Anyone have data or observations regarding capping unused rocket engines on space planes?

[ajburges]

For 720 an engine per launch you could cap the ends of your SSTO rockets with a disposable nose cone that can stage away when the engines start. For 320 you could skip the separator and just burn it off. Would the drag savings be worth it or even noticeable?

How much of a difference would it make for the flight profile? I can't imagine it making too much of a difference, but if significantly reduces the thrust needed for breaking the sound barrier that would be worthwhile.

[G'th]

Unless you have rockets pointing the wrong way, capping the nozzles shouldn't do much, and if anything will be detrimental as you'll be carrying pointless weight.

[Mic_n]

I don't know that the aero modelling cares about vortex drag like that.

If you wanted to know for sure, make some simple test vehicles to compare with and without. Grab a fuel tank with nosecones front and back and some radial-mount engines, set it at full thrust, point it straight up and launch. See how far it gets. Take the rear nosecone off, add something of the same weight into the stack that isn't going to alter the aerodynamics, do the same flight profile, and see what difference it makes.

[RSwordsman]

SSTO...stage away

I had to point out that once you drop parts during ascent it's no longer SSTO.

However this could be easily tested with MechJeb and a dV calculator I imagine.

[Red Iron Crown]

I made a quick little test rig like the one Mic_n described (craft), two test rockets identical except for one having a tail cone and decoupler (focus stays on the tailconed variant in the pics):

Initially, the heavier tailconed design falls behind due to lower TWR and drag being a small factor at low speed:

Approaching the transonic range the tailconed rocket catches and passes the lighter rocket due to reduced drag:

Once the atmosphere thins enough that drag becomes less significant, the lighter rocket overtakes again due to superior TWR (it would have been wise to decouple the tailcone before then to negate the TWR penalty and keep the altitude and speed advantage):

It looks like tailcones do reduce drag significantly, enough to overcome the mass penalty at supersonic speeds until out of the thick atmosphere, but they should be jettisoned once the atmosphere thins (or the rocket engine is needed). Not sure if the extra cost of the tailcone and decoupler is saved in fuel savings or extra payload, that will vary by design, I think, but it would seem to be a useful technique for planes that stay in the atmosphere at high supersonic speeds.

Edited June 16, 2015 by Red Iron Crown

[Mic_n]

Nicely done. Now repeat it a bunch of times under different circumstances to make it statistically significant

Good to know, though.

[Foxster]

I was messing a while ago with with 'staging' parts without actually staging by deliberately overheating the part to be jettisoned. That would probably work here, just put the nosecone on the bottom of the engine and fire up the engine - bang goes the nose cose.