# What is the burn time for the F9 boostback / reentry / landing burns?

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!).

• This should give you something to work with, note the timecode in the gray bar. youtube.com/watch?v=riU3DZmU-jE Commented Jan 25, 2016 at 17:40
• Be aware that not all of those burns are full thrust Commented Jan 25, 2016 at 18:20
• The first two are full thrust on 3 engines. The final burn is 1 throttled engine (1-D can throttle 70-100%). Commented Jan 25, 2016 at 18:26
• 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. Commented Jan 27, 2016 at 17:52
• @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. Commented Jan 27, 2016 at 22:00