Throwing Away Our Future

A Russian Soyuz rocket launching a scientific satellite exploded half a minute after launch on Tuesday.

It carried no crew, but it was of a similar type (though out of a different launch site) to the vehicle that delivers Russian cosmonauts (including paying space tourists, such as Dennis Tito and Mark Shuttleworth) to the International Space Station. Had the finances worked out, teen-heartthrob Lance Bass would have been aboard one this month.

As I see it, a major problem with the 'reusable vehicle' concept is that while a pound of 'stuff' that doesn't have to be recoverable adds a pound of spaceborne weight, a pound of stuff that does have to be recoverable adds considerably more. Since every pound of spaceborne weight adds well over a pound of launch weight (since it requires the addition of more fuel to get aloft, which in turn requires more fuel, etc.) adding the extra shielding etc. to make a vehicle reusable adds enormously to the amount of fuel that's required to get it aloft on each and every flight.

And of course we are not willing to invest in alternative launch approaches. Lots have been thought out, and could be tested, but some require some big infrastructure investments.

One example (there are a bunch) is this: bore a tunnel through a mountain from the base to the peak (or near the peak). Run some type of rail system up the tunnel. Mount the launch vehicle on the rail car. Accelerate the rail car up the tunnel. Ensure that the tunnel is evacuated of atmosphere before the run (or at least to a weak vacuum.) As the rail car reaches the exit, open the door. Launch vehicle is already traveling at 1/2 mile per second. Separate from rail car and launch. Rail car commences deacceleration, rail(s) now dip down (like from the peak of a roller coaster) and the rail car returns to bottom of the mountain. Launch vehicle goes on up to orbit.

Sound wild? It isn't - just requires precision in timing. High initial cost - bore the tunnel, big vacuum pumps, lots of power, administration and maintenance infrastructure, etc. But once it is built - much lower cost to orbit, because instead of gaining the 1/2 mile per second from using tons of fuel and a disposable 1st stage, just - race up the tunnel.

Could it happen? No - because it would take several billion in investment, and require all kinds of government regulatory agency signoffs. (Imagine the effect on the local wildlife, pollution, etc. Heck - you couldn't build the shuttle facility today!)

I say no it couldn't happen, but in fact if a small group of billionares decided to do this instead of playing with the stock market - they could probably make it happen. I'm not holding my breath.

I think that if you actually run the numbers, this logic fails. It's still cheaper to recover hardware than to throw it away, if the flight rate is high enough. If the flight rate is low, it's not worth developing anything new.

Making the flights more frequent won't help if the marginal fuel cost on each flight exceeds the cost of disposable launch vechicles.

It may be worthwhile to have certain parts of a launch system be reusable, but the extra costs associated with such reuse are substantial. Given that weight is at an absolute premium on space flights, designing things for serviceability is expensive.

This is especially true with certain types of materials and systems which can be inspected as they are produced/assembled, but which cannot be effectively disassembled without destroying them. If you are trying to make a component reusable, it will have to survive repeated stress cycles, each of which is much worse than the stress cycle that must be survived by a disposable component (re-entry will almost certainly produce stresses on the reusable component. In addition, certain failure modes at certain parts of the mission may be acceptable in a disposable system but not in a reusable one (e.g. a failure of one part of the system which damages another part which is no longer needed for the current mission may be acceptable in a disposable system, but would make reusing the parts impossible).

To be sure, the lack of thousands of samples for QC analysis may make it hard to figure the best compromise among weight, cost, and reliability. Even here, though, disposable parts probably have an advantage. It's much easier to design a part to survive the fatigue induced by one or two heating/cooling cycles than to survive that induced by twenty or fifty. The more cycles a part will have to experience, the more possible modes exist for progressive weakening and failure.

If a particular type of component will be used on 100 missions, the cost of engineering that component will be spread over 100 missions whether that component is reused between missions or not. If the design/testing of a reusable component costs twice as much as that of a disposable component, that is a substantial increase in the per-mission cost of that component. If mission rates were high enough, this particilar aspect of the cost might be effectively amortized over enough missions as to be reasonable. On the other hand, the more missions a component has to survive, the more engineering it will require.

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