Re: Griffin Calls STS, ISS "Mistakes"

On Wed, 05 Oct 2005 18:24:58 -0500, Pat Flannery <flanner@xxxxxxxxxx>
wrote:

>Josh Hill wrote:
>>
>>If you're talking about a recovery system, you start to lose some of
>>the weight and cost advantages of a disposable tank. I wonder if it
>>wouldn't make sense to go with LFBB's at that point: if you used
>>tripropellant engines in the orbiter, they could supply it with
>>kerosene. (OTOH, if you used tripropellant engines in the orbiter, you
>>might not /need/ a drop tank or LFBB's . . . )
>>
>>
>Drop tanks will be ar cheaper to design and build than even the most
>basic LFBB.

True, though I wonder how great the difference is if you're talking
about drop tanks that have recovery systems and don't flake foam and
take into account the savings from a smaller, lower tech orbiter.

>about the only time you would get a cost advantage with a LFBB over drop
>tanks is if you had an extremely high launch rate, which leaves one
>wondering what exactly one is going to be launching at such a high rate.
>And even if your LFBB is an pretty low performance vehicle that doesn't
>need any TPS on it for its return to the launch point, you still are
>going to have to store it between missions and keep it in serviceable
>condition- that costs money.

>There's another problem here: if you lose a LFBB for some reason, it can
>seriously affect your launch schedule due to the fact that you now have
>fewer LFBBs than orbiters to ride on them.

These are among the reasons I like the Russian LFBB proposal --
they're versatile and modular enough to be used in a multitude of
applications, which allows you to spread development cost among
several vehicles and more launches. If the LFBB's had about half the
thrust of an SSRB, you could potentially use one for the first stage
of a 10K satellite launch, two for a reusable crew vehicle, and four
for an HLV. So you're paying more than you would for SSRB's, which are
already on hand, but you get more for your buck, plus the operational
cost savings.

I assume you'd keep at least one spare ready-to-go LFBB on hand.
OTOH, I'm concerned that it would be impractical to design a vehicle
of this sort with engine-out reliability.

>>Almost speaking of which, since you don't need full thrust all the way
>>up, it seems to me that you'd do better by combining
>>sea-level-optimized kerosene and conventional, plug, or expandable
>>nozzle LH2 boosters in an orbiter without the design cost and
>>technology risk of tripropellant engines.
>>
>>
>You might run into plumbing weight problems with that approach (three
>propellant tanks), along with the fact that you have the parasitic
>deadweight of the LOX-kerosene motors to drag along into orbit.

Plumbing weight would certainly be a problem, though you also have
smaller tanks and overall vehicle size, so I suspect in the end you'd
save on dry weight.

I'm assuming that both the kerosene and LH2 engines would fire at
takeoff, so if you're going to drag the engines into orbit, it seems
to me that the dead weight of the higher T/W ratio kerosene engines
would be lower than the dead weight of hydrogen engines use in the
same configuration. In effect, you have a shuttle-like arrangement.

>I like the idea of a Lox-hydrogen or Lox-LPG motor system where the fuel
>also drives turbojets to accelerate the the vehicle from runway to
>rocket motor ignition. In this way the weight of oxidizer you save by
>using atmospheric oxygen may weigh more than the jet engines you add to
>the design.

So do I. But as I understand it, turbojets don't pay for themselves in
an SSTO, which means you need turboramjets at a minimum. (Though that
figure is for kerosene engines rather than hydrogen-fueled jets with
their higher specific impulse.) Not really a problem in that we know
it can be done, but it does add to the development costs.

>Scott Lowther dug up this device that allows you to use a multi-mach
>ramjet for a runway takeoff:
>http://www.up-ship.com/apr/extras/serjx15.htm

Clever idea! I'd thought of using a brute force approach, that is,
using the rocket engines to get up to speed, then switching to
ramjets, then switching back to the rocket engines. Not a very elegant
arrangement, though: you lose some of the Isp advantages of air
breathers, you have a noisy takeoff which would preclude the use of
many existing airports, and you reduce the safety/marginal cost
advantages of air-breathing engines.

--
Josh

"It was amazing I won. I was running against peace and prosperity
and incumbency." - George W. Bush
.

Relevant Pages

  • Re: NGB (was Stealth AWACS)
    ... cost on a recurring basis, and we know what cost projections are worth. ... principle SCRAM engines to date. ... A supercruise airframe is possible with current engines. ... Handing out a bushel basket of money and a blank F-size sheet of paper, saying "gimme an airplane" isn't the way to fix those things. ...
    (rec.aviation.military)
  • Re: Half a million tons per year to GEO
    ... engines did indeed have a long history of incremental improvements. ... The current problem with access is *cost*. ... I don't see much market though for lifting negative mass. ... I expect the energy demand to fall rapidly as the world ...
    (rec.arts.sf.science)
  • Re: H2 burner
    ... Where Th=hot temperature ... If the exhaust temperature is say 500K then efficiency ... Generally speaking lowest cost and lowest mass per unit weight is ... Here is a partial list of the heat engines that have been developed ...
    (sci.energy.hydrogen)
  • Re: Mosquito vs B-17
    ... >>cubic root of power increase then the B17A if fitted with the engines ... >>340 mph. ... > The YB-17 without supercharging had a top speed of 256 MPH at ... probably other factors such as weight increases. ...
    (rec.aviation.military)
  • Re: High Cost of Sportplanes
    ... I don't think the Rotax is much of a bargain at all. ... Rolls Royce acquisition of Walter engines in the Czech Republic. ... The companies building the LSAs are ... As far as the cost of materials goes, sheet aluminum is probably the best. ...
    (rec.aviation.homebuilt)