I've been doing some work on a rewrite of 'Seeing Further' and decided to go with a near OTL STS shuttle (save for a pair of jet engines in the wing roots for more cross range), and needed an HLV for a Mars mission (nuclear). So I've done some math and design for an SLS lite, by merely sticking the STS engines to the bottom of the external tank and add the payload on top.
Shuttle parts are from the NASA 3D model repository.
Now I ask you--why not take the thing on the right, and use it for every launch? If you want to send up cargo, send up cargo. If you want to send up people who of course require at least the option of safe return to Earth, make a space bus of some kind, one that mounted up there on top, in addition to advantages e of pi listed, also can more possibly have some kind of escape system. If you need to retrieve something from orbit--not done all that often OTL--make a reentry shell of some kind to launch empty (or with a bit of cargo to make up the maximum launch weight rattling around inside) and stuff the down mass into that when the cargo is stowed. It shouldn't generally be necessary to have crew accompany the down-mass carrier if the down mass comes from a space station. If it is a question of going out to some defective satellite in an isolated orbit and hauling it down, it might be necessary to provide some crew to do the stevedore work, but this just means that 1) a means of this crew escaping a launch gone bad needs to be provided and 2) more mass for general habitability, and mission related equipment.
So here I'm listing 5 or 6 different types of payload. One is unmanned payload as such. OTL the STS designers promised prospective customers for launch that the Orbiter would provide services for the various payloads that would allow them to simplify construction. Also, the limited G-load aimed at in the design (under 3.5 Gs in all circumstances) would ease it as well, so if we want to meet those promises, we need to design the launcher accordingly--it costs in gravity loss hence a bigger system than if we ignored that G limit and shoved stuff into space with peak Gs around 6-8. (This is one reason the Soviets could launch big payloads in the multi-ton range with the relatively modest R-7 rocket base--that system put pretty high G loads on, as did/does Proton). Anyway in addition to modest G loads STS promised cargo support, and the way I envision an STS variant that would be dramatically more economical would involve the payload package, whatever it is, being placed initially in a very low orbit, so some of our total initial mass has to be devoted to fuel and engines to move the the package up to the desired orbit. This is relatively modest, but I've been thinking an all up rule of thumb is that only about 60 percent of the package can be actual revenue payload, on the terms STS promised anyway. We can kick that up near 100 percent by telling the customers their payloads are their business, all STS promises to do is deliver them to that initial low orbit and they have to make their own way from there. But assume for the moment NASA promises the coddling, then a "cargo" load still involves a vehicle bus massing some 10 tons for every 15 tons of revenue payload carried. This is a vehicle, and it is going to cost.
But hey, if you read the Shuttle Decision book it is clear that NASA required work for all their subdivisions; designing a disposable cargo bus and then tailoring it for each launch keeps Marshall and perhaps JPL or Ames busy!
Then, assuming we have a space station, we have need for the station bus; this is a system to deliver some large number of people. If we are working with 120 tons, we will be delivering a hell of a lot of people to what must be some gigantic space station! The trouble here is that it is irresponsible to design a crewed spacecraft that does not have survivable abort contingencies covering the entire launch. Escape from an exploding stack requires substantial delta-V and in the early stages, requires it to be applied very fast, within a second or so, to get adequate distance separation from the disaster in progress. Then it has to land somehow. If we are looking at 8-12 people, it can be done. Several hundred, and no.
It probably makes a lot more sense to limit the size of a pure human transport bus to something realistic launch escape systems can handle and use any extra capacity (out of a 120 tons it would be a lot) for cargo as usual. Presumably this is all cargo destined for the same space station so that works out well. The crewed ship sits on top, the cargo, to be sacrificed in an emergency situation, is below.
Which tells us how missions typical of STS OTL would be designed. A people bus ship with full abort capability and spartan survival conditions rides ahead of a crew expanded habitably/workshop module. We can design the latter to be a disposable cargo with no provision for return to Earth (except in the form of sending it to destruction in the atmosphere for disposal) or we can integrate the core crew bus into a larger landable craft for reuse. This flexible concept again keeps design teams busy for individual missions.
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Anyway, your cargo launcher version sacrifices the main engines, being attached to a non-returnable tank. My proposal is to do your thing on the right (on a different scale probably, though I'd sure hope to work up to yours, and beyond!) but put the engines in some kind of capsule to allow them to return. I think that an SSME can be contained in a structure all up less than 15 tons, perhaps as little as 12--10 would be pushing it I'd admit. And we can approach solving this problem from several directions. Some sort of Gemini or Apollo type capsule that folds its upper cone side like petals of a flower around the exhaust bell of an SSME would be a compact structure that still could allow the 11 degrees gimbaling of the engines STS required, and we'd just design different size tanks to bolt them on to for different sized missions. More elegant, if perhaps heavier, would be to make some sort of space plane or lifting body for each engine and then they could perhaps fly back to runway or pond landings at the launch site after a once-around orbit.
If this system were implemented, you would have to deduct somewhere between 30 and 60 tons from payload for a 3-engine system, probably closer to 36-45 tons. But the engines would go on being reused.
Or of course if you could return the tank too, with engines attached, the reusability would be greater but probably the payload penalty higher.
In addition to matching the reusability of the OTL Shuttle while allowing delivery of larger payloads, this approach allows tailoring of different size of launch vehicle. Unless the greater economy going with greater payloads raises demand tremendously over OTL, causing the "space rush" Shuttle Decision planners assumed would happen, 120 tons to LEO, even with 40 deducted to leave 80, is a bloody huge payload for one launch--enough for a whole Skylab, anyway most of one. There won't be a market for that kind of launch very often! But you can downscale, make a 2 engine or 1 engine tank, order smaller SRBs, and launch on scales the market is more responsive to. Such small scales are also more appropriate for a manned craft; something with all the habitability of Orbiter but carrying no main launch engines and no internal cargo for delivery elsewhere, only stuff to be used on the ship on the mission, would be much lighter. Two SSMEs, or even just one, should be adequate to orbit that. We'd only need the heavy version for grand projects--or if space emigration really takes off!
At which point I'd strongly urge NASA to look into developing launch loops or something comparable
The Wet/Dry workshop concept was developed during the 1950s to use existing rockets that had expanded their fuels as stations by opening up the tanks, meaning that they were 'wet' during ascent. A dry workshop is 'Dry' during ascent, the best example is Skylab....Though it (Shadow Master's one-shot use of SSMEs attached to the tank}makes using the external tanks as wet workshops a tad harder.
Well, a big wet lab is something you'd want to develop into something grand over the years, and it could use some engines to move it. Or those engines can be detached and installed on some deeper space vehicle.
So what would we call a structure that is lofted as an "outer hull" for the ET or payload. ...
I think e of pi makes it clear enough how problematic this sheath thing is. One possible justification for it would be that one is going for "wet lab" structures on a grand scale, as with the tank-based space station concept. And so, smuggling an outer one-layer "tank" outside the real tank to orbit gets you two tanks. But of course not for the price of one! The thing (assuming we base all this on standard OTL STS sizes) would mass another 30+ tons altogether, so it is two for the price of 2. It is payload, it comes out of your payload budget. If your payload budget is really 120 tons, it is a small price to pay--if what you want is another great big tank. Or big structural panels I guess. Based on STS of OTL, with returnable engine module(s) you are not going to have 120 tons and it makes a bigger hit. I daresay the extra sheath will improve the insulation properties of the tank a bit, and might justify a lighter load of foam insulation, which saves some trouble when you unpack it--you are obliged to prevent the foam from becoming space junk, a layered design might allow elegant detachment and stowage of the stuff.
Also, such a notional “Bigger than OTL ET”, could be fitted with a scaled up return capsule for larger objects to be returned in a cost effective manner to Earth. Did someone mention SSME’s?
Me, me, me mention SSME and returning them! I think the idea of reusing the SSMEs is central to the whole idea of calling something a "reusable" space launch system in the first place. Which is why I urge designing a system to return them all after launch immediately. In early days, when launches are infrequent, it would be essential to incorporate the engines into their capsule system from the get-go so this idea of carrying a separate return module to unship and pack the engines into in orbit is redundant--then. If we get a high volume of launches shifting over to it might save weight and thus further enhance payloads. But I'm not at all sure it would work out to be a lot lighter than integral engine/capsule with immediate and simple detach and return process. The latter sure does save a lot of astronaut or robot tug work in orbit, which is at a premium! If we have a great big space station with hundreds or thousands of residents, the labor cost question shifts in favor of doing work in orbit of course.
It's worth noting that "wet workshops" have never panned out. It turns out that the difficulty and expense of re-fitting the tanks (which require long hours of EVA work by highly, highly paid astronauts who only get to the work site by being lofted into space on billion-dollar rockets.
It has been far cheaper in practice to launch whole dry workshops that are pre-built on Earth by mere high paid workers who don't need to go on an EVA to work inside the construction site and can commute to work in a $9000 car.
Powerful arguments to be sure. It depends though on how much investment we are making for our station, how long we want it to last, whether it will grow into something really really grandiose or if all we want is something like Skylab or a Salyut. In the latter case, of course it is not worthwhile to devote a lot of the limited ration of manned visits to actually building the thing!
But suppose Skylab had been designed with an eye for operating for a decade or more, with a plan to evacuate waste for disposal and ongoing launches to deliver not only crew but a stream of upgrading equipment along with the supplies. Skylab was launched with a variation of the Saturn V that used the second stage to fully reach orbit. Which means that at Main Engine Cut-off, the "spent" Saturn second stage was in orbit along with the Skylab itself. Now suppose that at some modest sacrifice of Skylab's initial capabilities, a tunnel and equipment and reserve air supplies were provided for the first crews to cut into the upper fuel tank (which has been opened to vacuum to vent out any residual hydrogen) and start fitting it out with more stuff? And we have an ongoing supply of upgraded S-1B type rockets and modified Apollo SMs that are lighter, so every mission carries cargo--mostly supplies but also additional stuff to fit the hydrogen tank out with. Gradually we work our way to exploiting the oxygen tank as well. To keep orbit, firing even just one of the J-2 engines even briefly is probably overkill, so those engines are pretty much useless, but compared to the larger stage mass the five of them don't mass all that much so we can afford the dead weight. Or if it is problem, pre-design the attachments with explosive bolts and blow them off, each (or ganged together somehow) with a small deorbiter solid engine to drop them into the atmosphere at a chosen moment.
Doubling up Skylab like this has costs to be sure; we have to invest more to get more. But the crew aren't doing EVAs, they are working inside an air-filled shirtsleeve environment. It isn't 2 for 1, but close to it; we start out with less than 1 considering that we have to dedicate some of Skylab's "dry" mass to refitting the tank from the get-go, but with a dozen missions or so we work it up to nearly 2 as we keep attaching stuff to the tank interior. Conceivably the plan could be to make this version of Skylab the core of a permanent space station, and then the spent stages come in very handy indeed.
Imagine what might be if every module of ISS that was brought up by STS came accompanied by the ET from the launch bringing that component! It would take some extra investment to use each tank but clearly it can pay off.
Since my post to your thread explaining how I attributed 150 tons of orbital capacity to STS (and admitting I was off by 10 or maybe more tons) got lost in various shenanigans, let me say right now--I was counting the mass of the fuel tank, for purposes of comparing to Saturn V lift capability.
Generally we don't want it, certainly not with a space program as constrained as OTL. But if a superior approach to launch logistics might enable a "space rush" compared to OTL, and feedback causes a much grander program, we will find these "disposal" items more useful than they are on our current scale of operations. Labor in space will become relatively less costly and justify more projects like this.
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I think the main issue with wrapping the station walls around the ET for launch is it then begs the question of how the SRBs attach and how the orbiter attaches.
fasquardon
Remember Shadow Master is proposing an in-line launch system. The solids remain and the problem you point out remains with them, but the main hydrogen engines are on the tail of the stack, not sidesaddle. (Where I think they belong!)
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Also consider: With the sheathing method, it becomes necessary to carry the ET up to orbit since it's hard to jettison it from inside the can. That actually eats up something like 20-30 tonnes from the orbital payload....
I seem to have missed quoting the person down thread--bahmaut IIRC--who corrects you on this. The STS launch mode, OTL, required the tankage to be quite adequate to put the mass of the tank as well as Orbiter into orbit; it was then necessary to choose an elliptical orbit the SSME burn put the whole thing into that would dump the Tank into the atmosphere. This meant of course the Orbiter also was at that point on a collision course with the atmosphere!
But, I can't stress this enough, not because the delta-V generated by that point was not adequate to achieve full orbit, as was the case with the second stage of the 3 stage Apollo Lunar version. No, the STS as designed could have put every Orbiter/tank combination into LEO. Lower LEO than the Orbiter generally went to, but high enough that the combo would stay in orbit for a considerable time before decaying. To then deliver a tank massing at least 27 tons on to a high orbit where it would be stable would be a major chore to be sure. But by sacrificing the payload in the cargo bay, it could be done--if necessary, by using some of that volume and mass capacity for supplemental OMS fuel, which might require a redesign never done but certainly could be doable.
Let's see here--say we are in a 100 km orbit initially-that's lower I think than the combination could reach on SSME burn alone, but let's go with it a moment. We can't stay there long, atmospheric drag on the tank would be significant, but let's say we don't dally around; as soon as the orbit is verified, we wait to phase orbits for transfer to let us say ISS. This means we launched to a 52 degree inclination, and are lighter than usual unfortunately, but too bad. We have a full 10 ton load of OMS hypergolic fuel. The Orbiter masses 120 tons, the tank masses 27, all up 147. We need to go from a 100 km orbit to one averaging around 413 km altitude, that's a 313 km ascent, call it equivalent of 300 bearing in mind we are above Earth's surface some distance and the gravity field has fallen off a bit. Roughly speaking, we need to add half the necessary energy to achieve a transfer orbit, then the other half up there to circularize and rendezvous. We need about 94 m/sec delta-V immediately and then another burn of that amount later. The ISP of the OMS was 316 sec so we'd need to use about 8.6 tons of propellant to haul the tank along with the Orbiter to the station. That leaves darn little to deorbit the Orbiter, so it seems safe to say we need some extra out of the payload mass. But not a tremendous amount I'd think; a few tons would do it.
So no, as others have pointed out, depending on just how high an orbit the SSMEs alone can reach for a 52 degree inclination, and how much delta-V was actually lost with each launch by going for the swan dive maneuver to control disposal of the tank, we might actually come out ahead, anyway all it costs is well under 5 tons or so of payload. At worst.
This is why I felt justified in counting the tank mass as mass delivered to orbit. And I admit to being off by maybe 10 tons, so it is really 150 tons in these terms--not 160. But not no measly 120 either!
STS delivered comparable, and superior, total mass to orbit versus Saturn V, and massed nearly a third less on the pad...2050 tons versus 2850.
It is superior technology, well worth developing.
Although I will admit the only reason for the SSMEs to be so damn cutting edge with a 200 atmosphere chamber pressure was to enable a hydrogen engine to operate at all efficiently at sea level launch. And ground-lighting was only necessary because there was no suitable abort mode for the Orbiter crew. Develop an alternate in-line approach with the hydrogen burning upper stage only lighting well up in altitude, and we can get by pretty well with an engine like the J-2S, with a modest 30 atmosphere chamber pressure! improve its ISP a bit and we could have something practically as good as SSME at a lower price in every way--easier to make reusable, or vice versa cheap enough to dispose of.
But I suppose the hydrogen-burning technology of SSME helps explain how come STS seems something like 50 percent more effective than even the mighty Saturn.
The inefficiencies come from demanding a payload a Saturn 1B could have delivered be nannied into orbit by a spaceplane it takes Saturn V power to lift there. By decoupling the engines from the spaceplane we can make it much lighter, leaving capacity for much more cargo. Eliminating the spaceplane completely greatly increases it. With such economies at work we can consider developing the means to bring back the essential engines--which being air-lit, can be much lighter and thus easier to recover, if we haven't decided to just dispose of them instead.
The right Shuttle Decision, in retrospect, it seems to me, was to focus on developing a flyback booster equivalent or somewhat superior to the capability of the Saturn 1B booster, to enable a reusable stage of the same launch capacity as the disposable Saturn 1B second stage to loft something like 20 tons of cargo on a single disposable upper stage using a J-2S engine. And then gradually come to grips with the question of how to get the upper stage engine back home too, maybe with the tankage as well. Meanwhile reusable manned spacecraft will be a side development.
Had they gone that way, it might be a long time before they work up to a 120 ton payload to orbit capacity but when they do, it will be on a well tested basis.