From: Philip Witham (p.j.witham@ieee.org)
Date: Fri Aug 14 1998 - 14:39:08 MDT
At 04:14 AM 8/14/98 -0700, Mark wrote:
>While that's certainly a worthwhile route with old technology, it has quite
>a few flaws. Firstly, there's a limit to how far you can reduce the cost
>that way, because you're still throwing away the booster every time you
>launch it.
Truax's designs are re-useable. He was probably the first proponent of re-useable rockets, and argued in favor of such during the Apollo project. He complained that we were building a stunt rather than an ongoing space transportation capability. His Big Dumb Boosters recover both stages in the ocean, and they are then towed back to dry dock for re-use. Many people have a hard time believing that you can recover a rocket in the ocean, but it has been proven and demonstrated many times. It does require a pressure fed design (or solid rocket motor, such as the Shuttle boosters), for the strong tank walls. Retaining the pressurizing helium in the tanks after shutdown results in the tank walls always remaining in tension during impact, eliminating buckling forces.
>Secondly the payload is much, much too large for launching modern satellites;
Modern satellites are light in part because of the cost per pound of putting them up with "modern" launchers. As the cost of orbiting mass goes down, the cost of developing satellites will go down as well, as one can make simpler spacecraft if you have more mass to work with. As the total cost goes down, the risk goes down, and you spend less on reliability, and the cost goes down more. There is no reason why one could not put up a number of small satellites in one launch, as long as they are in similar orbital planes. The 77 satellite Iridium network, for instance, in six to ten launches.
> And just imagine putting a hundred billion
>dollar satellites on a booster with a 95% reliability rating; you've just
>added an average of $5,000,000,000 to the cost of every launch, because
>one time in twenty you'll lose the whole hundred billion dollar payload.
If the satellites cost $10^9, why put more than one on each launch? That's just a financial optimization question. You don't need to use the full payload capacity of a launcher every time. Truax says that as every new launcher has been fielded, the builders immediately wanted more capacity. Witness the many variations on Atlases, Titans, Arianes, Deltas, etc., with strap-on solid boosters and other gimmicks that raise the complexity, and increase the cost. Why not build it bigger than you need, from the start? This is again the point where most folks follow their intuition rather than the facts: The cost does not go up very fast with scale, but it does go up rapidly with complexity. Is it cheaper to buy a one-seat car because you usually don't need the extra seats? Not much cheaper, and soon you need a second car. And as you make a rocket bigger, you can build it simpler, because various effects give you better performance as the scale goes up.
>You save a lot of money with an SSTO simply because it can safely abort
>in most launch failures without losing the payload.
I don't mean to be nasty, but this is like saying you'll save money buying a Mercedes-Benz rather than a pickup truck because in an impact, your groceries are less likely to get splattered. Most launch failures in rockets, by the way, are explosions.
-PW
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