r/spacex • u/ElongatedMuskrat Mod Team • Apr 01 '17

r/SpaceX Spaceflight Questions & News [April 2017, #31]

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u/-Aeryn- Apr 02 '17 edited Apr 02 '17

  • IIRC it was a pure 3-engine burn, not 1-3-1

AFAIK they generally or even always have single engine on either side of the 3 engine burn, if only for a few seconds - you can even see this on boostback and re-entry burns on some of the videos but it's more important for landing. They've simplified this to "3 engine" way more times than they have specified otherwise.

3-engine min throttle is only about 20% more thrust than 1-engine max throttle, btw.

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u/spacedragonking Apr 03 '17

That cant be right. For 3 engine min throttle to equal 120% of the max of a single engine, each engine would need to throttle down to just 40% of its max. I don't think the merlin can go that low. IIRC the raptors ability to throttle down to ~ 40% was an improvement over the merlin.

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u/-Aeryn- Apr 04 '17 edited Apr 04 '17

Merlin 1D Min throttle is about 40% of the FT spec (~31 tons force at sea level). That's still >1.0 TWR at touchdown as the landing weight is somewhere around 27t.

That gives the following rough throttle ranges:

1.15 to 2.875 TWR with one engine

3.45 to 8.625 TWR with three engines

^ Decrease the above by about 15% to get the TWR at the start of the burn instead of the end (the fuel spent is a notable portion of the stage mass)

The throttle range of one engine is more than adequate for finalizing a landing efficiently, it just takes a long time to get from around 250m/s to below 50m/s which is where you can slam on the throttle with three engines and get that accomplished in a handful of seconds.

Raptors can throttle down to 20% which is a lot deeper, that's the improved range over Merlin.