Saturday, November 02, 2002


Remember model rockets when you were in middle school? The biggest engines I ever launched were Es. These guys started with Is and ended up using Os.

What strikes me as odd about this stuff, though, is why all these rockets are mainly subsonic. They go transonic or slightly supersonic at the end of the boost stage, but they really aren't getting into the many thousands of feet per second that even suborbital stuff would. And why not? These engines deliver around 200 seconds of impulse, which is a pretty serious amount of bang. If they could get their rockets down to less than the weight of the engine, they should be good for 4500 ft/sec in a vacuum. Surely a few seconds of air drag wouldn't wipe out 70% of this delta V?

What I'd really like to build would be a solid fuel ignited propane ramjet. The idea is to use Kelly Johnson's design for the back end of the Blackbird engine, but with gas pressurized propane for the afterburner and a solid rocket motor instead of the turbojet as an ignitor. Getting proper mixing of the exhaust, air, and fuel might be tough... I think it's significant that those SR-71 motors are so long.

...and you gotta love the web. Georgia Tech has a web-based design tool for just this kind of thing. They call what I want to do a Supercharged Ejector Ramjet. Can't handle a solid-fuel core, but what the hell, let's try a O2-JP5 engine instead.

Work the specs:

C3H8 tank: 5cm dia 100cm long - 1900cc
Energy: 48 MJ
Fuel Mass: 1.1 kg
Solids: 200 seconds impulse, 2 kg fuel
Burn time: 10 sec
Thrust: 392 N (4 G acceleration off the pad)
Delta-V: 294 m/s
Rocket: 6.9 kg mass

Required thrust? 1000 N (about 10 G acceleration)
Inlet area? 45 cm^2
Cowl height? 3 cm
Nose to cowl? 100 cm
Compression angle? 10 degrees
Forebody friction? 0 (uh oh)

Primary Area ratio? 15
Chamber pressure? 6000 KPa (Can't be as high as the swoopy stuff)
Fan pres ratio? 1 (no fan)

Defaults after this...