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I'm trying to figure out the timing of the 3 recovery burns and the total fuel reserve needed to perform them. [Answers Inlined]

Boostback: [30] seconds of 3-engines at full-thrust.

Reentry: [20] seconds of 3-engines at full-thrust.

Landing: [32] seconds of 1-engine throttled between 70-100% (Merlin 1-D throttle range). They probably target somewhere between the midpoint (max margins) and higher throttle (better efficiency), so maybe 90% on average?

Merlin 1-D+ (F9FT) fuel flow rate is 270.7 kg/s.

S1: 270.7*3*30 = 24,363kg (wow)

S2: 270.7*3*20 = 16,242kg

S3: 270.7*0.90*32 = 7,796kg

Total: 48,400kg of fuel reserve (wow!).

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  • $\begingroup$ This should give you something to work with, note the timecode in the gray bar. youtube.com/watch?v=riU3DZmU-jE $\endgroup$ – Hobbes Jan 25 '16 at 17:40
  • $\begingroup$ Be aware that not all of those burns are full thrust $\endgroup$ – Hohmannfan Jan 25 '16 at 18:20
  • $\begingroup$ The first two are full thrust on 3 engines. The final burn is 1 throttled engine (1-D can throttle 70-100%). $\endgroup$ – Dave Dopson Jan 25 '16 at 18:26
  • $\begingroup$ Is that RP-1 only or does it include the LOX? When you look at the photos of the landed stage you can see roughly how much LOX was left when the stage separated. $\endgroup$ – Hobbes Jan 27 '16 at 17:52
  • $\begingroup$ @Hobbes - it's total mass flow. Given ISP and thrust, it's straightforward. "ISP * 9.8m/s2 * fuel_flow = thrust". To sanity check, 162 seconds of runtime on 9 engines gives ~394 mt of fuel consumed by the primary firing of the 1st stage. Total F9 weight is 540mt, so that sounds about right for the 1st stage fuel capacity. $\endgroup$ – Dave Dopson Jan 27 '16 at 22:00
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These are the data for the Orbcomm-2 mission where the stage landed back on Cape Canaveral:

Meanwhile, the first stage performed a roughly 30 second boostback burn beginning about 3 minutes 50 seconds after launch and a 20 second reentry burn about 8 minutes after launch, both using three engines. A final, roughly 32 second single-engine landing burn using only the center engine took place just before the landing, about 10 minutes after liftoff.

Falcon 9 first stage trajectory

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