go through it. Most ablative materials are excellent insulators, and are used as such in heatshields – meaning that in addition to boiling away, they act to protect the structure behind from conducted heat.
It’s not unreasonable to assume that this would be the limiting factor in this case, as the faster the capsule enters, the more ablator is used, and the longer the entry takes, meaning more time for heat to conduct through the ever-thinning shield.
Given that it was designed for 12.5km/s, worrying about 14km/s seems fairly reasonable – it’s not just an extra 25% energy, it’s a greater heating rate on a thinner shield for longer, so it’s not totally implausible for heat loads as transmitted to the structure of the capsule to be doubled. There’ll be a good design margin, but for example if the structure is Aluminium, and a “normal” entry heats it up to 100C, the high-speed entry might heat it to 180-200C, at which temperatures the strength of Al is starting to drop rapidly.
-Skip entries are more interesting with reusable (or semi-reusable) heatshields where heating rates are important. They are also useful in providing down/cross range capability.
They do have another advantage – lower peak G-loads. Clearly that is related to heatshield loading, but for the higher speed entries, crew comfort/safety starts to enter the equation. There are lots of papers on the subject e.g. https://arc.aiaa.org/doi/abs/10.2514/1.20351
which covers some of these issues (and has lots of nice graphs).
There is another reason why they would be concerned about a high speed entry in the story, although in the interests of keeping it readable and entertaining, I don’t think I’ve emphasised it yet. It’s to do with lift/drag ratios and entry corridors (essentially accuracy of re-entry). They’re very concerned about peak G-loads, and in these circumstances 14-15km/s is quick for an Apollo-shaped vehicle. But a bit more to come there.
Victorious Earth
The first of March 2001 was the fourth anniversary of the day of The Comet. Around the world, economies and societies were recovering. The reconstruction efforts of 1999 and 2000 had all but wiped out the collapse of 1998, and the benefits of repaired infrastructure and new technology were starting to be felt. In addition to these practical improvements, there was a spirit of co-operation across the nations of Earth, and even though not everyone thought it would last, the prospects for a long period of economic growth were very real.
Nevertheless, this anniversary was the first time that the world paused to mourn and remember on any scale. In previous years, the memories were too fresh and people’s minds were too focussed on survival and rebuilding to allow time to deal with anything else. The 1st March 2001 would be a quiet, contemplative and rather sad day for many, but with the consolation that a week later, there should be something to look forward to.
With the exception of the day of The Comet, no event in human history has been as keenly anticipated as the return of James Cartwright, Felix Dairmuir, David Lutterell and Hiram Markham, who will start their re-entry shortly after midnight on the 7th March 2001. The exact time will depend on the results of the deceleration burn, but orbital mechanics dictate that they will encounter the atmosphere high over North Africa, before blazing a trail to the East to reach a splash down point some hundreds of miles off the coasts of southern Arabia.
Irrespective of the success of their attempts to slow down while they are still in space, they will be re-entering at a higher speed than their command module “Odyssey” was designed for, and their chances of survival have been increased through a series of changes to the spacecraft, made on the long journey back from Jupiter.
Most importantly, this involved the removal of surplus equipment; although lightening the capsule would not affect the speed of re-entry, it would reduce the pressure and energy that the heatshield would have to withstand during the dive through Earth’s atmosphere. Tragically, even if helpfully, the ship would be lighter due to the absence of the two members of the Mars surface crew and the samples they would have brought up from the planet. Their two flight couches had long since been removed – unbolted piece by piece and used for parts in the hydroponic garden, or to repair one of the many bits of equipment and fixtures that had broken in the years since they left Earth. The few samples that they would carry back to Earth are biological; tiny amounts of waste, tissue and plant matter that had been carefully stored over the years. Aside from the crew themselves, these would be the only physical results returned by the mission, and investigators on the ground are keen to see how both plants and astronauts have been affected by years in space. After numerous generations exposed to the radiation environment and zero-G, there were beginning to be visible changes in the more recent vegetables they had harvested from the hydroponic “farm”.
In the weeks prior to their return to Earth, they had increased their calorie intake by using their carefully horded stocks of contingency food. Back when they left Mars, a 12 man-day supply of survival rations had been left in reserve aboard the Odyssey, just in case of emergency. Thanks to their efforts growing their own food, little of this had been touched and it was possible to increase their daily rations by about 20% in the weeks leading up to re-entry. In turn, this meant they could do more exercise, to try to condition their bodies for re-entry and the 1-G environment on the surface. During this time, they had also been venting their waste liquids overboard. On the way out to Jupiter, these precious fluids were retained and recycled where practical, but for the deceleration burn they would be nothing more than deadweight.
The plan for the last hour of the flight is the most complex sequence of events they have dealt with in years; it calls for them to use the FireStar drive to slow down just before their closest approach to Earth. There are backup schemes in case the drive doesn’t work, or if it fails during the burn, but the best outcome would be to run the engine for as long as possible; and that means draining every available drop of propellant from the tanks.
In addition to their gas recovery efforts on the long trip around Jupiter, they have one last source of propellant, although in normal V-Ship operations it was never thought of as such. The central liquid Hydrogen tank is equipped with a tubular baffle down its middle, which stays full of LH2 even when the rest of the tank is drained. Hydrogen is an excellent radiation shielding material, and in normal conditions, the presence of this column of liquid means that the FireStar drive could be run continuously at full power without giving the crew anything approaching a dangerous radiation dose. On their flight so far, total firing time has been little more than a day, and the radiation they have received from the engine is insignificant in comparison with the natural background of space. However, this central shield column contains 5.4 tons of LH2; theoretically, enough to run the engine for about 500 seconds.
To protect equipment near the back of the ship and reduce radiation-induced heating of the propellant tanks, the reactor also has a solid shield of Tungsten and Lithium Hydride, but this alone is not sufficient to protect the crew for long periods of engine operation. However, it will be enough to prevent them receiving a dangerous dose over a period of a few minutes at the end of the flight.
Without the LH2 column, they would receive a dose of about 60 milliSieverts/hour from the engine. However, it will take several minutes for the propellant to drain from the column, and so for much of the time, the shield will still be partly effective as the height of the fluid column gradually falls. Consequently, the dose they will receive due to “burning the shield” should be under 10 milliSieverts, a level that would be measurable under any terrestrial radiation protection scheme, but is a trivial addition to the amounts they have already soaked up during their years in space, and far below the level that would show any measurable effect on their health. On the other hand, it might seem reckless; during their long flight, they had already broken the limits on every occupational radiation protection scheme ever devised, and have received more than anyone except the most severe radiotherapy cases. However, unless they slow down as much as possible, the risk of burning up in the Earth’s atmosphere is severe and immediate, while being hit by a few more energetic photons is merely another long-term risk.
Using the liquid from the shield carries other risks, as there are no sensors to monitor the level of propellant remaining in the column. In zero-G, this inner vessel is usually sealed to prevent fluid drifting out into a partly-filled main tank. Normally, when the engine fires, a set of low-power pumps circulate LH2 from the rest of the tank into the base of the column, and venting is allowed at the top. This is done to maintain equal temperatures and pressures throughout the tank, and to minimise the formation of bubbles caused by radiation-induced heating within the shield column.
For the ship’s final manoeuvre, this system will be used in reverse to allow fluid to leak back through the pumps and into the main tank. Once the level in the tank drops sufficiently, the circulation pumps’ intakes will be uncovered, and without any fluid to pump they will overspeed and automatically shut down. The acceleration provided by the FireStar drive will then be enough to allow fluid to escape from the column at a rate greater than the engine’s fuel use, until the level falls and the pressure head is reduced.
The problem is that this process will only start after the propellant level in the main tank has dropped below the position of the lowest liquid level sensor. Normally, there is a safety system to shut down the engine shortly after this happens, but that has been disabled. Without any way of directly measuring the amount of propellant remaining, the best that can be done is to use a timer. The best projection is that there is 5,380kg of LH2 in the shield column, and there will be about 750kg remaining in the outer tank at the time the circulation pump intakes are uncovered. Theoretical and physical models on Earth have been used to establish estimates for how fast liquid will escape from the column. As the level falls, the force of the acceleration head driving the liquid out of the column will reduce, and at some point, the outflow rate will become too low to supply enough propellant to match the engine’s consumption.
Based on the results of these tests, the safe thing to do will be to assume that the engine can fire for 481 seconds after the circulation pumps shut down; in this time, it will use most of the liquid in the column, while still leaving an adequate margin for errors in the estimates.
There will still be some propellant left over in lines, valves and manifolds, but this head of fluid is needed to ensure that the main engine pumps continue to function properly until they are commanded to shut down. A disruption to the smooth flows in the pumps’ intakes could cause them to cavitate, stopping coolant flowing to the engine and causing to it rapidly overheat. There could never be a nuclear explosion, but thermal damage to the “washing machine” drums could result in unacceptable thrust transients, loss of attitude control or even the ejection of radioactive material at exactly the time when the crew will need to undock from the ship in a carefully controlled manner.
The total propellant available is therefore a minimum of 13,230 kg, which should be enough to run the main engine for 1,236 seconds, and to provide a velocity change of at least 1,460 m/s if they vent their waste water before the burn.
In late January, they had made one of their largest course corrections in years and had used the opportunity to test the main engines on both spacecraft. The FireStar reactor had been idling since they left Jupiter, and it hadn’t been fired at full power since they left The Comet behind more than three years earlier. Since then, they have only used low-power thrusters, which use just a few Megawatts, and the RCS jets which rely entirely on waste heat from the generators.
Just over 200kg of gas from the outer tanks was fired through the propulsion system to give the ship a shove of 8.7m/s. The reactor core’s power output was brought up to 30MW for the four-minute burn, but even this low level was sufficient to allow partial melting of the Uranium fuel, while an increase in the Neon buffer gas flow rate was used to check the stability of the rotating drums. Measurements of the reactor’s output and transient behaviour confirmed that it is still in good shape, even though the level of Uranium burnup is now beyond the original design limits, and larger than normal quantities of fission products will be trapped inside the solidified fuel due to the long period of low-power operation.
A few days later, they had performed a minor trim manoeuvre using Odyssey’s main engine. The Ares CSM’s chemically fuelled motor is far less efficient than the nuclear-heated Hydrogen thrusters, but it is essential to test the engine after so long in space. It hadn’t been fired since shortly before Odyssey docked with Victorious in Earth orbit, four and a half years earlier. The engine is so simple that no-one expects any serious problems, but they need to be sure of its performance well in advance of re-entry, as it will be used to target the capsule for its final plunge through the atmosphere.
Nine hours before entry, shortly after they pass inside the orbit of the Moon, they begin their final preparations to leave the ship. Years of thought and months of preparation have been leading up to the next few hours, and modifications have been made to air ducts, cabin fittings, control systems and program sequences to give them the best possible chance of a safe re-entry. One of the more bizarre pieces of improvisation is to their spacesuits. Normally, these would be lightly pressurised with air, and they would wear a water-cooled garment to help regulate body temperature. Months of meticulous stitching, gluing and sealing have completely changed this mode of operation. For the re-entry, their bodies will partially “float” inside a water-filled suit, with a carefully made seal around the neck to keep the water contained while the suits are being filled in zero-G. For safety, during this time they will have their visors open, while during entry and after splashdown, their heads will be slightly raised relative to the rest of their bodies, so they are in no danger of drowning. The purpose of this rework of their suits is to spread the loads of re-entry more evenly over their bodies, as flight surgeons are concerned by the levels of bone and muscle wastage that will have occurred during the long flight. It is regarded as highly unlikely that any of the crew will have the strength to be able to walk, or even to stand, once they return to Earth.
Ninety minutes before closest approach, they are strapped in aboard the Odyssey, with hatches sealed and suits plumbed in, ready to pull away from the Victorious at a moment’s notice. Their data displays are limited to readouts of the main ship’s navigation systems and a set of numbers showing the state of the engine, however, for the first time since they left Earth, Mission Control in Houston has real-time data and they will be monitoring most of the deceleration burn. Their current orbit will take them within 500km of Earth’s surface, and the FireStar burn is planned so that this altitude will fall only slightly due to the reduction in speed. It is vital for the crew to return to Earth, but it is equally important that the Victorious and her nuclear engine do not re-enter with them.
With 25 minutes to go before the burn, they pass geostationary altitude. The ship is now aligned for the burn, and their view of Earth is blocked by the bulk of the Hab. Other than for a few seconds after they separate from the ship, they will not see the blue planet again until they are floating on its surface.
At 23:36, the reactor startup sequence is begun, and 142 seconds later, they can see the numbers they hoped for; the Firestar is running at 98% of rated power, well within acceptable parameters. The output of one of the engine’s three turbopumps hadn’t built up as it should during the startup sequence, and it had been shut down, but the other two pumps are sufficient for normal operation. For the next 1,251 seconds, the engine fires steadily. The “Circ Pump Overspeed” event occurs a few seconds later than expected, meaning they have a few extra seconds of thrust. 479 seconds after those pumps shut down, the computer commands the reflector drums to open, and the chain reaction that sustains the fuel’s immense temperature rapidly dies away. Hydrogen continues to flow through the reactor at a low rate to deal with the effects of afterheat, but the crew do not have time to wait for a complete shutdown.
At two seconds after midnight on March 7th, it is time to abandon the ship that has sustained them on the longest voyage ever made by man. On both spacecraft, negative X-axis RCS thrusters fire to assist with the separation, and to null the effects of the cooling gas that is still flowing through the ship’s reactor. Command Module Pilot Hiram Markham pushes a switch, and four latches unlock at the top of the Odyssey. There is no time for ceremony or memorable speeches, and with two simple words from the Captain, they are on their way.
“Farewell Victorious”
Yes , although due to the deceleration she will stay well inside the orbit of Jupiter, so there is little chance of her being flung out of the system altogether.
The ship would be visible against the night sky over some of Europe and Africa, but the burn itself would be fairly unspectacular. All that is coming out is Hydrogen , plus a little Tungsten seeding, so there would be little in the way of visible plume.
Dawn
“Burn nominal minus 16 Zee, minus one X, zero Y”
“Odyssey, Houston, we confirm those residuals. Use program 115.”
“Rog. OK for program 115 after sep…”
“Negative Zed rates. Go for undock”
“Neg Zee RCS…
Four lights … undock … we’re loose”
“Farewell Victorious”
Twenty seconds after separation, Odyssey’s RCS starts to yaw her round to face almost directly away from the V-Ship. Once the turn is complete, she will follow one of a series of pre-programmed flightpaths that will carry the CM into the atmosphere. With all the uncertainty as to how much fuel remained on board the ship, and whether the main engine would perform as well as it should, controllers had developed 18 programs for the final manoeuvre and re-entry; ranging from a last-ditch attempt if the retrofire failed altogether, to options that assumed it had been as successful as anyone dared to hope. In the event, it was slightly better than expected, but only by 16m/s.
“In program 115. Plus one six two yaw by neg three pitch”
“…”
“OMS enable”
“Roger, yaw complete … ullage burn”
“Two, one, Ignition … we have good Pee-Cee”
“Banks A and B OK”
The small engine on Odyssey’s service module – a relative of the Space Shuttle’s manoeuvring engines – will now push them onto a course towards the atmosphere, leaving Victorious to hurtle past Earth and out again into deep space. Even if the big ship’s engine had failed, they would still have performed this burn, and accepted the risk of a very high entry speed.
Odyssey’s tiny fuel supply is exhausted in just over a minute, when she is just 6,076km above the surface, moving at nearly 11km/s. Despite the success of the two retrofire burns, the four men and their command module are still heading for Earth faster than anyone in history. If Odyssey’s OMS engine hadn’t worked, they could have used the RCS thrusters to nudge themselves into the entry corridor, but the greater fuel reserve and thrust of the main engine allows them to slow down a little more. Even so, the Service Module’s small engine can only provide a maximum delta-V of 320 m/s, but this is enough to shift their projected perigee down to 60.9 km and slow Odyssey down by a few more miles an hour.
Closely controlling their course to achieve a precise perigee is one of the most important parts of the entire re-entry sequence. They can cope with a range of re-entry speeds; theoretically, even 14km/s would be survivable (albeit with thin margins). They cannot cope with anything other than a very carefully controlled entry angle. Too shallow, and they will be unable to keep the capsule in the atmosphere for long enough to decelerate to sub-orbital speeds, leading to it skipping back out into space with only a few hours’ life support available. If they enter too steeply, the capsule will be unable to pull up out of the dive, and they will be flattened by extreme G-forces. As is so often the case, the ship could survive more than the crew; the capsule is structurally rated to survive 14G, but in their weakened state, the crew would be lucky to survive anything much more than six or seven.
In practice, these constraints mean that a perigee of over 65km will always result in them skipping out of the atmosphere. On very high-speed entries (above about 14km/s), their ability to dive into the atmosphere is also very limited, and at 14.5km/s, the lowest altitude at which they skip out and the highest altitude at which they would have to endure dangerously high G-forces coincide.
Based on the most likely range of re-entry speeds, 13.3 to 14.1km/s, controllers chose 60km as the target. Even if they were a little off, for a few kilometres either side of this altitude there are survivable entry paths throughout that speed range.
At 00:18:48 UTC, as the spacecraft is high over the Libyan desert, she reaches entry interface, the point at which the accelerometers measure 0.05G of aerodynamic deceleration.
Seven seconds later, at 96 km altitude, Odyssey reaches her maximum speed of 13,328m/s. For the next few minutes, they will be on autopilot. A variety of re-entry programs had been modelled and tested in the months leading up to their return, and the most relevant one was selected after the magnitude of the deceleration burn was confirmed. Apart from the precision that is possible with computer control, there is a very significant risk that the crew will black out during re-entry, and so the entire sequence is pre-programmed, from the initial orientation of the capsule to the post-touchdown parachute jettison.
The first goal of the program is to ensure that they do not skip out of the atmosphere. Their entry angle is such that if they do not use the capsule’s offset centre of gravity to fly “down”, they will punch through the thin upper layers of the atmosphere without slowing sufficiently to be captured by the planet’s gravity. As the pressure builds on the heatshield, roughly a third of the drag force is converted to lift, bending their flightpath deeper into the atmosphere. Thirty-six seconds after interface, they are experiencing something they have not felt in almost five years: One G.
Twelve seconds later, the acceleration has doubled, and the computer’s projections of future rate-of-change of acceleration tell it to fire the RCS thrusters to start to roll the capsule and use the lifting forces to pull them South rather than down. When the roll is complete, they are still diving, but the descent rate starts to drop as inertia continues to overwhelm gravity; they are still moving at 12.9km/s.
The re-entry program now switches to its next important goals; to balance the need to avoid a skip-out with the need to minimise the G-forces they will experience. They will fly swiftly across North Africa and Arabia, to reach a splashdown point in the Indian Ocean, south of Oman. The method is to establish a stable altitude, at an initial deceleration level that is tolerable. Eighty seconds after interface, they are experiencing the peak deceleration rate of 4.67G as they pass through 55km. The thrusters fire again, rolling the capsule back down to prevent it gaining height, and this is adjusted several times over the next minute. For two minutes after peak-G, their altitude barely changes and deceleration drops below 3G as they continue to slow down. 203 seconds after interface, they are unquestionably heading home; their speed has dropped below that required to stay in low Earth orbit.
Now that they cannot become lost in space, the entry program rolls them over to generate upwards lift. For the next part of the flight, the program’s goal is to extend their flight to ensure that the capsule skips over the tip of the Arabian Peninsula to a safe splashdown. Altitude increases as speed continues to drop, and deceleration falls to just 1.3G in the thin atmosphere at the top of their brief climb. As the dynamic pressure drops, lift can no longer counterbalance weight and they begin their final inevitable descent back towards the surface. Now on a slightly steeper dive, they experience their second period of higher G-forces, peaking at 3.06G, 257 seconds after the skip manoeuvre begins.
Forty seconds later, they are down to aircraft-like speeds, a mere Mach 4, and the capsule stops its manoeuvring and turns to generate maximum lift. G forces are falling, and the capsule must be held steady in preparation for parachute deployment. At Entry+597 seconds, she decelerates through the sound barrier, and thirty seconds later the 12’ drogue parachute is fired out of its canister near the top hatch. The increase in drag quickly slows them down to just a couple of hundred miles an hour and stops almost all lateral motion.
Due to the need for a variety of re-entry plans, there was a degree of uncertainty as to where the CM would splash down, and four recovery vessels are stationed near each of the four most likely sites, with support ships from eighteen nations available to cover all other reasonable possibilities. With such an arrangement, there is fierce but friendly competition to be the unit that recovers the greatest space mission in history. The caprices of time and gravity dictated that it would be the crew of the support ship INS Ranvir who would pick up Odyssey’s radar signature as she ended her plunge through the atmosphere.
At 00:30:11 UTC, the three parachutes are fired out of their canisters. One hesitates to open, although it does eventually inflate. Even if it hadn’t, the other two would be adequate to slow the capsule and lower it safely into the sea. One of the reasons for so many recovery vessels is that unlike all previous manned splashdowns, this one will happen at night. With only a few minutes’ warning, it was never going to be possible to cover all possible landing sites with fully capable recovery ships, so ships such as the Ranvir have the task of alerting and guiding the heavy units. Once it is confirmed that the capsule is nearing the surface, the frigate fires off star-shell and signal rockets to help illuminate the scene. In the bright glow of the flares, lookouts spot the brilliant white parachutes. The ship’s helicopter is already in the air, carrying divers to help stabilise the capsule after it splashes down, and to attach lines to allow it to be recovered.
Barely two minutes after Odyssey thumps down into the water, the first greeting the crew have received in person in nearly five years comes from Leading Seaman Chopra Vaas, whose grinning wet-suited face appears outside one of the capsule’s side windows. He later describes the moment he waved and got a thumbs-up in reply from Felix Dairmuir as “the greatest honour of my life”.
There are just a few minutes’ wait before a heavy-lift Sikorsky arrives from the carrier USS Eisenhower to pull the capsule from the sea and gently lower her onto the big ship’s deck.
As the first light of dawn creeps over the Indian Ocean, the recovery team’s first action is to encase the CM in an inflatable plastic tent, which will help to provide a sterile environment for the crew and the heavily screened technicians and medics who will care for them until they reach land. After so much time in space with no exposure to Earth’s various diseases, doctors are concerned that even a common cold could overtax their weakened immune systems. Access to the crew will be restricted for weeks, and closely monitored after that, but images of them being carried out of the Odyssey on board specially-designed water-filled stretchers are flashed live to every corner of the world.
Asked “how does it feel to be back on Earth”, the Captain mocks both himself and the traditional view of stuffy naval culture. Shakily making the “peace” sign with both his hands, he smiles broadly before replying replies “Heavy, man … heavy”.
For all of them, the return to Earth means round-the-clock treatment in the best facilities available anywhere. For three of them, a combination of care, diet and medication is enough, but for David Lutterell, it is not. As the G-loads built up, his suit had developed a leak. The water that spread the forces on his body soon drained away, and he had to endure the entry in a suit that had been adapted to contain water. For any normal person, this would have been an uncomfortable experience, but for an under-fed, weightless-adapted man it led to too many complications.
Broken bones and damage to blood vessels could have healed, but he was so worn-down by the years of spaceflight that his body couldn’t cope with the injuries and the treatment that was needed for the infections that soon followed.
The success of their mission is beyond question, although to the 13.62 million people (according to the official United Nations estimate), it certainly made no difference at all as their towns were flattened or burned, their homes were buried by landslides or washed away by tidal waves. To the estimated half-billion whose lives were ended or shortened by the disruption and famine that followed, they had been of little help.
To everyone else, and to everyone who will ever live, they are, and always shall be, the men who saved the world.
Six months later, at a ceremony for all six members of the crew in Canberra, the capital closest to the centre of The Comet’s impact pattern, the leaders of 114 nations agree to the founding of a global astrophysical research agency. JARA will have many detailed objectives, but its purpose is perhaps best described by President Fuller, who invokes, and gently mocks, one of his predecessors.
“Our most basic, common link is that we all inhabit this small planet, we all breath the same air … and because we cherish our children’s future, we must now break that link.”
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Fantastic.
And from the last line, I take it that the sequel is an attempt to establish off-Earth colonies?
And from the last line, I take it that the sequel is an attempt to establish off-Earth colonies?
Too right it isn't!
That makes me laugh. I know that very well, and yet...
There's the need for proof-reading in a nutshell. I must have re-read that sentence a half-dozen times since I wrote it, yet such a glaringly obvious error never popped into mind!
Glad you enjoyed it.