totm nov 2023 SpaceX Discussion Thread

[Majk]

When Starship is done, exactly where will the Super Heavy Booster be recovered? The sources (such as https://www.space.com/spacex-starship-super-heavy-landing-plans) seem to indicate that it will be caught by the very same launch tower from which it started. However, this doesn't make much sense to me from orbital mechanics point of view.

• The booster doesn't have enough delta-v to become orbital on its own, so it cannot complete a full orbit to come back to the launch site. 
• The booster must gain some horizontal velocity. If it was merely lifting the second stage to orbital altitude, this would be incredibly wasteful. Furthermore,  the upper stage has about 7km/s delta-v, which is less than the orbital velocity at LEO, so it wouldn't become orbital even if it used up all of its fuel.
• Will the booster simply cancel all horizontal velocity it got, gain a comparable amount of horizontal velocity in the opposite direction to come back to the launch site? Again, this sounds incredibly wasteful, even one could argue that by staging the payload, the dry mass decreases significantly, thus improving delta-v of the booster.

A naive solution would be to have a separate landing site to "catch" the booster. This is not without issue, because either you need to have a network of evenly spaced launch sites all around the world (clearly impractical), or haul the behemoth over land. Furthermore, you're severely constrained in the inclinations you can launch into. Last but not least, this would contradict the claim here in https://techcrunch.com/2020/12/30/elon-musk-says-spacex-will-attempt-to-recover-super-heavy-rocket-by-catching-it-with-launch-tower/

Quote

Another potential benefit raised by Musk is that it could allow SpaceX to essentially recycle the Super Heavy booster immediately back on the launch mount it returns to — possibly enabling it to be ready to fly again with a new payload and upper stage (consisting of Starship, the other spacecraft SpaceX is currently developing and testing) in “under an hour.”

What am I missing?

[Nothalogh]

12 hours ago, tater said:

Yeah, that's absurd, Starliner will fly, there's no if.

 

 

The one thing that gets me with the whole Starliner thing is that Boeing convinced NASA to pay them an additional $287.2M for additional crew missions (within their contract anyway) because it looked like SpaceX was falling behind.

When you maximize your bluff skill

[Hannu2]

9 minutes ago, Majk said:

When Starship is done, exactly where will the Super Heavy Booster be recovered? The sources (such as https://www.space.com/spacex-starship-super-heavy-landing-plans) seem to indicate that it will be caught by the very same launch tower from which it started. However, this doesn't make much sense to me from orbital mechanics point of view.

Return to launch site needs delta-v but I think possibility of rapid reuse is more important than optimizing mass of single launch. It takes days to bring rocket from sea to launchpad and gives more restrictions if there is bad weather on sea.

[Hannu2]

On 5/3/2021 at 12:55 PM, Meecrob said:

Well lets at least be fair here and let Boeing complete their test program before you go declaring SpaceX the winner. Its not a race, first into space is not the superior vehicle.

Of course situations may vary. It is the reason why NASA wants to have more than one company capable of their operations. Final results will be clear in future when both ships have retired and replaced by newer ships. But so far Dragon has been clearly cheaper (If I remember correctly Boeing has been given much more money) and more productive.

[Lukaszenko]

17 minutes ago, Majk said:

 

• Will the booster simply cancel all horizontal velocity it got, gain a comparable amount of horizontal velocity in the opposite direction to come back to the launch site? Again, this sounds incredibly wasteful, even one could argue that by staging the payload, the dry mass decreases significantly, thus improving delta-v of the booster.

It's exactly this. Yes, it is incredibly wasteful, but of fuel, which if everything else works out is of little concern. As long as there's plenty of performance left over for launching the Starship (there should be) and it allows for rapid reusability (it should), then so be it.

The math is pretty straightforward: if sacrificing 50% of a launch's payload capacity allows you to have 100000% more launches, then it's stupid not to do it (I pulled these numbers out of my ass, but that's the jist of the argument).

Keep in mind that Falcon 9 already does this on missions where there's margin left over.

[JoeSchmuckatelli]

For those checking in: they are currently speculating on attempting after Noon, today.  Primarily concerned about the wind conditions.  

 

 

 

 

[tater]

https://www.windfinder.com/#7/26.2195/-96.8939/2021-05-04T18:00Z

 

 

[SOXBLOX]

8 hours ago, tater said:

120 a month, anyway (assuming a goal of 2 launches per month)

Nuts!

[Minmus Taster]

 

[RCgothic]

4 hours ago, Majk said:

When Starship is done, exactly where will the Super Heavy Booster be recovered? The sources (such as https://www.space.com/spacex-starship-super-heavy-landing-plans) seem to indicate that it will be caught by the very same launch tower from which it started. However, this doesn't make much sense to me from orbital mechanics point of view.

• The booster doesn't have enough delta-v to become orbital on its own, so it cannot complete a full orbit to come back to the launch site. 
• The booster must gain some horizontal velocity. If it was merely lifting the second stage to orbital altitude, this would be incredibly wasteful. Furthermore,  the upper stage has about 7km/s delta-v, which is less than the orbital velocity at LEO, so it wouldn't become orbital even if it used up all of its fuel.
• Will the booster simply cancel all horizontal velocity it got, gain a comparable amount of horizontal velocity in the opposite direction to come back to the launch site? Again, this sounds incredibly wasteful, even one could argue that by staging the payload, the dry mass decreases significantly, thus improving delta-v of the booster.

A naive solution would be to have a separate landing site to "catch" the booster. This is not without issue, because either you need to have a network of evenly spaced launch sites all around the world (clearly impractical), or haul the behemoth over land. Furthermore, you're severely constrained in the inclinations you can launch into. Last but not least, this would contradict the claim here in https://techcrunch.com/2020/12/30/elon-musk-says-spacex-will-attempt-to-recover-super-heavy-rocket-by-catching-it-with-launch-tower/

What am I missing?

Yes, it is wasteful. Return to launch site comes with a hefty performance penalty as Superheavy has to reserve a sizeable amount of fuel for the boostback burn. Not two thirds or even half its fuel though, as relieved of the mass of the Starship Upper Stage it has a very much more favourable mass ratio and can therefore extract a lot more DV from the remaining propellant.

It's notable that even with this performance penalty Starship Superheavy manages almost twice Saturn V's mass to LEO.

Edited May 4, 2021 by RCgothic

[RCgothic]

Scrub. No SN15 flight today.

 

 

 

[CatastrophicFailure]

Revenge of the Fifth it is, then. 

[tater]

 

 

Regarding RTLS for SH, Musk said a while ago that for F9, downrange recovery was a ~18% performance hit to LEO, and RTLS was a 40% hit.

I want to say Musk said SS/SH total mass was like 5000t. If we assume 100t for SS, 150t for cargo, 150t for SH dry mass, that gives SH 3900 m/s pushing the whole stack. It needs a lot less props to RTLS empty, so it seems to work out that it gives SS more dv than F9 gives the upper stage, and can still kill that and boost back.

 

[cubinator]

I think this next Starlink launch will have good viewing opportunities in the US this evening.

[RealKerbal3x]

 

[Dfthu]

 

[Deddly]

2 hours ago, tater said:

 

 

Regarding RTLS for SH, Musk said a while ago that for F9, downrange recovery was a ~18% performance hit to LEO, and RTLS was a 40% hit.

In addition to the points you make, Starship doesn't need a re-entry burn and it saves a lot of dv by belly flopping. On top of that, I would expect it to have a greater engines-out cross-range ability than the Flacon 9. 

[tater]

Liftoff!

MECO/Sep/SES-1/Fairing all nominal

I thought that was gonna miss. Wow.

 

Landing

 

And not just landing, like a dead on bullseye.

[tater]

 

[cubinator]

Right in the middle! Looks like it made an impressive translation to get there.

How long will the second stage stay in orbit? Would it still be up there after five or six orbits?

[mikegarrison]

1 hour ago, Deddly said:

I would expect Starship to have a greater engines-out cross-range ability than the Falcon 9. 

Well, that will help make sure that the impact point isn't in the middle of a city or something, but otherwise I'm not sure what the point is for "engines-out cross-range ability" when all that means is an ability to direct where the "BOOM!" happens.

[sevenperforce]

36 minutes ago, RCgothic said:

Yes, it is wasteful. Return to launch site comes with a hefty performance penalty as Superheavy has to reserve a sizeable amount of fuel for the boostback burn. Not two thirds or even half its fuel though, as relieved of the mass of the Starship Upper Stage it has a very much more favourable mass ratio.

It's notable that even with this performance penalty Starship Superheavy manages almost twice Saturn V's mass to LEO.

I can't remember if I've done the math on this before or not....

120 tonnes dry mass, 30 tonnes landing reserve prop, and 100 tonnes payload to LEO, so m_f is 250 tonnes. 1200 tonnes prop at staging but that presumably includes those 30 tonnes in the header tanks so m₀ is 1420 tonnes. So dV expended to LEO is 6473 m/s. We know that Starship needs all six engines to light at staging to help avoid overmuch gravity drag, which means a fairly lofted trajectory and some gravity drag losses -- probably as high as 600 m/s. So we can ballpark staging velocity at LEO - 5873 or roughly 2 km/s.

Superheavy needs to cancel its downrange velocity (but not its upward velocity) and head back to the pad. A lofted trajectory helps with this because it only needs enough speed to traverse the same distance during its hang time. We can assume that Superheavy, like Falcon 9, will have a landing burn that kicks off just after the vehicle goes subsonic, around 310 m/s, but just as Falcon 9 it will need about 200 m/s more to account for gravity drag during the landing burn.

In the NROL-108 mission, Falcon 9 needed about 550 m/s for the landing burn (it was a single-engine burn) and also burned all three engines for an entry burn for 28 seconds. It is 27 tonnes dry so that is 6 tonnes of prop for the landing burn, and a Merlin 1D has a mass flow rate of 305.4 kg/s so burning three of those for 28 seconds was about 25 tonnes, so total mass before the start of the boostback burn would have been 58 tonnes. Its boostback burn lasted 37 seconds and thus would have consumed 33.9 tonnes of propellant. Since the vacuum specific impulse of the SL Merlin 1D is 311 seconds,  the boostback burn provided about 1400 m/s (staging velocity was 1600 m/s but part of that was an upward component, because lofted trajectory).

The SL Raptors on Starship light up at 330 seconds and reach somewhere around 356 s by burnout. It won't need an entry burn. I'm estimating Superheavy at around 290 tonnes dry and 3690 tonnes wet, so it burns around 1700 tonnes (49% of its prop load) getting to Mach 2, another 1450 tonnes (43% of its prop load) getting to staging, about 229 tonnes (6.7% of its prop load) on the boostback, and about 49 tonnes (1.5% of its prop load) for the landing burn.

[Deddly]

41 minutes ago, mikegarrison said:

Well, that will help make sure that the impact point isn't in the middle of a city or something, but otherwise I'm not sure what the point is for "engines-out cross-range ability" when all that means is an ability to direct where the "BOOM!" happens.

You would prefer the BOOM to be in an uncontrolled location? But seriously, I think you know as well as I do that it is too early to judge whether the flip maneuvre is going to work reliably. All of the early landing attempts of Falcon 9 also crashed and exploded, and look what that resulted in in the end. Would have been a shame if they just scrapped the whole idea and said "What's the point of grid fins when all that means is an ability to direct where the explosion happens?" 

I still can't believe they nailed the belly flop and skydive maneuvre right from the first attempt. 

[mikegarrison]

1 minute ago, Deddly said:

You would prefer the BOOM to be in an uncontrolled location? But seriously, I think you know as well as I do that it is too early to judge whether the flip maneuvre is going to work reliably. All of the early landing attempts of Falcon 9 also crashed and exploded, and look what that resulted in in the end. Would have been a shame if they just scrapped the whole idea and said "What's the point of grid fins when all that means is an ability to direct where the explosion happens?" 

I still can't believe they nailed the belly flop and skydive maneuvre right from the first attempt. 

It's the "engines out" part that I was really referring to. Whether Falcon 9 or Starship, "engines out" is an unrecoverable failure.

[sevenperforce]

13 minutes ago, mikegarrison said:

It's the "engines out" part that I was really referring to. Whether Falcon 9 or Starship, "engines out" is an unrecoverable failure.

Engine-out is unrecoverable for Falcon 9.

Engine-out is fine for Starship as long as it happens at relight (not mid-landing-burn) and the failed engine doesn't frag the others.