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#12
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Pulse Detonation Engine, first stage or ..
In article ,
Jake McGuire wrote: A VTVL SSTO has the raw delta-V capability to perform a lunar mission. Can its main propulsion system start in zero and 1/6 g? 1/6 G, almost certainly, provided it isn't dependent on ground support equipment to start at all. Free fall is a harder question. Can it handle the thermal environment of trans-lunar cruise? Probably. Overheating is the usual thermal problem for large vehicles, and the translunar cruise environment is colder than LEO. Can it handle the thermal environment on the lunar surface? Most likely, although this might impose restrictions on surface operations (e.g., might restrict it to an Apollo-style profile of landing in early morning with the Sun low, and leaving after only a short stay). Do its engines throttle deeply enough to allow sane lunar landing trajectories? Very probably, given that they have to throttle deeply enough to get it into orbit with nearly empty tanks, i.e. at a total mass far smaller than takeoff mass, without crushing the crew or breaking the structure. (The tanks would not be empty at lunar landing.) The continuous throttling needed for controlled lunar touchdown may be a bit more of a challenge, but probably isn't a big deal. Is it capable of weeks of on-orbit propellant storage? Probably, unless its designers were misguided enough to use LH2. :-) Remember that you don't need "weeks" -- Apollo flights only lasted about ten days. Or weeks of on-orbit power generation? Or a week of life support for the crew? Power would certainly need attention, perhaps a deployable solar array as part of the payload. Life support likely would need extra tankage, which again can be in the payload. (In many SSTO designs, some or all of the crew accommodations would be payload in any case.) How will you get down to the lunar surface from the cargo/crew compartment? Depends on where it is, but ladders are not heavy. And that's only the things that I could think of faster than I could type. You missed what is probably the biggest issue: can its heatshield handle a deep-space reentry? That is a *much* more severe thermal environment than a LEO reentry. For example, shuttle-style tiles are not up to it. For that matter, structural strength is also an issue for reentry, since higher Gs are normally involved too. Although much of an SSTO's structure would have to be sized for heavily-loaded takeoff conditions, so it might not be a big issue. ...you'd probably have to address them by modifying one (or a couple) vehicles specifically for the lunar trip. If you can't dedicate an existing vehicle or two to it, you surely can't justify developing a new one to be entirely dedicated to it! At which point it might very well end up that the modifications would cost more than designing an in-space transport from scratch... Depends on how often you plan to use it and how reusable it is. Full development (which is a whole lot more than just design) of a new vehicle is a big up-front expense. Unless your SSTO has a very small payload, the only part of the above which would deeply concern me would be the heatshield requirements. The rest either should be manageable or would involve only a modest reduction in payload. (Note that Apollo payload was only a hundred kilograms or so, counting the two-man crew as part of the vehicle.) There is no question that you'd *eventually* want a dedicated vehicle, as traffic picked up and the compromises inherent in using an existing vehicle became more annoying. But as an interim step, adapting a VTVL SSTO is not unreasonable. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#13
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Pulse Detonation Engine, first stage or ..
In article u,
Roy Stogner wrote: You missed what is probably the biggest issue: can its heatshield handle a deep-space reentry? That is a *much* more severe thermal environment than a LEO reentry. For example, shuttle-style tiles are not up to it. Does it help if the vehicle doesn't try to reenter on it's first perigee? Probably at least somewhat... but then you have other problems, like repeated passes through the Van Allen belts and an aerobraking program that's likely to take weeks. (The initial orbit has a period of a week or two, so if you're going to use more than a couple of passes, the earlier intermediate orbits are still going to be fairly long.) How much it helps would depend on the type of heatshield. Some aspects of lunar reentry are qualitatively different from LEO reentry -- it's not just "the same thing only more of it" -- and this would affect different technologies differently. -- MOST launched 30 June; science observations running | Henry Spencer since Oct; first surprises seen; papers pending. | |
#14
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Pulse Detonation Engine, first stage or ..
Moon ships certainly don't need to be aerodynamic around the
Moon, but there's a healthy advantage to aerodynamics on Moon ships. At some point you're going to want to go from the Moon back to Earth. And that means either a direct descent to the Earth's surface, aerobraking into Earth orbit, or propulsive injection into Earth orbit. The last is the only option that doesn't require aerodynamic parts on a Moon ship. Well, finallly back to square one after Pusch made the inane objection to a lifting body moon ship. He thinks a moon transportation system needs at least two ships: 1] earth to LEO, 2] LEO to Lunar orbit and maybe 3] LO to Lunar surface! Not everyone agrees with this expensive course. Those who disagree with Gordon say that a moonship would have to leave from earth and return to earth. One ship, one set of development costs and operations, one spaceport, no docking maneuvers. Simple is cheaper. I feel sure that this lifting body moonship would have to be boosted toward LEO by a really good air launcher, powered by fanjets, with their high Isp's, or a really efficient pulse det rocket, which may or may not prove to be beyond the capabilities of entrpreneurs to perfect for economical usage. ^ //^\\ ~~~ near space elevator ~~~~ ~~~members.aol.com/beanstalkr/~~~ |
#15
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Pulse Detonation Engine, first stage or ..
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