Reusable Rockets

Do "reusable" rocket systems make sense? From a mailing list discussion ....

> We have to stop throwing expensive hardware in the ocean after one use.

Consider the essence of your systems, and revisit your assumptions.

Rockets impart huge kinetic energies to relatively small payloads. 700x the specific energy of an airliner, 100,000x the specific energy of an automobile, a million times the specific energy of the oil tanker that brings the petroleum manufactured into fuel for the rocket. These are all driven by combustion engines, but have very different goals and implementations.

Rocket components have a spectrum from light to heavy, cost-dense (M$/kg) to cost-light ($0.1/kg LOX), inherently durable (microprocessors) to fragile (balloon tankage). Reuse the light, cost-dense, durable stuff first. Expend or discard the heavy cheap stuff intelligently, the moment it becomes an expensive liability. Staging is a very intelligent way to do so.

The big pop-cans that hold the propellant and constitute the bulk of the dry weight of a rocket are relatively inexpensive. Aluminum burns. Is there some way to use the tanks as extra reaction mass? Probably not ... but if there is, that's $$$$$$$ for some clever inventor.

Avionics and turbopumps are expensive, but design for multiple re-use is even more expensive. You are accelerating added mechanical weight (needed only in the far future) with huge accumulated delta V over time, many hundreds of kilometers per second (orbit-stop-orbit-stop...). Can the turbopumps and avionics (a small fraction of total dry weight) be "gently" reentered using some of the tank as heat-shield and first stage "parachute", then refurbished and re-used? Design the heaviest sub-components to wear out with each launch, mass produce them, and replace them on automated inspection and re-assembly lines. Don't make them long-lasting and even heavier, beyond the needs of reliability. For heaven's sake don't bury them deep in a non-inspectable structure.

Expansion bells are "dumb", inherently sturdy and can reenter on their own. Can they be buoy equipped, sonar-tagged and trawled out of the ocean, then refurbished and re-used? The sooner a properly designed red-hot expansion bell gets quenched in seawater, the less warping and crystal-grain growth occurs.

Reentering fragile (mostly pop-can) structures intact makes no sense. Stiffening them to survive reentry subtracts mass from the payload. Adding weight to survive many launch cycles subtracts far more.

And every component subject to such extreme stress cycling is a potential accumulator of lethal defects, unless you do a 100% disassembly, detailed microscopic inspection, and reassembly. That is almost as expensive as making a new one.

Two space shuttle crews were killed by the compromises inherent in re-usability. How many more crews must we kill before we learn that rockets are not airliners and should not pretend to be?

> A reusable transportation system is the only way to the human expansion into space.

Agreed, that is why I promote launch loop ( ), which accelerates only payload, and does not expend reaction mass.

For the foreseeable future, rockets are what we've got. They are the epitome of non-reusable, since 95% of the mass on the pad turns into smoke. The ideal for a self-contained reaction-mass-expending rocket is that every kilogram leaving the pad, besides the payload, turns into high speed exhaust, and contributes to payload delta-V. Orbit-stop-orbit-stop of a huge structure doesn't help at all, besides perpetuating the lethal pretense that rockets are merely fast airplanes that go to orbit.

Rockets (last edited 2012-12-11 01:46:42 by KeithLofstrom)