posted on Aug, 7 2017 @ 09:36 PM
originally posted by: Blackfinger
Thing I find with Rocket propelled planes is "fuel usage"...The Me163 had about 3min fuel and something like the shuttle or X38b had to have external
tanks to achieve altitude.. A hybrid engine system is the only thing I think of.
The Shuttle had to have external tanks because 1) they wanted to recover the expensive rocket motors for reuse and the Shuttle was the only part that
made the round trip, and 2) they were going all the way to orbit (around 7.8 km/sec).
The rocket equation tells you what fraction of the starting weight of your rocket has to be fuel in order to achieve a particular final speed,
depending on what fuel/oxidizer choice you make. Because the rocket equation is exponential, the propellant mass fraction grows very fast. For
example, if you tried to go all the way to orbit (7.8 km/sec) with the propellant combination used in the ME-163, in a single stage rocket, the
rocket equation says that about 97% of the starting mass would have to be fuel, meaning that only 3% would be left for tanks, motors, payload, and
everything else. Nobody knows how to build a rocket that light weight.
However, if you only want to boost the speed by about 1/3 of orbital velocity (about 2.6 km/sec) and you could use a propellant combination about as
good as LOX/Kerosene, the propellant mass fraction would be about 55%. If you started the rocket portion of the flight with an initial speed of
around Mach = 1 you would burn out at a speed of around Mach = 9, but outside the atmosphere. So you would have a horizontal hypersonic glide
trajectory from about M=9 down to about M=2.
For reference, the SR-71 could take off with a propellant mass fraction of 60%, so this seems feasible.