Eyes Turned Skywards

[Shevek23]

Addendum/clarification of previous post, lest the point be lost...

...I understand that the LEO-L2 transfer you have Artemis program proposing is not Farquhar's minimum-cost orbit; the fact you describe it as "hyperbolic" demonstrates that! And while clearly a hyperbolic, exceeding-escape velocity TLI is a higher delta-V than the 1960s-70s Apollo program used, I think by now most of us here understand that the delta-V increment at that point is probably pretty small, and might be little more than the incremental improvements in engine efficiency and structural lightening between the 1960s design period of Apollo and the 1990s of Artemis allow for. So the payload reduction to L2 is not much, and I suppose the transit time might even be in the same ballpark as the time to LOI of Apollo.

But given the faster transfer, Farquhar's figure for halo orbit injection must clearly be out the window! If the period of time between passing by Luna at perilune and reaching the region of L2 is a lot less than Farquhar's 3 days, then that suggests to me the halo orbit insertion burn has to be a much higher one, to brake down the excess speed. So that might be where the big difference in payload to L2 comes in.

And then, there we are, IIRC E-M L1 & L2 are roughly as far from Luna as geosynchronous orbit is from Earth, around 40,000 km. That's awfully far from Luna, and obviously it will take more time to get to and from Luna. The time savings by going hyperbolically instead of on the fast ellipse OTL Apollo used to reach Luna probably can't offset the extra time to go on from there to L2, and then there's no margin left to offset the extra time to and from the Lunar surface.

Artemis missions are going to be much longer in duration than Apollo missions. The radiation exposure issues are not being mitigated, but increased.

I suppose longer transfer times are not so bad in the context of the mission that proposed to stay on Luna longer anyway, weeks instead of days. But the transfers are still going to be a significant chunk of their deep-space time.

As I understand it, L2 has advantages over L1 for more far-sighted purposes than sorties to Luna's surface--for deeper space missions, and because it can be reached (via slow orbits) more economically than L1. But since Artemis program will not build up any reused infrastructure in cisLunar space, for Lunar or deep-space purposes, and since the craft that goes to the Lagrange point is the manned one and uses a faster, less economic trajectory anyway, have you given any consideration, if LLO is out, to staging out of L1 instead?

L1 is of course closer to Earth rather than farther away; a craft in a tight halo orbit there would always be in line of sight of Earth, never eclipsed; given the choice to use fast transfers, L1 can be reached sooner than near-Luna space rather than after it. And in terms of reaching any desired point on the Moon it is pretty much equivalent to L2.

So even if the choice of L2 staging is driven by an unspoken NASA agenda to prepare the way for future Lunar-region bases and deep space missions that would actually require the outer point, for one-shot sorties that seek to develop the technique of staging out of such saddle points, L1 should be good enough for this stage of operations, teaching most of the lessons we'd need to use L2.

The economics, and the time required to go to and from Luna, will unfortunately remain just as oddly disadvantageous, but at least we ought to save some time getting to the Lagrange point, which makes the other costs easier to bear.

 

[nixonshead]

Great update! And very nice diagrams, especially the mission chart - it really helps to get a feel for the mission.
It also set me to wondering how much delta-v it would cost to break the Pegasus stage into L2 halo orbit rather than discard it. Maybe do a mission with and extended Mission Module and tools instead of an LM, and you've got the potential for a nice little Wet Workshop...

 

[Bahamut-255]

Great update! And very nice diagrams, especially the mission chart - it really helps to get a feel for the mission.
It also set me to wondering how much delta-v it would cost to break the Pegasus stage into L2 halo orbit rather than discard it. Maybe do a mission with and extended Mission Module and tools instead of an LM, and you've got the potential for a nice little Wet Workshop...

Well in that instance, IMHO, you'd be better of with dedicated Station Modules and a small Centaur-derived braking stage that gets them into the stable EML2 point. IIRC Wet Workshop Stations were dismissed in the 1960's on account of difficulties with regards to modifying spent stages in low-zero G conditions, and haven't been seriously considered since IOTL.

But the question here would be why? Why put a large (or even a small) station there? Maybe if you want something to support a crew in case of total failure of the CSM to get back, it could help support them until a fresh rescue craft can be sent to them, but even that is unlikely when you take into account that NASA would make all practical efforts to make certain that such a scenario simply won't be allowed to happen.

If you want a station within easy reach to test a few "critical" technologies with regards to manned BEO Mission (Minus Luna), e.g. Artificial Magnetic Field, Long periods without resupply, then there is a little sense here. But the above examples are just as easily accomplished in LEO as they are at EML2.

So again, why?

 

[su_liam]

The LCLM does use a cryogenic descent stage, yes, and has anti-boil-off measures designed in. It only needs to keep the hydrolox liquid for a week or so, though, so it's not a huge deal.

According to some documents I've read from NTRS and ULA, it looks like six months with fairly minor boiloff is pretty doable. These were, I think, ca. 2005-ish, but a week or two seems reasonable for the 90s.

 

[Shevek23]

According to some documents I've read from NTRS and ULA, it looks like six months with fairly minor boiloff is pretty doable. These were, I think, ca. 2005-ish, but a week or two seems reasonable for the 90s.

It won't need anything like that; hours, days at the most, recalling that the TLI stage will be launched either just before or just after the manned, mission-modules stage, and they will rendezvous immediately.

Now I'm wondering just how that will work--the stack and all that. Presumably the mission-mods-manned stage is stacked, on top of the Heavy second stage, bottom to top like this: Lunar excursion vehicle, already mated to the Mission Module, L2 Service module with the CM mated to it on top, escape tower ready to pull the CM loose as usual in case of abort. Then upon reaching orbit (instead of waiting for completion of TLI boost as per the old Apollo moon program) the CSM pulls loose, turns and docks to the MM.

Now what? Does the CSM pull the big stack of disposable stages out of the second stage with its maneuvering thrusters as per old Apollo? Or does the crew move down, into the MM and ascent/habitation stage of the Lunar lander set and take over controlling the stack from there? And use the Lander's thrusters to break loose?

And then--which way does the stack dock with the TLI booster stage? I'd think backing the Lunar lander into a docking frame, securing it to the booster the way it was originally bolted to the ascent second stage would be sensible, because the lander needs some kind of "straight down" view for the pilot to land it on the Moon anyway, whereas the view from the CM is blocked by the big Lander stage. From the picture of the Boeing/Grumman lander though that view is apparently by some kind of TV anyway. It might just as well be switched to the CM control stations I guess.

If we assume the crew must remain in the CM to ride out the TLI boost, and that the rendezvous and docking with the TLI rocket will happen pretty quick, in a matter of hours at most, I suppose they could hold off on turning around and mating up with the MM until after TLI is done as per old Apollo after all. Otherwise, with the CSM perched nose to the MM, and the crew riding it out there, they'd be eyeballs-out, strapped in their couches upside down during the burn. Alternatively if they backed the SM into a docking cradle with the Lunar stack's bottom facing forward during the burn, there's the combined mass of the MM and Lunar landing stack, all balancing on the CM's nose--that strikes me as quite a juggling trick! It all depends on the thrust of the TLI stage of course, but it seems dubious to me.

So I suppose they delay the MM mating and all stay confined in the CM until after undocking the LM from the ascent stage on LM thrusters, then steering the whole stack, still in Earth launch configuration, to dock with the TLI rocket, LM feet down to it, using the same cameras they have to rely on for Lunar landing. Now the modules are all "right side up" under boost, and the same structures that held the stack together during launch are still securing them during TLI. Then as per old Apollo, the CSM does its thing turning around.

In old Apollo, the Service module had the job of braking the whole stack of lunar vehicles to LLO, then boosting itself and the CM back to Earth out LLO. It seems clear, given the more ambitious Lunar mission and larger crew (and addition of the MM, though it is the Lunar lander stack that will be considerably bigger than the LEM of the '60s that makes the most difference I think) that here it will be the Lander engine(s) that maneuver the stack into station at the Lagrange point. Therefore if the crew remain in the CM for that maneuver they will be eyeballs-out.

And no matter what, we still have the dubious circus stunt, like a seal balancing a ball on the end of its nose, of the CSM wobbling around on 'top', in force terms, of the stack. I keep wanting to imagine auxiliary support struts being deployed, from the MM rim to the heatshield rim of the CM, or beyond (to avoid stressing the critical heat shield) to the SM, cradling the thing. Three seem like the adequate minimum, six don't seem excessive to me!

To be sure the halo orbit injection can be done at low thrust. The Lander needs enough thrust to brake its descent under lunar gravity, I guess 2 Lunar G's (or 1/3 Earth G) would be adequate though perhaps we'd want the margin of 3 lunar G, or about 1/2 Earth's--5 meters/sec^2 then. But that is at landing, when the Lander stage's propellant would mostly be depleted. Here it is full (except for what gets used up in halo injection) and there is also the mass of the CSM and MM. If those two put together mass as much as the LM (they may very well be much less though; they'd be about 12-15 tonnes but while that matches the old LEM all up, the new one will be considerably bigger I'd think) and if the descent propellant makes up about half the LEM mass we'd be down to just 1.25 m/sec^2 acceleration. And of course we don't have to use full thrust; the landing engine will need to be throttleable, and/or made up of clusters of small engines turned on and off as needed.

Still--12 or more tonnes of CSM, just wobbling around on one docking connection--wouldn't we all like to see some reinforcing struts to keep it steady? I sure would! They needn't mass a lot, and can be conduits for power, signals and fluids between the SM and the LEM.

(And I'd recommend the same thing for any further Apollo derivatives beyond the Block V that happen to have rather massive MMs.)

The crew will probably, upon assembling the full stack in mission configuration, come to regard the Lander ascent stage as the real command center of the ship; once TLI is accomplished at the latest they'll want to move in there, which will after all be their home on the Moon's surface anyway.

So in fact I suggest, forget the separate MM between them! Just use the ascent stage of the Lander as the mission module. That means of course having to boost its mass back to Earth from the Lagrange point, but the proposed mission profile does include the MM in that anyway. The ascent stage would be more massive and so require more propellant in the SM, and that's storable propellant so it masses more per delta-V times spacecraft mass. But the ascent stage's all-up mass at launch would be greatly depleted by the ascent propellant used up returning to the Lagrange rendezvous, and other mass can be jettisoned on the Moon or after rendezvous with the CSM; what is left (the habitation core mainly) probably won't outmass the MM by much and would serve the MM's purpose better.

Finally, to return to responding to su_liam's post: even without these revisions I'm suggesting, it seems clear that it is the Lander descent engines that will be the workhorse of the mission in the way that the SM was in old Lunar Apollo. So it would be that stage, of all the translunar ones, that would most profitably be hydrogen-oxygen cryogenic instead of storable fueled. Its propellent would be expended early in the mission--but not in a matter of hours after launch as with the TLI rocket. It needs to coast out to L2 over a period of time that (I fear! no one has gotten back to me yet on the question of how much longer an L2 transit takes than old Apollo transit to LOI) would probably be close to a week, then do up to four burns (a burn at perilune if something like Farquhar's trajectory applies, halo orbit injection at L2, departure of just the Lander stack from L2 to lunar surface destination, and finally landing) over a period of days.

So the news that several weeks can involve little boil-off is good to hear, and your six months or so even better, but I suppose those figures were for a more massive TLI rocket, not for the relatively petite Lander stage. (Even if the Lander tankage is significantly larger than just the landing alone needs, what with all those other burns first). Square-Cube law suggests the Lander's boiloff period would be shorter, so any advanced insulation or rechilling tricks we can manage would be best applied there.

It sure would be nice to be able to use hydrogen-oxygen for the ascent stage and even the SM as well. The latter is out, not just because it has to hold out the longest while having even worse square-cube issues than the Lander, but because the program mandate is to minimize redesign from established Apollo family practice, and the SM has always used storables. Redesigning a hydrogen SM is clearly off the table (though it wouldn't be starting from zero; the old Apollo lunar program did examine the concept back in the 60s). A case might be made for the Ascender rocket being hydrogen, using any nifty rechiller we might have in the Descent stage to keep a reserve stock of hydrogen in a tank on the Ascender cold during Lunar daylight, powered by solar power--at night, the problem of hydrogen boil-off would be much mitigated.

I expect the arguments and caveats I made against staging through L2 have already been considered by the authors. But until they say more, we don't know enough about the proposed delta-Vs and transit times of the mission stages to judge just what capabilities the various stages of the translunar stack need to have.

I'm in favor of as much hydrogen-fuel usage as can possibly be managed, myself.

 

[Michel Van]

Interesting Point, Shevek23

on Earth orbit rendezvous
Pegasus could reach LM&CSM in 4~6 hour after his launch see current ISS approach an docking.
Pegasus is covered in thermal protection foam to prevent boil off for those hours.

i look on Apollo TLI burns source http://www.braeunig.us/apollo/saturnV.htm
they take around 6 minute with a maximum of 1.45 G

1.45 G sound not much, but hardware has support 1,45 more time it's own weight !
With modern materials no problems, but we need support and holders form CSM&LM to Pegasus stage.
That the LM has a docking ring with supports to it, would make the task easier
the ring and supports would remain on pegasus after jettison
LM two stages frames, makes also the supports for force transfere during LTI between CSM and Pegasus

The Halo orbit insertion (HOI) make around 140 m/s that can make the CSM with own RCS thruster !

on Apollo 13 type "problems" and solution
CSM malfunction: abort to Lunar surface, there a supply lander waiting them anyway. so they can send a new CSM to Halo or new supply lander until CSM bug free.
LM malfunction: the CSM is than lifeboat making a lunar fly by back to Earth.
and the CSM & LM need compatible system for exchange in case of problems, see Apollo 13 problem with incompatible carbon dioxide scrubber cartridges.

 

[Workable Goblin]

CSM malfunction: abort to Lunar surface, there a supply lander waiting them anyway. so they can send a new CSM to Halo or new supply lander until CSM bug free.

This is impractical because of the small number of missions planned per year (one) and the resulting shortage of prepared supply landers and launch vehicles needed to allow this option. In any event, the very large delta-V capability needed by the landers and the fact that the lander ascent stage is designed to support life alone for a long period of time anyways means that the nominal response is essentially to follow the Apollo 13 profile, with various options to accelerate the return if necessary.

And, yes, all essential components, including the air filters, have been designed to work in either spacecraft, so far as possible.

 

[e of pi]

Still--12 or more tonnes of CSM, just wobbling around on one docking connection--wouldn't we all like to see some reinforcing struts to keep it steady? I sure would! They needn't mass a lot, and can be conduits for power, signals and fluids between the SM and the LEM.

It's well within the margins for which the Apollo probe and drogue was designed in the 60s--and they never got rid of those margins. While you may be troubled by their lack aesthetically, any added "struts" would be complex (they'd need to "latch on" after docking or be installed on EVA, and yet be separable in a hurry) and the Block V Apollo, not counting any MM, masses less than the original Apollo LM--which of course was "wobbing around" on the end of an Apollo P&D on the original Apollo flights, so this is actually well within the original design parameters for the Apollo P&D.

(And I'd recommend the same thing for any further Apollo derivatives beyond the Block V that happen to have rather massive MMs.)

As mentioned, the Apollo probe and drogue system could manage the 15-ton LM, so anything up to that should be doable. Frankly, by the time you'd get there, it'd be better to just design a new, larger capsule--perhaps with integral thrusters like Dragon for re-use, and certainly more room in the pressure hull for downmass cargo or crew.

So in fact I suggest, forget the separate MM between them! Just use the ascent stage of the Lander as the mission module. That means of course having to boost its mass back to Earth from the Lagrange point, but the proposed mission profile does include the MM in that anyway. The ascent stage would be more massive and so require more propellant in the SM, and that's storable propellant so it masses more per delta-V times spacecraft mass. But the ascent stage's all-up mass at launch would be greatly depleted by the ascent propellant used up returning to the Lagrange rendezvous, and other mass can be jettisoned on the Moon or after rendezvous with the CSM; what is left (the habitation core mainly) probably won't outmass the MM by much and would serve the MM's purpose better.

We'd examined this before, but we'd had concerns about the impact on the propellant requirements--it wasn't within the prop capacity of the Block IV Apollo tanks. However, we gave it a second look this weekend, and it actually looks like it's within the tank capacity of the Block II Aarvark SM--which is the Block IV SM with optionally expanded tanks. Given the deliberate commonality between the systems, migrating lunar-mode Block Vs to this configuration is pretty easy. It turns out it adds about another 750 kg of required propellant to the TEI burn to get the ascent stage back in place of the MM we were looking at, but it saves the 2 tons or so of the MM on TL2I and halo orbit insertion, so it does (narrowly) save mass on balance and greatly expands the volume and capabilities available to the crew on the return-to-Earth leg.

I expect the arguments and caveats I made against staging through L2 have already been considered by the authors. But until they say more, we don't know enough about the proposed delta-Vs and transit times of the mission stages to judge just what capabilities the various stages of the translunar stack need to have.

Shevek, did you see Workable Goblin's post on the end of the previous page? He addressed a lot of our assumptions and your earlier comments there.

 

[Bahamut-255]

We'd examined this before, but we'd had concerns about the impact on the propellant requirements--it wasn't within the prop capacity of the Block IV Apollo tanks. However, we gave it a second look this weekend, and it actually looks like it's within the tank capacity of the Block II Aarvark SM--which is the Block IV SM with optionally expanded tanks. Given the deliberate commonality between the systems, migrating lunar-mode Block Vs to this configuration is pretty easy. It turns out it adds about another 750 kg of required propellant to the TEI burn to get the ascent stage back in place of the MM we were looking at, but it saves the 2 tons or so of the MM on TL2I and halo orbit insertion, so it does (narrowly) save mass on balance and greatly expands the volume and capabilities available to the crew on the return-to-Earth leg.

Am I to take this as meaning the the Ascent Stage on the new Lunar Lander is now an AS/MM Combo unit?

If yes, then does the Ascent Stage feature jettisonable propellant tanks to trim mass at the penalty of increased complexity? Or are they kept fixed for a simpler design with the acceptance of an increased propellant requirement for the CSM/AS?

And could you clarify this point for me which has had me thinking for a while now? from what I gather, the new Lander is being treated as a Taxi for getting the crew to and from the Lunar Surface, yet I see no mention the supplies vehicle featuring it's own Crew Support (as in actually housing them). So which one does the crew actually reside/sleep in while on the Lunar Surface?

 

[Michel Van]

Am I to take this as meaning the the Ascent Stage on the new Lunar Lander is now an AS/MM Combo unit?

If yes, then does the Ascent Stage feature jettisonable propellant tanks to trim mass at the penalty of increased complexity? Or are they kept fixed for a simpler design with the acceptance of an increased propellant requirement for the CSM/AS?

And could you clarify this point for me which has had me thinking for a while now? from what I gather, the new Lander is being treated as a Taxi for getting the crew to and from the Lunar Surface, yet I see no mention the supplies vehicle featuring it's own Crew Support (as in actually housing them). So which one does the crew actually reside/sleep in while on the Lunar Surface?

I think this Artemis is like the Apollo application Program lunar mission.

there land a unmanned LM shelter first, later manned LM taxi near it
the crew power the LM Taxi down and use the LM Shelter for 14 days.
while the CSM make Lunar-surface mapping like Apollo 15-17
then the Crew shut down the LM Shelter and power up the LM Taxi and take off to rendezvous the CSM

i think Artemis mission goes almost the same*
either a tin can for 4 person or inflatable Transhab on Supply lander.

* except the CSM Lunar-surface mapping, here is CSM store in Halo orbit, the mapping is made by cheaper unmanned probe.

 

[e of pi]

Am I to take this as meaning the the Ascent Stage on the new Lunar Lander is now an AS/MM Combo unit?

Yeah. It's got most everything it needs already.

If yes, then does the Ascent Stage feature jettisonable propellant tanks to trim mass at the penalty of increased complexity? Or are they kept fixed for a simpler design with the acceptance of an increased propellant requirement for the CSM/AS?

Fixed. It's well within the CSM fuel margin, and the loss of a ton or so of dry weight (which translates to a fuel savings of about 100 kg) isn't worth the complexity increase.

And could you clarify this point for me which has had me thinking for a while now? from what I gather, the new Lander is being treated as a Taxi for getting the crew to and from the Lunar Surface, yet I see no mention the supplies vehicle featuring it's own Crew Support (as in actually housing them). So which one does the crew actually reside/sleep in while on the Lunar Surface?

Among the main "cargo" that the cargo lander is landing is the surface habitat. The crew lander is mostly doing just that--landing the crew, the ascent module, and maybe a few hundred kg of miscellaneous basic surface equipment, but the surface hab and everything else for the mission proper comes down separately in advance on the cargo lander.

 

[Dathi THorfinnsson]

If, IF they used lh2/lo2 all the way, it's 'easy' to get oxygen from the regolith, which means return visits only need to carry lh2 for the ascent, which is hugely lighter. It only rally works for a permanent base, though, and youd have to dedicate at least one flight for the o2 factory.

 

[Bahamut-255]

If, IF they used lh2/lo2 all the way, it's 'easy' to get oxygen from the regolith, which means return visits only need to carry lh2 for the ascent, which is hugely lighter. It only rally works for a permanent base, though, and youd have to dedicate at least one flight for the o2 factory.

Well in that instance, they'd only need a fuel that can properly burn with O2, any fuel that they can store for a protracted period on the Lunar Surface. That means Kerosene, L2CH4, LCH4, or LH2 would make viable Ascent/Earth Return Propellant with Lunar LOX Oxidiser.

And like you said, that's only likely to happen if it's either a very-long stay exploration or a permanent base.

 

[Michel Van]

Tuesday, Arts Day

The Europa 2-TA (official Canon now)

Some Explanation:

The Blue Streak is modified, the sub-systems are installed inside Stage so the two side container are removed
the RZ.2 engine have protective cover against booster exhaust

The Booster are first stage of french SSBM S3 and later Black Diamant. (the ESA internal designation is P-16)
The two booster configuration need aerodynamic fins for stabile launch, do to higher acceleration.
The four booster configuration acceleration is much slower because only one pair burn, after it's jettison, the second pair ignite.

the Data in ETS Wiki on Europa 2-TA are the four booster configuration.

 

[Bahamut-255]

Nice to see the Europa LVs illustrated now, but I do have a question concerning it.

It would appear that the Europa 1st Stage has 6 attachment points for the two booster configurations available to it. Why is that the case? My best guess is that it involves Launch Pad integration along with keeping the booster nozzles away from what appears to be the 1st Stage Engine Preburner Exhausts, but I can't be completely certain that that is the case.

In any case, I wonder how many LVs are left to be illustrated now. I suspect that there's still the Europa-2 with LOX/LH2 upper stage for ESA, plus the upcoming Chinese and Indian LVs.

 

[nixonshead]

Tuesday, Arts Day

The Europa 2-TA (official Canon now)

Wonderful work once again! It's great to see the Europa rockets overcoming their OTL difficulties. Shame to see those distinctive pods go, but the distinctive ribbing remains in place.

In any case, I wonder how many LVs are left to be illustrated now. I suspect that there's still the Europa-2 with LOX/LH2 upper stage for ESA, plus the upcoming Chinese and Indian LVs.

Well, I'm looking forward to seeing Europa 3 and 4 (with Minotaur perched on the summit!), not to mention Delta-4000 and Caravel. Then there's Japan's efforts too.

 

[Michel Van]

Nice to see the Europa LVs illustrated now, but I do have a question concerning it.

It would appear that the Europa 1st Stage has 6 attachment points for the two booster configurations available to it. Why is that the case? My best guess is that it involves Launch Pad integration along with keeping the booster nozzles away from what appears to be the 1st Stage Engine Preburner Exhausts, but I can't be completely certain that that is the case.

In any case, I wonder how many LVs are left to be illustrated now. I suspect that there's still the Europa-2 with LOX/LH2 upper stage for ESA,

those "Engine Preburner Exhausts" are the RZ.2 turbo pump exhaust
the one reason why the booster are position like that, the other is rocket stand at launch pad.
thanks to the many ribs on Kerosine tank, the Blue Streak is easy to adapt with multiple attachment points configuration, especial for P-16 Booster

For the moment i work on EUROPA 1 & 2
i wait on EUROPA 2-HE details like Payload faring, next on list is then EUROPA 3 & 4.

plus the upcoming Chinese and Indian LVs.

ehh more rockets for me ?

 

[Dathi THorfinnsson]

Well in that instance, they'd only need a fuel that can properly burn with O2, any fuel that they can store for a protracted period on the Lunar Surface. That means Kerosene, L2CH4, LCH4, or LH2 would make viable Ascent/Earth Return Propellant with Lunar LOX Oxidiser.

And like you said, that's only likely to happen if it's either a very-long stay exploration or a permanent base.

Not really, no. For the other fuels, oxygen is only half the propellant total, which makes payback much longer. With LH2, oxygen is like 85% of the total, which is huge. Its even then worth lifting oxygen into orbit to refuel spacecraft. If all that comes from earth is LH2, you can massively improve beyond LEO access.

 

[Bahamut-255]

Not really, no. For the other fuels, oxygen is only half the propellant total, which makes payback much longer. With LH2, oxygen is like 85% of the total, which is huge. Its even then worth lifting oxygen into orbit to refuel spacecraft. If all that comes from earth is LH2, you can massively improve beyond LEO access.

I've checked the Optimum Propellant Mass Ratios for a few propellant mixtures using LOX as the Oxidiser:


LOX/LH2 - 6:1

LOX/LCH4 - Can't find the right number here, but I suspect the Optimum Oxidiser/Fuel Mixture Ratio is about 3.5:1

LOX/Kerosene - 2.56:1

LOX/UMDH - 1.67:1


In all of the above cases, LOX accounts for over half of the total propellant mass. Even with UMDH, LOX accounts for over 62% of the propellant mass if my math is correct.

In any instance though, I'd suspect in such a scenario there would be quite a few people pushing for the LOX/LCH4 option. Namely on account of providing a means to get LOX/LCH4 Engines properly matured ahead of a Manned Mars Programme they'd be pushing.

 

[Shevek23]

...
Shevek, did you see Workable Goblin's post on the end of the previous page? He addressed a lot of our assumptions and your earlier comments there.

No, I totally missed that post until late last night, and then could not think about it much because of work stuff. Which has me bogged down all week, until Saturday after noon.

It came between my two posts, and while my second was meant to indicate I understand that your orbits are quite different from Farquhar's by being more energetic, I never tried to do the math until tonight.

Having spent some time doing so, I can readily see how the vicinity of EML2 can readily be reached within the 3 day timespan you allot.

OTOH I could really use some clarification about the halo orbits and the plan to reach them with really low delta-V's like just 150 m/sec or so. Figures as low as you cite for that part, while plausible for Farquhar's minimal energy trajectory (which puts the craft in something near a 6 day LLO-L2 Hohmann orbit, which would have no radial velocity to kill on arrival 3 days after perilune and a transverse velocity somewhere in the ballpark of 1/4 the total needed, which relative to Luna is small, under 200 m/sec) seem low by an order of magnitude for what is needed to park after a fast 3-day transfer all the way from LEO. It seems to me that if you take a lot less than 72 hours to get from near the Lunar surface to L2, you'd arrive where you want to match to a halo orbit with a lot more outward radial velocity than you'd want and not enough transverse--and this despite being quite clever in using the Moon's gravity to curve the trajectory to lower the former and raise the latter.

As I said here (IIRC) years ago, it is an attractive approach for many reasons, but the logic of it seems far more compelling in terms of a base-building program, where the economical 9-day-out orbit can be used for cargo to supply both a Lagrange Point base and supporting exploration and possible exploitation activities at many points on the surface; none can be reached with anything in less than days, but any can be reached at any time.