Again, SS2 is already vacuum optimized, so a nozzle extension wouldn't produce noticeable performance gains... especially given the already low specific impulse of the hybrid motor, the increased mechanical complexity, and the fact that it would likely disrupt the entry aerodynamics.
Honest opinion: the choice to use a hybrid motor instead of a liquid fuel engine is going to be the Achilles heel of SS2, if it isn't already.
Agree with you they should have used a liquid fuel engine from the beginning.
I was trying to find engine specs on SpaceShipTwo but they weren't available. I was only able to find them on SpaceShipOne here:
SpaceShipOne.
Propulsion was by a SpaceDev hybrid rocket engine, selected after competitive testing against an eEC design. The enormous liquid nitrous oxide tank dominated the fuselage interior. The HTPB fuel burned in the presence of the nitrous oxide to produce a chamber pressure of 37 atm, expanded through a 25:1 nozzle. Thrust loads were transmitted from the fuel casing, through the nitrous oxide tank, and then to the spacecraft through rubber that bonded the tank to the fuselage.
http://www.astronautix.com/t/tierone.html
I assume the SS2 engine specs will be analogous. Virgin Galactic was going to switch the fuel for SS2 to a type of nylon for better performance, but decided to
switch back to the same rubber-based fuel of SS1.
Then a 25:1 expansion ratio is larger than that common for a sea level engine but is not that of an optimized vacuum engine. For instance the
Merlin 1D sea level engine has a 16:1 expansion ratio, and for the
Merlin Vacuum it's at 165:1.
A 25:1 ratio for the SS2 engine is rather at an intermediate level, appropriate for the 45,000 feet ignition point.
That Astronautix page gives the SS1 engine Isp as 250s. For whatever reason, though this was sufficient for SS1 to reach 100 km, it's not for SS2.
Hybrid engines do not scale up easily, which Virgin Galactic discovered much to their chagrin. So perhaps the SS2 engine simply can't reach the 250s performance level. Or it might simply be that having a larger passenger cabin on SS2, instead of just a pilot section, increased the dry weight and therefore lowered the mass ratio.
In any case I used the
Rocket Propulsion Analysis(RPA) program to estimate the vacuum Isp for a vacuum optimized nozzle for the N2O/
HTPB propellant combination. I set the vacuum expansion ratio at the 280:1 level of the RL10-B2. Then I got a 318.5s Isp at optimum expansion.
SS1 at a 3,600 kg gross weight and 1,200 kg dry weight has a mass ratio of 3 to 1. Then at its 250s Isp it had an ideal velocity, i.e., the velocity without gravity loss, air drag loss, and air pressure loss on Isp, of 250*9.81Ln(3) = 2,690 m/s.
But at 318.52s Isp, the ideal velocity would be 318.52*9.81Ln(3) = 3,430 m/s. That 740 m/s increase in the ideal velocity would translate to a significant increase in altitude.
Bob Clark