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Pulse Detonation Engine, first stage or ..



 
 
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  #11  
Old April 27th 04, 03:34 AM
Christopher M. Jones
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Default Pulse Detonation Engine, first stage or ..

(John Carmack) wrote in message . com...
(Gordon D. Pusch) wrote in message ...
Your last statement is simply absurd. Moon ships need to be "aerodynamic"
like fish need bicycles.

[snip]

I certainly agree with you regarding the value of a lifting-body, but
I would contend that there are circumstances where a single vehicle
design may still win out for moon missions. A VTVL SSTO can do the
LEO to lunar surface and back trip after refueling in orbit. While it
certainly wouldn't be optimal for it, the cost of a new vehicle
development could completely swamp the operational inefficiencies of
using the existing design unless flight rates got quite high.

The argument would also hold, perhaps more so, with a boosted
nearly-SSTO, with the nearly-SSTO optimized for vacuum and reentry.


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. It's also
one of the most costly. Compare the weight of the S-IVB
booster to the Apollo heat shield, for example. OK, that's not
really fair since the S-IVB had to boost a whole bunch more
mass than the CM. Compare the ratios then. The fueled S-IVB
took up 73% of the mass of the S-IVB + CSM + LM stack. The
Apollo heat shield however took up 15% of the mass of the CM,
and provided much greater "effective delta V".
  #12  
Old April 28th 04, 09:07 PM
Henry Spencer
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Default 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  
Old May 5th 04, 12:57 AM
Henry Spencer
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Default 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  
Old May 6th 04, 03:55 AM
Allen Meece
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Default 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/~~~
 




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