High delta-V rocket aircraft

Well, we presented at Space Access... off we go!

The basic premise of our presentation was about mass ratios, and how making high mass ratios easier to achieve is perhaps more important that achieving a high Isp (or engine efficiency). The basic measure of a rocket craft system is the amount of delta-v (velocity change) provided to a set payload. So for discussion, let's compare options for a 9,000 m/s delta-v rocket vehicle.

NASA tends to focus on liquid hydrogen/liquid oxygen. Given liquid hydrogen's outstanding performance, only 87% of the rocket's take off mass has to be propellant. This leaves 13% of the mass to be rocket. Using somewhat standard numbers for other masses as a percentage of take of mass:

1% Engine
10% Tank
1.5% Heat Shield
3% Landing Gear

These add up to 15.5% - so you have to trim some mass through clever engineering. This can be done, but finding almost 20% of mass to cut will be difficult - and even at that point, the payload will be a tiny fraction of the vehicle.

A lot of NewSpace companies focus on denser fuels, such as liquid hydrogen/RP1 (essentially kerosene). The combination has lower performance, so 93% of the rocket's take off mass has to be propellant. This leaves 7% of the mass to be rocket. Using somewhat standard numbers for other masses as a percentage of take of mass:

1% Engine
2% Tank
1.5% Heat Shield
3% Landing Gear

Notice that the tank is now much lighter. These add up to 7.5% - so you still have to trim some mass through clever engineering. Finding 7% of mass to cut will be easier than the liquid hydrogen case - but still a challenge, and the payload will be a small fraction of the vehicle.

Our design uses solid rocket propellant, but completely eliminates the tank. Solid rocket propellant has even lower performance, so 97% of the rocket's take off mass has to be propellant. This leaves 3% of the mass to be rocket. Using our design numbers for other masses as a percentage of take of mass:

0.5% Engine
0% Tank
0.5% Heat Shield
0.1% Landing Gear

The tanks are now gone, and everything else is much lighter. The engine can be lighter because the maximum thrust requirement has dropped. Takeoff has the highest thrust requirements, and during takeoff most of the propellant mass is supported by ground equipment. The heat shield is much lighter because much of a standard heat shield's mass is spent protecting the tanks, and there are no tanks. The landing gear is also sized by the takeoff load, and during takeoff the ground equipment supports the propellant.

These masses add up to 1.1% - so you have a 63% margin. You can use this as payload, or simply increase the sturdiness of your vehicle. Either way, you can now build a vehicle that masses less than the payload it carries!