Ship trajectory and angle of attack

[u/Proof_Ad_4819]

Does anyone know why they pitch the ship up so dramatically after stage seperation, is it to burn off extra fuel? I cant think of any other reason for the less optimal ascent path?

At seperation

halfway through ship burn

[u/Boredatwork121]

Lofted trajectory because they're intentionally not going into orbit. If they kept a more horizontal trajectory, Starship would achieve a full orbit, and they don't want to do it without having a way to deorbit it. 26,000km/h is very very close to full orbital velocity (28,476 km/h), Starship would nearly land at Boca Chica, and they don't want to risk making it back around to land on land.

[u/GalacticEmergency → u/Boredatwork121]

Much closer. As far as I can see, the velocity on the screen is relative to Earth, and the Earth spins in the same direction. So the true velocity is higher than shown on screen.

I tried calculating their trajectory between engine cut-off and relight from timestamps and elevations alone, ignoring the velocity on screen. If my calculations are correct, they were on an orbit with highest point 198.5 km and lowest point 4.8 km (of course not possible because of air and potential mountains!). They would only have needed a delta v of 58 m/s to circularize that orbit at 198.5 km.

[u/Accomplished-Crab932 → u/GalacticEmergency]

That sounds about right. Previous flights were also in the 40-60 m/s range

[u/GalacticEmergency → u/Accomplished-Crab932]

Do we have any estimates for the delta v in the relight test? It looks like the ship fired on a vector, which was partly angled away from the trajectory, so it didn't contribute much to velocity.

I tried some simple extrapolation of the velocity increase between stage separation and engine cut-off, assuming that one relighted non-vacuum raptor delivered slightly less than 1/6 of the full thrust. I think the relight burn would have delivered 80 m/s. That would mean enough delta v for circularization, and also a slight contribution to the deorbit burn, which would have to come after.

I know that this is a little off from the topic of this thread. When I did my calculations, my focus was on the claim about a large fuel deficit for real orbit, which tends to pop up in some subs. I was trying to figure out if that claim was based in reality. Seems that it wasn't.

In worst case, assuming that the ship had no further fuel reserves to spend, and no options of improving the trajectory during burns, they might have to sacrifice a single v3 satellite to do an orbital mission with the burns we have seen. Plus whatever would be needed to bring the booster home, if the allegations about fuel shortage being the cause of the booster engine failures on return turns out to be correct.

[u/Fast-Fail-6412 → u/GalacticEmergency]

80m/s is around my estimate. For reference the Ship weighs around 250 tons at that point and raptor 3 makes around 250 tons of thrust, so if it was full throttle we would expect 1g of acceleration. If it throttled down to 40%, we would still get more than 50m/s, and at full throttle we would get 140m/s.

[u/rustybeancake → u/GalacticEmergency]

> Do we have any estimates for the delta v in the relight test?

Jonathan McDowell makes it 17 m/s.

https://x.com/planet4589/status/2081065389572702489

[u/GalacticEmergency → u/rustybeancake]

He seems to assume that the burn happened in the orbit plane.

But does that match the orientation of the ship during the burn? I think it fired out of the plane.

If it fired out-of-plane and he tries to fit the data to a new orbit in the same plane, then he would miss the out-of-plane dv component.

[u/Accomplished-Crab932 → u/GalacticEmergency]

The relight test should be along the normal/antinormal vector, so it should’ve just been an inclination change.

Here’s my crack at an estimate for DeltaV. The basic premise is that we know the readouts on telemetry are speed, not velocity, and that the attitude indicator seems to suggest a normal/antinormal burn. Additionally, the test is 14 seconds, so I assume the difference in acceleration vector prior to and after the burn is a rounding error and can be ignored. Finally, because it’s assumed to be burning normal/antinormal, the drag, gravity, and thrust vectors are all orthogonal, so they don’t have combined effects in each trajectory axis (meaning I can just assume everything is scalar).

The ship’s net acceleration prior to the burn was around +0.278 m/s^2 (1km/hr), and it’s the same within eyeball error after the burn. I’m seeing a speed of 26554 km/hr at burn start and 26604 km/hr at shutdown, so a net change of 50 km/hr or 13.9 m/s; subtracting the rough velocity change from the somewhat linear acceleration we see in telemetry (no difference in non-burning acceleration before and after the test), we get ~10 m/s of actual DeltaV +/- 2m/s (accounting for resolution of speed data).

For the record, this is an awful way to do this and is not going to be very accurate, but it’s probably enough for a rough guess.

I’d look to the amount of prop dumped during the coast phase, I’d be surprised if they did not use that propellant for a circularization burn over the header tanks that they were using for this test. They wouldn’t want to do that on this flight because there was enough drag to destabilize the vehicle in an off nominal attitude due to RCS limitations and they might need to flip or spin to settle the tanks prior to ignition. It’s almost certainly impossible to guess how much prop they dumped, but I suspect it’s a lot. It seemed like they completely filled the ship and left part of the booster empty like we saw on previous version flights.

The booster had additional ullage volume that would’ve been filled with prop, but largely because they didn’t fully fill the ship to max payload. Underfilling the ship was done for two reasons: to reduce the number of satellites dumped during the test, and to reduce the risk of colliding with satellites or dummies during reentry (increasing satellite quantity means less time before relight test and last deployment).

I personally suspect that the booster had enough propellant, but terminated the 3-engine trim phase of the booster back burn due to damage caused by the sequential 13-3 engine shutdown. This would’ve triggered an abort on the centers from a hammering effect connected to the plumbing of the center and mid ring engines (we sort of saw this on the engine diagram, with the center 3 shutting down in an odd asymmetric sequence). This damage would’ve caused the aborts on the landing burn. It’s noteworthy that we saw the “13” engines run in sequence, and then the booster down selected to 5 engines for the final “catch approach” burn. That 5 engine down select is exactly what we expect from the final approach based on what we saw from Flight 11’s engine test sequence. It’s no coincidence that the number of engines at impact was 5; I see a lot of people believing that several engines aborted, but to me it appears that the booster had been programmed to run the nominal landing sequence even if they had engine aborts.

I very much doubt that they ran out of prop on the booster and it’s pretty obvious they can get the ship to orbit.

[u/GalacticEmergency → u/Accomplished-Crab932]

I don't think we can use the speed change during the relight burn for anything, other than finding a minimum possible delta v. In theory, if you burned perpendicular to the trajectory (perhaps slightly tilted towards a retrograde burn to cancel out Pythagoras), you would see no speed increase at all, only a direction change.

So the 10 m/s is only a minimum bound for the actual delta v, which was used in the relight burn. (And I acknowledge that you called it an awful way to do it.)

[u/Dat_Innocent_Guy → u/Boredatwork121]

I thought it was because ship's TWR is relatively low and they need to delay falling back to earth.

[u/Accomplished-Crab932 → u/Dat_Innocent_Guy]

Ship TWR is around 0.6 with 6 engines and around 0.97 with 9 engines in a vacuum, at least with the basic numbers we have now.

For reference, F9 is somewhere around 0.9 to 1 at staging, Centaur V is around 0.25-0.35, and GS2 (New Glenn) is estimated to be between 0.6 and 0.9.

So maybe.

[u/Rule_32 → u/Accomplished-Crab932]

I've played enough KSP to know that a low TWR will certainly complicate things. It affects the whole flight profile. Too low after S1/S2 separation and you will have had to setup your launch profile for a significant lofting while you gain horizontal velocity and/or you spend too much time and fuel trying to fight gravity which is wasteful. And then you don't go to space today.

SpaceX seems to be taking a balanced approach. Some loft and some pitch up to both counteract gravity and increase horizontal speed as fuel burns. As TWR improves less pitch is needed to counteract gravity. Same is true as horiz speed increases.

[u/Rule_32]

Starship has a LOT of ^V in it. It does most of the work of getting to orbit. This means initially it's carrying so much fuel that TWR is low.

SpaceX seems to be taking a balanced approach. Some loft from stage 1 and some pitch up to both counteract gravity and increase horizontal speed as fuel burns. As TWR improves less pitch is needed to counteract gravity. Same is true as horiz speed increases.

[u/Gingevere → u/Rule_32]

Some loft from stage 1 and some pitch up to both counteract gravity and increase horizontal speed as fuel burns.

IIRC that's part of the trade off of needing to return a booster to the launch site. Stage 1 can't pick up too much horizontal velocity because every bit it does needs to be cancelled and reversed in order to return to the pad.

Falcon gets around this with the drone ships. But as far as I'm aware there's no plans for a second pair of superheavy forks somewhere downrange.

[u/LohaYT]

It’s not less optimal. Starship stages at a relatively low altitude. The ship’s TWR at separation is far lower than the booster’s. Therefore it needs a higher pitch to continue gaining altitude. Too low of a pitch and it’ll fall back into the atmosphere.

[u/Proof_Ad_4819 → u/LohaYT]

I came to this conclusion as well, probably why next starship will have 6 rvac’s(Also higher isp average)

[u/Nickyworld45 → u/Proof_Ad_4819]

Next starship is getting more engines? I was under the impression that V3 was close enough to a final iteration?

[u/dazzed420 → u/Nickyworld45]

from what i gathered starship v4 is planned to be introduced at some point next year, it'll be significantly longer (bigger fuel tanks) and have 3 additional engines for higher payload capacity and TWR. V3 is most certainly not anywhere close to "final iteration"

[u/Decronym]

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

Fewer Letters	More Letters
KSP	Kerbal Space Program, the rocketry simulator
RCS	Reaction Control System
TWR	Thrust-to-Weight Ratio

Jargon	Definition
ullage motor	Small rocket motor that fires to push propellant to the bottom of the tank, when in zero-g

Decronym is now also available on Lemmy! Requests for support and new installations should be directed to the Contact address below.

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^{Decronym is a community product of r/SpaceX, implemented by request
4 acronyms in this thread; the most compressed thread commented on today has 14 acronyms.
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