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Old November 16 2012, 08:02 PM   #353
gturner
Admiral
 
Location: Kentucky
Re: Envisioning the world of 2100

One simplification you can make is that the thrust vector doesn't need to stay vertical (rotating the nozzle). In free space a side-thrust will pivot the rocket around a fixed point located at the center of percussion relative to the center of the applied thrust (and the thrust can be distributed across multiple engines, which are summed for the calculation), as if the rocket was somewhere along the second hand of a watch. Once in the air, it doesn't need to remain above a fixed point and moving sideways is fine, even helping it clear any ground support equipment. You can also include a significant unpowered free-fall period to allow the completion the rotation, since the powered phase will give you a both vertical velocity and a rotational velocity. The stage will continue the rotation until it's stopped by another motor, either the main engines after ignition or a de-rotate motor, trusting Isaac Newton to once again complete what you've set in motion. But if you've already decided on pivoting half the rear-engines, igniting the remainder as the orientation becomes more vertical, you could improve on this.

I'd been crunching some numbers on the tank's bending stress, and one side may need to be thickened to handle an increased tensile load, but there may be some things you can take advantage of involving the fact that the tank is normally pressurized and already handles large hydrostatic loading. During the horizontal lift-off the hydrostatic pressures aren't very prevalent yet (the pipe is on its side), so you've got some margin, especially lower down toward the tail.

If you were gong for a completely re-usable liquid lift, you could trade off the weight savings of using a common bulkhead between fuel an oxidizer with going back to seperate bulkheads and exploit the empty area between for an engine mounting location, reducing the bending loads on the stage. Of courrse, if you really wanted to get wonky you could run a line of small pressure-fed engines all down the sides (your basic thruster), fed from the main engines' turbopumps, avoid the bending loads altogether and leaving you plenty of redundancy, with only a small fraction of the same thusters firing for landing.

But speaking of side-loadings, what happens to the Shuttle SRB's on SSME ignition is brutal! A beam clamped at one end, swaying to absorb the sudden application of a side-load. That's two feet of deflection in bending. As one of the Shuttle's designers said, if they didn't time the SRB ignition to coincide with the stack swaying back to vertical, and lit them while fully deflected, the SRB's might just blow up. You could say the Shuttle was an experiment to see how many catastrophic failure modes could fly in a single vehicle, which is probably one of the reasons emergency thrust termination on the SRB's was built right in. Wouldn't want one of those running loose!

ETA:

Earth to Sojourner. We've been terminating the thrust of solids on command since before I was born. It's critical for precisely targeted ballistic missiles. It's also trivially simple.
I think Byeman has already answered this.
No, he said we don't use it, not that commanded thrust termination isn't there. Range safeties require the ability to be built in to any really large solid.

Any shortfall in the solid booster's thrust is compensated for by the subsequent stages, but on solid-fueled ballistic missiles (and some final stage applications), you have to terminate the thrust so you don't overshoot the target. To kill the thrust you simply blow open vents at the foward end of the motor, which drops the chamber pressure, and thus exponentially drops the burn rate.

Last edited by gturner; November 16 2012 at 08:14 PM.
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