Comment by InclinedPlane | Hacker News Reader

Comment by InclinedPlane | Hacker News Reader

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Reusability trumps everything.

EVERYTHING.

When you throw away the entire rocket every flight you throw away tens of millions of dollars in hardware. If you can recover and re-fly stages you can get away with a lot of compromises elsewhere. Especially since the first stage, for SpaceX anyway, represents 3/4 of the hardware cost of the launch vehicle.

Even reusing the first stage once means cutting the overall hardware cost of a flight by nearly 1/3. For SpaceX that translates to about $20 million per flight in savings. And that's from one and only one reuse flight of the first stage. In comparison the fuel is nothing. In comparison even a massive payload hit is acceptable. As long as the payload reduction is less than the cost reduction, everything is golden, and the rest is profit margin. For 2 reuses (3 total flights) the hardware cost per launch drops to 1/2 of current costs. For 5 or more uses per first stage the cost drops below 40%.

SpaceX's launches are already cheaper than the competition, dropping below 1/2 of their current cost floor makes it impossible for the competition to keep up and would enable them to own the launch market.

Well, cost trumps.

My comment above wasn't shooting down the concept of reuse, but showing that the lateral range of the stage 1 Saturn V booster was pretty significant. To re-land at Kennedy, you'd likely have to:

1. Change the launch trajectory to gain more initial vertical range.

2. (Possibly) reduce the first stage size to decrease its range. This means increasing comparatively the 2nd and 3rd stage sizes.

3. Use of strap-on boosters (themselves independently recoverable) which would reduce the mass of the remaining recoverable stage 1, and hence the momentum that would have to be re-vectored to KSC.

And while reusability is good, it's a bit like Amdhal's Law: your initial gains are the biggest, and likely you're going to see a cost function something like:

    (fixed booster cost + reusability engineering cost) / reuses +
    additional fuel cost + refurbishment costs + launch risk

The first two elements are going to decrease with reuse. The reusibility engineering costs will likely themselves be a function of reuses. And at some point you are below the increased incremental fuel, refurbishment, and launch risk (failure) costs.

Which doesn't say that the exercise is futile. Only that past 3-4 reuses you're gaining little for what's likely a large additional expense, or phrased differently, there's a minimum cost if you want to go to space today.

You could achieve very large savings in many things if you fly a lot and the oparations are routine. Also risk should drop a lot.

It's still not inevitable that such things will happen with reusability automatically. Most likely the first reusables have high maintenance. But they can be improved.

Launch risk does seem to be a function of experience -- the Russians have been flying pretty much the same rocket for nearly 50 years. That provides a lot of opportunity for incremental improvement. Combine this with early Russian practice (based largely on lack of sophisticated testing equipment) of testing rocket designs through live flight.

But risk in general is also a function of use and operational time. This has been borne out in many contexts, including both space flight and aviation. Systems degrade in nondeterministic ways over time, increasing failure risk. Even very minor variations in design -- a few mm of protrusion in a fuel-oil heat exchanger in the Boeing 777, implicated in the British Airways 38 Heathrow crash, given a specific set of circumstances in in-flight ambient temperatures and engine throttle settings resulting in ice-induced fuel starvation -- can have profound impacts.

https://en.wikipedia.org/wiki/British_Airways_Flight_38

It'll be interesting to see how re-use, risk, and cost play out with Space-X.

Which would then force anybody else who wants to be in the launch biz to develop their own version of the technology. Sounds like a win for humanity.

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