7 ms·

Now that Falcon 9 landings have become so routine, it feels surprisingly wasteful to see that mission profile with the center booster burning up without even tr

by usrusr 4y ago

Now that Falcon 9 landings have become so routine, it feels surprisingly wasteful to see that mission profile with the center booster burning up without even trying. I wanted to write something about that lack of center booster reuse on falcon heavy feeling irrationally worse than a single F9 launch in the non-reusable profile (for a slightly higher payload iirc), but then I discovered that the non-reusable F9 profile hasn't happened in quite a while. Nice! When they started with the reuse, I was expecting the full payload profile to be a far more regular occurrence. (even if in hindsight I see that none of my reasons for that expectation make any sense)

wiredfool 4y ago
Edit: Units... I'd be the one going in mph through the surface of mars. ~The booster was going 12km/sec at MECO, which is more than escape velocity. What they're doing with the second boost, they're not telling anyone.~
- xcskier56 4y ago
  The booster was doing 14,000km/h at MECO which is about 3.8km/s. Orbital velocity is 7-7.5km/s depending on altitude and escape velocity is even higher, so no, it was not at escape velocity or anywhere near it.
- cecilpl2 4y ago
  No, it was going 14,240km/hr according to the video, which is about 4km/sec. Not near escape velocity.
- s1artibartfast 4y ago
  Isn't orbit tracking trivial? Also, orbital velocity depends on altitude, so perhaps they are just going to a higher orbit.
- wumpus 4y ago
  > What they're doing with the second boost, they're not telling anyone. This is a direct-to-GEO launch, and some of the payloads are public.
BitwiseFool 4y ago
>"wasteful to see that mission profile with the center booster burning up without even trying." My understanding was that the intended orbit / mission profile means that they must fly the center core in expendable mode. Otherwise, the satellite wouldn't be able to reach the intended orbit with the intended amount of fuel for the mission's duration.
- bpodgursky 4y ago
  Hopefully once Starship launches, we'll never see a disposable core again.
  - verdverm 4y ago
    They are different classes of launch vehicles. This payload is not really a good one for starship, and conversely, starlinks will never fly on heavy
    - bpodgursky 4y ago
      I'm pretty sure the goal with Starship is that the economics are so much better that it's used even for awkward launches. I do not believe they are targeting Falcon Heavy launches in the long-term, assuming the best-case Starship scenarios.
    - bryanlarsen 4y ago
      Yes, it's really odd to see a GEO launch rather than a GTO at launch. GTO is a geosynchronous transfer orbit with apogee at GEO and perigee at LEO that uses the satellite's thrusters to circularize to full GEO. Starship can't do a GEO directly, it would need refueling or a kick stage to do GEO. Since we really don't know why the Space Force wanted direct GEO we can't know for sure if refueling or a kick stage would have met requirements or not. Would it be good enough as long as the satellite to stay hidden inside the fairing during refueling?
      - ErikCorry 4y ago
mercurywells 4y ago
It doesn't have enough spare fuel for the descent/landing burns when lifting such a large payload.
mensetmanusman 4y ago
Have to consider overall risk mitigation waste impact. E.g. if the additional complexity of adding a center booster increases risk of full mission failure, then, integrated over time, it can be a more wasteful decision if the goal isn’t to test that. Especially considering the satellite that would get destroyed and all the impact of having to rebuild that across the global supply chain.
dotnet00 4y ago
I think that due to how much F9 has managed to eat into FH's market, FH flights we see are more likely to only happen when the payload is heavy enough to need the center core to be expended. For example, for a GTO insertion, an expended F9 can do 8.3t while a completely reusable FH can do 8t, but the expended F9 is going to be cheaper overall. Thus FH is only worth using when the payload is heavy enough to require at least the center core to be spent. This might be different for Lunar payloads though.
- bryanlarsen 4y ago
  Do you have any source for this? I would expect that a FH launch that reuses all 3 first stage cores would be cheaper than an expended F9. A few hundred thousand extra in fuel costs for the side boosters is cheaper than the few million dollars that the 9 expended engines would cost. But I still believe you -- the infrequency of an FH launch probably increases the ancillary costs. I'd just like to see your source.
  - dotnet00 4y ago
    My payload numbers are from the FH Wikipedia page: https://en.wikipedia.org/wiki/Falcon_Heavy#Capabilities https://en.wikipedia.org/wiki/Falcon_Heavy#Capabilities As for the price, the official numbers are $67m for a new F9 and $97m for a reusable FH (from their respective Wikipedia pages). Additionally, my reasoning is (as you mention) related to cadence. FH flies very infrequently and requires more ground expenses to prepare, recover and then refurbish 3 boosters. Additionally there's the opportunity cost of keeping a center core on standby (as it's purpose built for FH [1]) and the opportunity cost of keeping the side boosters either on standby or converting them to F9s. Elon has previously mentioned that with Falcon rockets the main costs per flight are the second stage (~$10m, expended), fairings (~$3-4m, reusable), refurbishment (~$1m) and fixed ground costs, which can be reduced by increasing cadence [2]. In the case of FH the second stage and fairing costs are the same, the refurb costs are tripled and due to much lower cadence the ground costs are higher. Meanwhile, F9 flies frequently, so it has lower ground costs per launch and the booster's cost can be more easily amortized via several flights before the expendable launch. Additionally, [2] mentions that the marginal cost of building an F9 booster is ~$15m, so all the extra costs of FH only need to be greater than $15m to make it worth just using an expendable F9. [1] https://spaceflightnow.com/2022/01/27/spacex-gives-converted-falcon-heavy-side-booster-new-life/ https://spaceflightnow.com/2022/01/27/spacex-gives-converted... [2] https://www.inverse.com/innovation/spacex-elon-musk-falcon-9-economics https://www.inverse.com/innovation/spacex-elon-musk-falcon-9...
bell-cot 4y ago
When you are watching SpaceX launch videos, pay attention to the velocities at which the various engine burns begin and end. Reaching higher velocities (for comparable payloads, etc.) requires burning more fuel - so there is less fuel left in the tank to perform boost-back burns, entry burns, and landing burns. Not enough fuel for a boost-back burn (the typical situation) means "barge landing". Not enough fuel for an entry (& landing) burn means "expended first stage" - which also means "remove the weight of the useless landing legs before launch". And if the customer is paying SpaceX to take a heavy payload to a high velocity / orbit...well, they are paying for it. Today's launch was actually the third-most-performant (and -expensive) Falcon Heavy launch mode. #2 has both side boosters landing on barges (so the boosters don't need fuel for boost-back burns). #1 expends everything - just like all of SpaceX's competition does.
- pfdietz 4y ago
  If you watched the liftoff, after the vehicle was above the fog, the plume from the core looked much weaker than from the boosters. I think they throttle that stage down so it's using less fuel, leaving fuel to be burned after the boosters are done. This will also reduce the velocity at which the boosters are staged so they have an easier time getting back. After the boosters stage the core will be throttled back up (if it hadn't been already).
  - bell-cot 4y ago
    Correct, and that throttle-fiddling is required to get a good performance boost from using side boosters.
    - pleo__ 4y ago
      Additionally, the center core is throttled to make sure the vehicle safely gets through the dangerous "max-Q" part of the launch where stress from the atmospheric drag is highest.
      - pfdietz 4y ago
willis936 4y ago
It's not impossible, but it's certainly trickier. You'd have to save plenty of fuel to kick back into the atmosphere and land, re-entry is much more strenuous, and you'd have to complete a full orbit to land back where you launched. Nothing is a showstopper, but it would take development.

by usrusr 4y ago

wiredfool 4y ago

Edit: Units... I'd be the one going in mph through the surface of mars. ~The booster was going 12km/sec at MECO, which is more than escape velocity. What they're doing with the second boost, they're not telling anyone.~

xcskier56 4y ago
The booster was doing 14,000km/h at MECO which is about 3.8km/s. Orbital velocity is 7-7.5km/s depending on altitude and escape velocity is even higher, so no, it was not at escape velocity or anywhere near it.
cecilpl2 4y ago
No, it was going 14,240km/hr according to the video, which is about 4km/sec. Not near escape velocity.
s1artibartfast 4y ago
Isn't orbit tracking trivial? Also, orbital velocity depends on altitude, so perhaps they are just going to a higher orbit.
wumpus 4y ago
> What they're doing with the second boost, they're not telling anyone. This is a direct-to-GEO launch, and some of the payloads are public.

BitwiseFool 4y ago

>"wasteful to see that mission profile with the center booster burning up without even trying." My understanding was that the intended orbit / mission profile means that they must fly the center core in expendable mode. Otherwise, the satellite wouldn't be able to reach the intended orbit with the intended amount of fuel for the mission's duration.

bpodgursky 4y ago
Hopefully once Starship launches, we'll never see a disposable core again.
- verdverm 4y ago
  They are different classes of launch vehicles. This payload is not really a good one for starship, and conversely, starlinks will never fly on heavy
  - bpodgursky 4y ago
    I'm pretty sure the goal with Starship is that the economics are so much better that it's used even for awkward launches. I do not believe they are targeting Falcon Heavy launches in the long-term, assuming the best-case Starship scenarios.
  - bryanlarsen 4y ago
    Yes, it's really odd to see a GEO launch rather than a GTO at launch. GTO is a geosynchronous transfer orbit with apogee at GEO and perigee at LEO that uses the satellite's thrusters to circularize to full GEO. Starship can't do a GEO directly, it would need refueling or a kick stage to do GEO. Since we really don't know why the Space Force wanted direct GEO we can't know for sure if refueling or a kick stage would have met requirements or not. Would it be good enough as long as the satellite to stay hidden inside the fairing during refueling?
    - ErikCorry 4y ago

mercurywells 4y ago

It doesn't have enough spare fuel for the descent/landing burns when lifting such a large payload.

mensetmanusman 4y ago

Have to consider overall risk mitigation waste impact. E.g. if the additional complexity of adding a center booster increases risk of full mission failure, then, integrated over time, it can be a more wasteful decision if the goal isn’t to test that. Especially considering the satellite that would get destroyed and all the impact of having to rebuild that across the global supply chain.

dotnet00 4y ago

I think that due to how much F9 has managed to eat into FH's market, FH flights we see are more likely to only happen when the payload is heavy enough to need the center core to be expended. For example, for a GTO insertion, an expended F9 can do 8.3t while a completely reusable FH can do 8t, but the expended F9 is going to be cheaper overall. Thus FH is only worth using when the payload is heavy enough to require at least the center core to be spent. This might be different for Lunar payloads though.

bryanlarsen 4y ago
Do you have any source for this? I would expect that a FH launch that reuses all 3 first stage cores would be cheaper than an expended F9. A few hundred thousand extra in fuel costs for the side boosters is cheaper than the few million dollars that the 9 expended engines would cost. But I still believe you -- the infrequency of an FH launch probably increases the ancillary costs. I'd just like to see your source.
- dotnet00 4y ago
  My payload numbers are from the FH Wikipedia page: https://en.wikipedia.org/wiki/Falcon_Heavy#Capabilities https://en.wikipedia.org/wiki/Falcon_Heavy#Capabilities As for the price, the official numbers are $67m for a new F9 and $97m for a reusable FH (from their respective Wikipedia pages). Additionally, my reasoning is (as you mention) related to cadence. FH flies very infrequently and requires more ground expenses to prepare, recover and then refurbish 3 boosters. Additionally there's the opportunity cost of keeping a center core on standby (as it's purpose built for FH [1]) and the opportunity cost of keeping the side boosters either on standby or converting them to F9s. Elon has previously mentioned that with Falcon rockets the main costs per flight are the second stage (~$10m, expended), fairings (~$3-4m, reusable), refurbishment (~$1m) and fixed ground costs, which can be reduced by increasing cadence [2]. In the case of FH the second stage and fairing costs are the same, the refurb costs are tripled and due to much lower cadence the ground costs are higher. Meanwhile, F9 flies frequently, so it has lower ground costs per launch and the booster's cost can be more easily amortized via several flights before the expendable launch. Additionally, [2] mentions that the marginal cost of building an F9 booster is ~$15m, so all the extra costs of FH only need to be greater than $15m to make it worth just using an expendable F9. [1] https://spaceflightnow.com/2022/01/27/spacex-gives-converted-falcon-heavy-side-booster-new-life/ https://spaceflightnow.com/2022/01/27/spacex-gives-converted... [2] https://www.inverse.com/innovation/spacex-elon-musk-falcon-9-economics https://www.inverse.com/innovation/spacex-elon-musk-falcon-9...

bell-cot 4y ago

When you are watching SpaceX launch videos, pay attention to the velocities at which the various engine burns begin and end. Reaching higher velocities (for comparable payloads, etc.) requires burning more fuel - so there is less fuel left in the tank to perform boost-back burns, entry burns, and landing burns. Not enough fuel for a boost-back burn (the typical situation) means "barge landing". Not enough fuel for an entry (& landing) burn means "expended first stage" - which also means "remove the weight of the useless landing legs before launch". And if the customer is paying SpaceX to take a heavy payload to a high velocity / orbit...well, they are paying for it. Today's launch was actually the third-most-performant (and -expensive) Falcon Heavy launch mode. #2 has both side boosters landing on barges (so the boosters don't need fuel for boost-back burns). #1 expends everything - just like all of SpaceX's competition does.

pfdietz 4y ago
If you watched the liftoff, after the vehicle was above the fog, the plume from the core looked much weaker than from the boosters. I think they throttle that stage down so it's using less fuel, leaving fuel to be burned after the boosters are done. This will also reduce the velocity at which the boosters are staged so they have an easier time getting back. After the boosters stage the core will be throttled back up (if it hadn't been already).
- bell-cot 4y ago
  Correct, and that throttle-fiddling is required to get a good performance boost from using side boosters.
  - pleo__ 4y ago
    Additionally, the center core is throttled to make sure the vehicle safely gets through the dangerous "max-Q" part of the launch where stress from the atmospheric drag is highest.
    - pfdietz 4y ago

willis936 4y ago

It's not impossible, but it's certainly trickier. You'd have to save plenty of fuel to kick back into the atmosphere and land, re-entry is much more strenuous, and you'd have to complete a full orbit to land back where you launched. Nothing is a showstopper, but it would take development.