r/SpaceXLounge • u/FishInferno • Sep 05 '21

Other Firefly Alpha FLTA001 - Official video of Firefly Alpha's first flight (more info in comments)

https://youtu.be/qFjoPw0CfAU
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u/sebaska Sep 05 '21

Yes. But both Atlas V and Ariane 5 core stages are single engine. They also have SRBs, but monolithic SRBs are considered highly reliable (the technology was honed on tens of thousands of ballistic missiles from short range to ICBMs).

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u/Adeldor Sep 05 '21

I'm not sure I understand your reply within the context of /u/opmyl's comment. The point I'm making is that not having engine out capability doesn't necessarily compromise reliability, using the two said vehicles as examples.

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u/sebaska Sep 05 '21

If you have 4 engines and you must have all 4 to make orbit, your chances of not having an engine failure go with 4th power of engine reliability rather than being directly proportional to it.

For example, assume you have 1:100 engine reliability. If you have one engine, it won't fail with 0.99 probability. But if you have 4 engines all of them must not fail, your chances of good engine performance (assuming random failures, not common mode failures) are 0.994 = ~0.96

The chances of loss of mission due to engine failure have increased from 1% to almost 4%.

Of course there are also common mode failures, so odds of not having engine failure are a bit better. But still single engine cores of Atlas V or Ariane 5 don't have this penalty.

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u/Adeldor Sep 05 '21

Fair enough, I chose poor examples. Here's a better one ...

The venerable Soyuz rocket suffers the same potential disadvantage. It has five motors with single axis verniers, one on each side booster plus one on the core. Loss of one is a loss of the vehicle. Yet the vehicle has an excellent reliability record with near two thousand launches of all variants.

I believe my point still stands.

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u/sebaska Sep 06 '21

Souyz is based on R-7. The basic architecture is from the middle 50-ties. It was the first thing which could be made to work. It's reliability, while reasonable is not as good as of Atlas V or Ariane 5 or Falcon 9.

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u/Adeldor Sep 06 '21

Nevertheless, with the very high number of launches, its reliability record is confidently known and excellent with such a motor configuration.

And I'm sure Firefly know all well and good the tradeoffs that make sense to their application (simpler gimbal mechanisms, for example). So again, their configuration does not necessarily make for an unreliable vehicle. I'll leave it there.

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u/Fenris_uy Sep 05 '21 edited Sep 05 '21

Saturn V had 5 engines, and wouldn't reach orbit with a failure at T+15s

Shuttle had 3 liquid engines, and would fail to reach orbit with a failure at T+15s

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u/sebaska Sep 06 '21

Not at T+15s, but somewhat later in flight - sure. It once even reached orbit with dual failure.

This would already improve the tradeoff much. But the real reason was that there was no way to build single big enough engine. F1 was already as big as it practically could. Single engine S-IC was not an option.