SpaceX F9 - Starlink Group 4-18 - LC-39A

[JohnnyEnglish]

Can also WAG acceleration maths using time instead of distance. Using SX's tlm data I get about 8.2 m/s2 (or .84g)

Its worth pedantic-izing that this is all average acceleration. Max instantaneous acceleration on landing burn is probably a good bit higher--maybe 2-3g's? Maybe a bit less? Not totally sure how quickly Merlin can throttle--that would be one of the limiting factors on max instantaneous acceleration (Newtie's 2nd, and all). Its also worth pointing out that the above calculations are the total vehicle acceleration--the free body diagram, if you will. The acceleration from the landing burn also needs to offset gravity, so the F=ma math from just the Merlin thrust would be ~1.84g.

Leveraging that to rathole on Moar Math, its interesting to note that a single Merlin's minimum thrust on a completely empty falcon first stage results in an acceleration of ~16.2m/s2, or ~1.65g's. Its not a super useful number other than the fun thought experiment of a hovering falcon first stage: This "thrust results in more than 1g" lower bound means that once a stage is below ~4% fuel load, which is about where minimum thrust = 1g, it cannot hover--it will start to go back up. Put another way, the stage can't simply hover indefinitely until its fuel runs out.

***FTR that's not factoring momentum--obviously if the thing is still falling at 4% fuel load its a different story
***Also FTR, and of course Good Math willing, that 4% fuel load represents about 125 seconds at minimum throttle

Wow, didn't expect my original post to generate this much comment

Thinking it through a bit more I guess that at the start of the landing burn (when the booster is in freefall) gravity is being offset by the drag, which will quickly reduce as the velocity decreases. Then the landing legs are deployed. Pretty complex, which is probably why it took SpaceX a few attempts to get it perfect.

 

[Electroman]

The landing legs deploy almost at the very last few seconds. Just about 5 seconds before touch down. Why so late, given in an airliner the landing gear comes up a minute or more before touch down.

 

[Grendal]

The landing legs deploy almost at the very last few seconds. Just about 5 seconds before touch down. Why so late, given in an airliner the landing gear comes up a minute or more before touch down.

I expect it has to do with speed and stabilization.

 

[Cosmacelf]

The landing legs deploy almost at the very last few seconds. Just about 5 seconds before touch down. Why so late, given in an airliner the landing gear comes up a minute or more before touch down.

In an airliner, if the legs don’t deploy correctly, the pilot can abort the landing, go back up, and try various emergency procedures to get them down again. In a rocket, there’s no margin for error. There isn,t enough fuel to hover or go back up or whatever. So might as well deploy only when you need them since there zero point to deploying them early.

 

[ecarfan]

The landing legs deploy almost at the very last few seconds. Just about 5 seconds before touch down. Why so late, given in an airliner the landing gear comes up a minute or more before touch down.

It appears that they deploy about 2 seconds before touchdown. I can’t be sure as to why they don’t deploy earlier; as @Grendal noted it may be because above a certain speed they introduce instability and affect vehicle trajectory, and as as @Cosmacelf observed reusable rockets and aircraft are very different. Near touchdown an F9 booster has essentially zero fuel remaining so no ability to hover. If the a leg or legs fail to deploy, knowing that a minute before touchdown isn’t going to make any difference; the vehicle is fully committed to landing no matter what.

How many times has a booster leg failed to deploy? I can’t remember any but I suppose it happened during a very early landing attempt. I do recall that a leg has malfunctioned due to excessively high impact speed, but the other three legs worked correctly and the booster remained intact, though leaning.

 

[Cosmacelf]

It appears that they deploy about 2 seconds before touchdown. I can’t be sure as to why they don’t deploy earlier; as @Grendal noted it may be because above a certain speed they introduce instability and affect vehicle trajectory, and as as @Cosmacelf observed reusable rockets and aircraft are very different. Near touchdown an F9 booster has essentially zero fuel remaining so no ability to hover. If the a leg or legs fail to deploy, knowing that a minute before touchdown isn’t going to make any difference; the vehicle is fully committed to landing no matter what.

How many times has a booster leg failed to deploy? I can’t remember any but I suppose it happened during a very early landing attempt. I do recall that a leg has malfunctioned due to excessively high impact speed, but the other three legs worked correctly and the booster remained intact, though leaning.

AFAIK no booster has failed to land due to improper landing leg deploy.

 

[Grendal]

The legs have the crush cores and have been reasonably crushed from time to time. Never too bad on the landing itself. There have been a number of times the booster has arrived at port with a serious lean. Most of those have been due to rough weather in the transport back to port.

The only landing leg failure was for Jason 3. That was the first drone ship attempt after the first successful ground pad landing. That booster was a F9 v1.1. The successful landing for Orbcomm-2 was the first F9 FT rocket launched. Every launch and successful landing of a booster since has been, at minimum, a F9 FT with densified fuel and oxidizer. So historically the Jason 3 launch was the only time a non-FT booster successfully landed. The leg lockout failure caused the rocket to fall over and RUD. Which made the landing a failure.