High density propellants for the 1st stage pays high dividends later on. Once you get to the 'staging' conditions of say Mach 4 or 5, 35-40 km altitude, then other propellant combinations can takeover from there. LOX/LH2 can do it from that point.

One of the biggest problems facing reusable vehicles is that you fly 'useless' hardware all the way to orbit before it becomes useful. I have heard it described that the space shuttle is just a 3 billion dollar reusable payload shroud. I think this is an oversimplification because it has been very useful on more than one occasion to have a crew there onsite with the payload to do somekind of fix, workaround, or other improvisation. The human factor ought not be ignored..

Well actually I'm favorable to the basic MUSTARD concept.  In more of my previous discussions I suggested something like two X-33 vehicle that exhanged fuel.  I noticed on the like that the orginal MUSTARD concept would have orbited a fully fueled third stage that would then be able to go to the Moon.  So my two stage version would be approriate for orbital insertion.  I this version I didn't include cross fueling but that would also be a possibility.  One thing I like about the "MUSTARD" concept the possibilty of high commonality between the stages which would reduce R& D, production, and support costs.

But, my idea on propellant type is also about the difficulties with LH2.  Consider the fuel pump issues with the space shuttle.  Also, the type of vehicle I'm considering would not have an external tank like the shuttle.  If the propellant for both stages was contained within one most consider the geometries.  While one might think of 2H2 + O2 --> 2H2O as the process in reality the real mix is 5H2 + O2 --> 2H20 + 3H2 for the combustion kinetics.  That is a lot of LH2 at .14 the density of water it put inside an airframe and heat shield!  So for a desposable rocket the reasoning of RK-1 + LOX for the first stage and LH2 + LOX for the second stage is correct.  However, this second stage isn't able to fly back to earth like the shuttle.  I think that density might out way ISP even here.

GoogleNaut, I think that we are talk past each other here.  I fully agree with your point about upper stages like on Atlas-Centaur, Saturn IB, and the Saturn V.  The advantage is clearly with specific impluse.  But, for a reusable two stage space plane/shuttle concept you have consider other factors as well.  The very low density of LH2 (and the high fuel to oxidizer ratio) drive up  the size and weight of the orbiter.  I'm repeating myself here but this means a much heavier airframe and thermal protection cover.  I'm questioning whether this extra weight won't counter balance the advantages of the higher specific impluse in this particular application. 

The issue of economics is key.  Since that is what I do for a living perhaps I need to do some analyses on the economic issues.  One thing that jumps to mind about the current Shuttle program is the very small scale of activity about 115 flights in 26 year with five flying vehicles. Perhaps the problem is mostly an economy of scale issues.  Does NASA have a lot of "Maytag Repairmen", i.e. specialists that you need to have and have fully employeed but the bulk of the time they are just doing busy work wait for their real justification.  If we had more vehicles and flghts could the same infrastructure support this effort with small incremental costs?  (Of course these are the many problems with the vehicle itself...I understand)

One thing with disposable LVs is that you make a lot more of them and so you are a lot further down the learning curve and have a lot more favorable amoritization of fixed costs of production, and no maintainance costs.  That can counter balance the support cost of reusable systems at the very low rates of use.

On the other hand I really doubt we will every really have economic access to space with throw away vehicles.  Also, we need to consider all of the space junk we are generating as well.

I'm not totally opposed to LH2 in reusable Earth to orbit vehicle but like we have both said above there could be a case for backing down a little to save volume.  I guess some modeling is required here.  There are also some special issue with LH2 as well. 

-- Edited by John at 15:22, 2007-03-04

This is getting away from spaceplane propellants, into launch stack architecture.

GoogleNaut Mar 3
> Within the airframe of an orbiter, carrying LOX/LH2 is probably very inefficient as a propellant; The Space Shuttle only carries those chemicals as reactants for the onboard fuel cell power generation equipment.
> Trying to wrap a flyable airframe around something like an ET is dubious at best--I've seen some attempts even as far back as the late 1980s with NASAs "Pioneering the Space Frontier" Report.
> For a more compact design, it might make more sense to have the thing carry LOX and RP-1: RP-1 residuals could also make a nice fuel for a few turbofan engines to give a returning orbiter better cross range and control when returning...
> On the other hand, with something as complex as an orbiter, it is difficult to imagine that it could be quickly turned around fast enough for a return flight for a flight rate of 40 or more per year--which seems to be about the golden rate which actually ends up saving you money...
> I used to be a hardcore reusable vehicle guy myself--but now I'm not so sure. It takes a lot of effort to turn around an orbiter...
> The other thing--it is good to seperate the cargoes from the crews--it gives astronauts a much better means of escape if things get dicey. Perhaps an integral CEV-style crew cabin that can be ejected whole from the orbiter airframe?

I still have trouble buying into the concept of a 2STO for a few reasons. Mostly because even two stages to orbit is still pretty demanding, and because of the wide variety of needs for the orbital/upper stage. The K.I.S.S. principle seems to be inescapably violated, and you quickly run afoul of the "flying camel cost-overrun" killer that did in the "Hope", "Hermes", and other spaceplanes that tried to do too much and went from being a winged crew carrier to a "Mini-Shuttle"

Try this:
2 belly-belly planes as the core stack, the workhorses of the launch system architecture. These 2 have different requirements for different regimes, and only share fuel, if that.
1) Re-usable booster that uses RP+Lox, and doesn't get too high or fast before flying back. (low-demand on the design this way, and this is likely to be the biggest part)
2) A re-usable upper stage booster could be tripropellant, with a bit of RP for the boost if it's parallel burning (and extra for fly-back jets). Possibly trans-fueled from the booster (like the MUSTARD did).

These lift the orbital stage: Top-mounted on the 2nd stage, only re-usable if its the Crew Launch Vehicle.

Of course, it's now 2.5 or 3 stage, with the third being either a simple upper circularization motor for cargo, or the hybrid escape/circularization engine of the CLV. (might as well be dual-use: If its not needed for escape, use it for circularization and OMS. If its needed for escape, then why also carry an OMS? Escape delta-V is entirely adequate, if throttleable, for OMS and circularization)

By all means separate crew & cargo. Also separate crew and boost engines and the tankage associated with it. You need a safe, survivable crew carrying specialist stage, and it needs to be small and compact with the escape system integral or else firmly and closely attached.
We're looking at something like an old pre-Columbia OSI/Grumman/Northrup OSP concept. (Either the HL-42 or the early, nose-down Kliper model makes the most sense.)

The 2.5 or 3 stage is either the CLV or a dumb expendable cargo shroud. (it being lots harder to re-use it, and extra weight up here really cuts into the total cargo capacity the most of anywhere on the launch stack)

You don't want orbital workspace, EVA prep, and the robot arm/tool storage attached to the CLV. Best to lift this like any successful station of the past: As a cargo, a can on top of the launch stack, unmanned for launch, met by the CLV, and mated on-orbit to any heavy cargo which needs a crews' attention. Its unlikely this is best launched with the cargo (in any case, not all cargoes), and it certainly doesn't need to go up & down with the crew. Maybe it doesn't make sense to lift a new one of these every couple of times it's needed (if/as it can't be economically re-used or if it's in the wrong orbit) but it seems to be the best solution to multi-use needs for on-orbit workspace.








-- Edited by john fraz at 14:30, 2007-04-01