# Falcon boosters: thrust:weight during return flight? **URL:** \ **Category:** Factual Questions\ **Created:** [February 8, 2018, 1:09pm UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437 "2018-02-08T13:09:01Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 8, 2018, 1:09pm UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437/1 "2018-02-08T13:09:01Z")
AIUI, the Falcon rocket boosters (or the main first stage, for that matter) are mostly empty during their return flight: the upper stage has been delivered to earth orbit or beyond, and most of the booster’s fuel has been used during the ascent. [Here’s one image](https://i.stack.imgur.com/csOwN.png) indicating the phases of the booster flight during which the main engines are burning fuel in quantity: -boostback burn: the booster flips and thrusts to eliminate its downrange velocity. Consistent with other sketches I’ve seen, this one suggests that the boostback burn also adds some altitude, presumably so that a small westward velocity will have enough time to get the booster back to the barge or launch site. -Reentry burn: I assume this is to cut down on the freefall velocity so that the booster doesn’t get cooked by aerodynamic heating. -Landing burn: all the vertical velocity is scrubbed off for a gentle touchdown on the launchpad. [This video](https://www.youtube.com/watch?v=Z_kfM-BmVzQ&feature=youtu.be) shows the landing burn, starting at 0:20. A sonic boom arrives several seconds later, which means they’re doing more than 760 MPH before that burn starts. Just ten seconds after ignition, they’re traveling at touchdown speeds, a few MPH. So just how fast are they falling at the start of the landing burn? What sort of thrust-to-weight ratio do they have during the burn? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [February 8, 2018, 2:13pm UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437/2 "2018-02-08T14:13:02Z")
Here’s one analysis of a previous falcon launch. [http://rebrn.com/re/iridium-telemetry-3549502/](http://rebrn.com/re/iridium-telemetry-3549502/) ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [February 8, 2018, 2:26pm UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437/3 "2018-02-08T14:26:18Z")
> [@naita](#): > > Here’s one analysis of a previous falcon launch. [http://rebrn.com/re/iridium-telemetry-3549502/](http://rebrn.com/re/iridium-telemetry-3549502/) That is awesome, thank you. So it looks like it’s barely going Mach 1 when the landing burn starts. With a nice clean plot showing the acceleration I can estimate the landing burn’s thrust:weight ratio later today. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [February 9, 2018, 4:58am UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437/4 "2018-02-09T04:58:05Z")
IIRC the Falcon booster rockets can adjust their thrust, but there’s a lower limit on their thrust that is significantly greater than their weight. This means that they can’t do a “smooth landing” where they reduce the thrust gradually as they land; they basically have to finagle things so that they reach 0 m/s just above the landing pad, and then turn off the engines and “drop” the remaining distance to the ground. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 9, 2018, 6:00am UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437/5 "2018-02-09T06:00:17Z")
Check out [Flight Club](https://www.flightclub.io) for detailed analysis of many missions. For instance, [Iridium NEXT-4](https://www.flightclub.io/result?id=9634f1a7-cc39-45b3-921f-e8eb80036253&code=IRD4). You can see from the acceleration chart that the booster never goes below 1.7 gees, and peaks at about 4.5. **MikeS** is correct that the stage just can’t throttle any lower; they just have to time it so that they reach zero velocity at zero height. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 9, 2018, 6:04am UTC](https://boards.straightdope.com/t/falcon-boosters-thrust-weight-during-return-flight/808437/6 "2018-02-09T06:04:22Z")
Also: You can tell that they’re going over mach 1 just before the landing burn. You can generally hear a triple sonic boom before the engine noise ramps up. Sounds like dun…dundun. The first boom is from the bottom edge of the craft, and the next two closely-spaced booms are from the grid fins and the top edge of the craft. With the Heavy landing, you could hear six booms: two sets of the three.