From: henry@spsystems.net
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. | henry@spsystems.net
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