5 ms·

The booster was falling at 4500 Km/h 30 seconds before the catch with 2-3% fuel left. How is that amount of fuel remotely enough to stop the downward momentum?

stetrain 2y ago
Because the rocket weighs a small fraction of what it did at launch, specifically because it only has 2-3% fuel left. Fuel is the vast majority of the vehicle weight at launch, kind of like an empty vs full can of soda.
- justin66 2y ago
  The atmosphere does its bit to slow down the booster as well. It’d be interesting to see a plot of the power output over time on reentry but I’ve always assumed the motors aren’t doing a lot of work other than keeping the booster stable, until the very end.
  - stetrain 2y ago
    Yes, for most of the booster return it’s ‘gliding’ with the rocket engines completely shut down. They ignite a subset of engines just a few seconds before landing for the final slowdown and maneuvering. Edit: here is a video from further away that shows the rocket gliding in under control of the grid fins before the engines light and execute the final landing maneuver: https://x.com/shaunmmaguire/status/1845444890764644694 https://x.com/shaunmmaguire/status/1845444890764644694
    - justin66 2y ago
      Super heavy probably makes certain aspects (unrelated to the tower catching the booster) of landing much easier, by virtue of its greater mass. Timing with the Falcon 9 was always essential, because the minimum thrust (with one motor, throttled all the way back) of one of the engines was enough that if you left the motor running, the booster would start to rise. Time the beginning of the final burn incorrectly and you had a real problem, with the booster either crashing or rising at the end and... then crashing, but with Super Heavy, it might simply be a matter of having to compute a slightly different sequence for shutting off the engines. They sure made it look easy...
      - HarHarVeryFunny 2y ago
        Super Heavy is easier to control on landing because it's using 3 engines so has better directional control, and it can throttle those engines down into a sustained hover, which is what it does before being "caught" by the arms. Surprising to see this work first time though - I don't recall them doing any hover and lateral movement tests, but I assume they must have done. What's also wild is that the booster isn't being caught/supported by those giant grid fins, but rather by small lifting pins just below them, and seems to only have two of these (one on either side), so it also has to get it's rotational position right so those pins engage with (are supported by) the arms.
    - nordsieck 2y ago
      > Yes, for most of the booster return it’s ‘gliding’ with the rocket engines completely shut down. Watching the video, it looked like the bottom of the rocket was glowing hot, but the engines were cool. I imagine that means they were probably running some amount of methane through the engine bells to cool them.
- HarHarVeryFunny 2y ago
  Yeah, but even without fuel the booster weighs 200 tons. It's pretty wild to have 200 tons of steel incoming at supersonic speed, then nailing a gentle pinpoint landing like that !
  - chasd00 2y ago
    Yeah if the engines didn’t light and it hit the launch facility that whole place would be a crater. The whole thing was shocking to me but how fast it was coming down was extra shocking haha. I wonder what deceleration forces were at play because when the engines lit the booster slowed down very fast.
chedabob 2y ago
It weighs about 10% of what it did at liftoff, but half of the engines fire to slow it down. Also I don't think the telemetry on the feed is that accurate, so with all of the atmospheric braking, it was probably going a bit slower than the 1200km/h at engine reignition.
magicalhippo 2y ago
First off, the booster was going about 1250 km/h when it started its landing burn, it relied purely on drag to get it slowed down to that speed. Going by the telemetry of the seconds before the landing burn and noting the speed vs time, it seems drag was around 40 m/s^2 when it was going at around 3000 km/h. Since drag depends on velocity squared though, it had reduced to just above 10 m/s^2 just before the engines lit at 1250 km/h, and so would quickly become negligible once the engines lit. Going by Wikipedia, the Super Heavy[1] has 3400000 kg of fuel at launch, so 3% of that is about 102000 kg. For the landing burn, it used 13 Raptor v3 engines[2] to scrub speed. Each Raptor flows about 650 kg/s max, so 3% fuel is enough for about 12 seconds for the 13 engines. The empty mass of the Super Heavy is about 275000 kg, so about 377000 kg before the landing burn with 3% fuel. Using the sea-level vs vacuum performance of the Raptor v2 engines, one can estimate that each Raptor v3 produces about 2.45 NM of force at sea-level. So 13 of them would produce about 31.85 MN of force. Using Newton's second law, F=ma, this gives an initial deceleration of about 84 m/s^2 and about 104 m/s^2 when empty. If we do a rough spreadsheet integration, we get that a burn of roughly 4 seconds is needed to scrub the speed assuming no other forces. Now, comparing this with reality, the full 13 engines were lit for a little over 5 seconds. In my simplified calculations I was assuming full throttle the whole way, which obviously isn't realistic, and I also assumed 3% fuel. So over all I think that's a pretty decent estimation. [1]: https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines [2]: https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor
- rsync 2y ago
  "... Super Heavy[1] has 3400000 kg of fuel at launch ..." So, 34M kg of fuel has to be burned (in this booster alone) to facilitate a flight ... and I see that the propellant is CH4 / LOX[1]. Burning methane is much, much better than simply releasing methane but the release becomes CO2 instead ... What is the back-of-the-envelope conversion of 34M kg CH4 vs., for instance, 34M kg of kerosene/JP ? [1] https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines
  - CrazyStat 2y ago
jjk166 2y ago
Note that the energy of 3% of the propellant (~100 GJ) could theoretically get the empty booster (100,000 kg) to a little over 5000 km/h if properly applied.

stetrain 2y ago

Because the rocket weighs a small fraction of what it did at launch, specifically because it only has 2-3% fuel left. Fuel is the vast majority of the vehicle weight at launch, kind of like an empty vs full can of soda.

justin66 2y ago
The atmosphere does its bit to slow down the booster as well. It’d be interesting to see a plot of the power output over time on reentry but I’ve always assumed the motors aren’t doing a lot of work other than keeping the booster stable, until the very end.
- stetrain 2y ago
  Yes, for most of the booster return it’s ‘gliding’ with the rocket engines completely shut down. They ignite a subset of engines just a few seconds before landing for the final slowdown and maneuvering. Edit: here is a video from further away that shows the rocket gliding in under control of the grid fins before the engines light and execute the final landing maneuver: https://x.com/shaunmmaguire/status/1845444890764644694 https://x.com/shaunmmaguire/status/1845444890764644694
  - justin66 2y ago
    Super heavy probably makes certain aspects (unrelated to the tower catching the booster) of landing much easier, by virtue of its greater mass. Timing with the Falcon 9 was always essential, because the minimum thrust (with one motor, throttled all the way back) of one of the engines was enough that if you left the motor running, the booster would start to rise. Time the beginning of the final burn incorrectly and you had a real problem, with the booster either crashing or rising at the end and... then crashing, but with Super Heavy, it might simply be a matter of having to compute a slightly different sequence for shutting off the engines. They sure made it look easy...
    - HarHarVeryFunny 2y ago
      Super Heavy is easier to control on landing because it's using 3 engines so has better directional control, and it can throttle those engines down into a sustained hover, which is what it does before being "caught" by the arms. Surprising to see this work first time though - I don't recall them doing any hover and lateral movement tests, but I assume they must have done. What's also wild is that the booster isn't being caught/supported by those giant grid fins, but rather by small lifting pins just below them, and seems to only have two of these (one on either side), so it also has to get it's rotational position right so those pins engage with (are supported by) the arms.
  - nordsieck 2y ago
    > Yes, for most of the booster return it’s ‘gliding’ with the rocket engines completely shut down. Watching the video, it looked like the bottom of the rocket was glowing hot, but the engines were cool. I imagine that means they were probably running some amount of methane through the engine bells to cool them.
HarHarVeryFunny 2y ago
Yeah, but even without fuel the booster weighs 200 tons. It's pretty wild to have 200 tons of steel incoming at supersonic speed, then nailing a gentle pinpoint landing like that !
- chasd00 2y ago
  Yeah if the engines didn’t light and it hit the launch facility that whole place would be a crater. The whole thing was shocking to me but how fast it was coming down was extra shocking haha. I wonder what deceleration forces were at play because when the engines lit the booster slowed down very fast.

chedabob 2y ago

It weighs about 10% of what it did at liftoff, but half of the engines fire to slow it down. Also I don't think the telemetry on the feed is that accurate, so with all of the atmospheric braking, it was probably going a bit slower than the 1200km/h at engine reignition.

magicalhippo 2y ago

First off, the booster was going about 1250 km/h when it started its landing burn, it relied purely on drag to get it slowed down to that speed. Going by the telemetry of the seconds before the landing burn and noting the speed vs time, it seems drag was around 40 m/s^2 when it was going at around 3000 km/h. Since drag depends on velocity squared though, it had reduced to just above 10 m/s^2 just before the engines lit at 1250 km/h, and so would quickly become negligible once the engines lit. Going by Wikipedia, the Super Heavy[1] has 3400000 kg of fuel at launch, so 3% of that is about 102000 kg. For the landing burn, it used 13 Raptor v3 engines[2] to scrub speed. Each Raptor flows about 650 kg/s max, so 3% fuel is enough for about 12 seconds for the 13 engines. The empty mass of the Super Heavy is about 275000 kg, so about 377000 kg before the landing burn with 3% fuel. Using the sea-level vs vacuum performance of the Raptor v2 engines, one can estimate that each Raptor v3 produces about 2.45 NM of force at sea-level. So 13 of them would produce about 31.85 MN of force. Using Newton's second law, F=ma, this gives an initial deceleration of about 84 m/s^2 and about 104 m/s^2 when empty. If we do a rough spreadsheet integration, we get that a burn of roughly 4 seconds is needed to scrub the speed assuming no other forces. Now, comparing this with reality, the full 13 engines were lit for a little over 5 seconds. In my simplified calculations I was assuming full throttle the whole way, which obviously isn't realistic, and I also assumed 3% fuel. So over all I think that's a pretty decent estimation. [1]: https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines [2]: https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor

rsync 2y ago
"... Super Heavy[1] has 3400000 kg of fuel at launch ..." So, 34M kg of fuel has to be burned (in this booster alone) to facilitate a flight ... and I see that the propellant is CH4 / LOX[1]. Burning methane is much, much better than simply releasing methane but the release becomes CO2 instead ... What is the back-of-the-envelope conversion of 34M kg CH4 vs., for instance, 34M kg of kerosene/JP ? [1] https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines https://en.wikipedia.org/wiki/SpaceX_Super_Heavy#Engines
- CrazyStat 2y ago

jjk166 2y ago

Note that the energy of 3% of the propellant (~100 GJ) could theoretically get the empty booster (100,000 kg) to a little over 5000 km/h if properly applied.