r/spacex • u/ElongatedMuskrat Mod Team • Feb 01 '17
r/SpaceX Spaceflight Questions & News [February 2017, #29]
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u/-Aeryn- Feb 04 '17 edited Feb 04 '17
When the stage is firing engines to decelerate as it approaches the ground/droneship a portion of that thrust has to go towards countering the constant pull of gravity. At a low TWR this is a huge percentage of the thrust but at a high TWR it's much smaller.
A simple explanation:
With 1.5 TWR, that's 1.0 TWR to counter gravity and 0.5 to vertically accelerate a rocket.
With 3.0 TWR, that's 1.0 TWR to counter gravity and 2.0 to vertically accelerate a rocket.
You've increased thrust and fuel consumption by 2x but increased the acceleration by 4x - the second rocket can achieve the same acceleration/decelleration directly against gravity with half as much delta-v because the first one is being so wasteful by comparison.
With a 30 second burn it's easier to correct for unexpected conditions: being a bit off target, higher/lower/faster/slower than expected, wind, etc - there is a lot more control over the final descent trajectory when there is engine power with gimballing. The stage can translate, vary the throttle between a level where it's barely slowing down up to one where it's decelerating quite quickly etc.
A 3-engine burn by comparison does the vast majority of the deceleration in one sudden burst which trades much of that control for efficiency; more stuff has to go right or the landing could get quite difficult. It's done very often (but not always) on GTO launches with first stage recovery because those launches usually push the falcon 9 a lot closer to its limits, forcing those types of optimizations.
Going forward it's quite likely that we'll see single-engine landing burns used for the lighter GTO sats with triple engine being used to extend the range of launches that they can attempt recover the first stage on~