🚀 go-pugleaf

> 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
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