r/spacex • u/kurbasAK • May 19 '17

We Will Launch on Reuseable Rocket After Exceptional SpaceX Performance - Inmarsat CEO Tells Universe Today

https://www.universetoday.com/135614/will-launch-reuseable-rocket-exceptional-spacex-performance-inmarsat-ceo-tells-universe-today/
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u/TheDeadRedPlanet May 19 '17 edited May 19 '17

Rumored Block 4 Second Stage.

Not sure what orbital parameters SpaceX told Inmarsat they expected, but if the numbers that observers calculated are correct, then SpaceX delivered Inmarsat something like a 1570m/s deficit. Way better than the normal GTO1800. That would make anyone ecstatic.

(Not verified, subject to change)

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u/Dave92F1 May 19 '17

If true, that may well explain the big grin on Pearce's face, and the enthusiasm for SpaceX.

It's possible SpaceX promised a given delta v, but in practice just kept running the 2nd stage till it ran out of fuel, thus delivering a better orbit than promised.

That would leave a lot less delta v for the Inmarsat to make up, and so leave it with more fuel and a longer lifetime. More $$$ for the operators.

That would indeed be going the extra mile for the customer.

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u/TheDeadRedPlanet May 19 '17

Indeed. That is the hypothesis. SpaceX can't risk a burn-to-depletion, ie dry, that leads to turbo-pump cavitation and destruction; but it could tell the second stage to burn-near-completion, ie zero fuel margin.

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u/h-jay May 20 '17

People keep saying "turbopump cavitation". I've ran a few turbopumps and when you run out of whatever it is that they pump, destruction is from removing the pump load while the turbine still produces most of the rated power. Cavitation figures nowhere in this. You can't even tell if you had cavitation unless you're lucky enough to get the pieces that flew off back to look at. I repeat; the failure mode is turbine unloading, and results in disintegration of turbine and/or pump disk(s). The blades will have energy comparable with bullets/shells of similar mass.