From: rbw3q@helga9.acc.Virginia.EDU (Robert B. Whitehurst)
Newsgroups: sci.space
Subject: Re: Russian Engines for DC-Y?
Date: 5 Nov 92 05:59:45 GMT
Article-I.D.: murdoch.1992Nov5.055945.28439
Organization: University of Virginia

In article <17142@mindlink.bc.ca> Bruce_Dunn@mindlink.bc.ca (Bruce Dunn) writes:
>> Brad Whitehurst writes:
>>
>>         Av. Week had a second article on the NPO RD-701, a tri-fuel
>> engined which burns kerosene, LOX, and LH2, with continuously variable
>> fuel transition from kerosene to LH2.  Is this to customize the rocket
>> performance as a function of altitude, load, etc.?  The thrust figures
>> for all kerosene were significantly higher than with pure LH2.
>
>
>        This makes the RD-701 a "tripropellant" engine.  Numerous design

	Oops, yes "tripropellant", not fuel.  Brain was not fully
engaged. :-)

>studies done in the US have shown that using a tripropellant engine can lower
>the dry mass of SSTO vehicle by a modest amount.  The engine uses mainly
>kerosene/LOX during the early part of the flight, when specific impulse is
>not so important, and thrust and propellant density is very important.  Later
>in the flight, the engine switches to LH2/LOX because specific impulse
>becomes more important than thrust level and propellant density.
>      The upper stages of multi-stage vehicles or the later parts of SSTO
>flights are critically dependant on specific impulse, because all the
>propellant for these sections of the flight has to be boosted to several
>thousand meters per second before use.  If a given lower stage can only boost
>say 100 tons of propellant to say 6000 meters per second, then it is far more
>useful to have 100 tons of LH2/LOX than 100 tons of kerosene/LOX at this
>velocity.  Propellant burned however at the beginning of a flight does not
>have to be boosted through any great velocity increment and its specific
>impulse therefore is not too important.  However for an SSTO vehicle, the
>tank for the early-burn propellant still has to be boosted all the way to
>orbit, therefore it pays to have a dense propellant combination.  In this
>case, kerosene/LOX wins over LH2/LOX.
>        The design studies generally show about a 20 to 25% reduction in dry
>mass by using a tripropellant engine in an SSTO.  However, this reduction
>comes at the price of a 50% increase in the number of propellant tanks,
>piping systems, valves, and turbopumps, as well as the need for a complicated
>combustion chamber and/or nozzle arrangement to handle the two fuels.  The
>Aerojet Corp. did a number of design studies on a "dual expander"
>propane/LH2/LOX engine, but to my knowledge never built such an engine.
>--
>Bruce Dunn    Vancouver, Canada   Bruce_Dunn@mindlink.bc.ca

	The interesting thing about this article was that it said that
the RD-701 used only two turbopumps, with the LOX and kerosene being
pumped by one, and the LH2 by the other.  I read it quickly, so I
might have this wrong, but that sounds rather intriguing.  Is
fuel/oxidizer premixed in other engines?  They said the kerosene could
be throttled back simultaneously with throttle up of the LH2.  Sounds
like a real trick to me!  I gotta go read it again tomorrow...
-- 

Brad Whitehurst    |   Aerospace Research Lab
rbw3q@Virginia.EDU |   We like it hot...and fast.