Wiki Super Heavy/Starship - General Development Discussion

[ecarfan]

How is it the Starship does not have grid fins but is able to control precisely in the last minute and seconds of landing?

This wing like things (chines?) can control when the rocket is falling horizontally higher on the atmosphere, but once it gets vertical how is the rocket movement controlled through the air?

Starship uses a completely different re-entry method than the Super Heavy booster (which will use basically the same technique as an F9 booster). We’ve already seen most of the Starship re-entry profile during the various “hop” test flights, which you can re-watch on YouTube to refresh your memory if needed.

During the final seconds of a Starship landing — when the chopstick arms on the tower will catch it — it will hover and null out its velocity by throttling and gimbaling the center Raptor engines and use its side thrusters (initially cold gas but later hot gas from the autogenous pressurization system) to adjust its position horizontally if necessary.

At least that is my understanding.

 

[scaesare]

My guess is that the atmospheric flight modes are either 1) falling horizontally using the canards etc or 2) vertical using the gimballed thrust of the raptors. There's no real flight mode where its falling in the atmosphere vertically, un-powered.

I think the fact that the center 3 core engines can be gimbaled to a wide range angle individually also plays in to this.

From what I've read, Falcon 9 can gimbal all it's engines, but I've never seen anything dramatic other than the single center, Also unsure if they can be individually actuated?

The 2 that were lit for the Starship bellyflop tests were cranking pretty hard and at vastly different angles:

 

[mongo]

I think the fact that the center 3 core engines can be gimbaled to a wide range angle individually also plays in to this.

From what I've read, Falcon 9 can gimbal all it's engines, but I've never seen anything dramatic other than the single center, Also unsure if they can be individually actuated?

The 2 that were lit for the Starship bellyflop tests were cranking pretty hard and at vastly different angles:

View attachment 871690

Yeah, Falcon 9 engines are all individually steered (roll control). Center engine has larger range of motion (landing).

 

[ecarfan]

The 2 that were lit for the Starship bellyflop tests were cranking pretty hard and at vastly different angles:

Yes, and that looked rather odd to my definitely-not-an-aerospace-engineer eyes. But from that single camera view we can’t accurately determine the relative angle between the two engines nor can we know whether or not there was a small difference in output between the two. What was actually happening was certainly much more complex than what the video showed.

 

[mongo]

Yes, and that looked rather odd to my definitely-not-an-aerospace-engineer eyes. But from that single camera view we can’t accurately determine the relative angle between the two engines nor can we know whether or not there was a small difference in output between the two. What was actually happening was certainly much more complex than what the video showed.

Additionally, the engine bells can colide so running with split thrust like in the photo allows each engine to have more maneuvering room. Especially if the other's TVC fails.

Also, the engines are offset from the centerline. To avoid imparting rotation, the thrust vector needs to point through the center of gravity (which is impacted by sloshing propellant).

 

[ecarfan]

To avoid imparting rotation, the thrust vector needs to point through the center of gravity (which is impacted by sloshing propellant).

And that sounds like a very tricky problem! Fortunately as the engines null out the vertical velocity the propellants are forced towards the bottom of the tanks so hopefully they aren’t moving around too much in the final seconds of descent.

 

[mongo]

And that sounds like a very tricky problem! Fortunately as the engines null out the vertical velocity the propellants are forced towards the bottom of the tanks so hopefully they aren’t moving around too much in the final seconds of descent.

Indeed, and they will shift based on ship/ net acceleration angle. I think that's minimal for mostly vertical angles though.

 

[scaesare]

Additionally, the engine bells can colide...

I feel like we saw that happen in one of the early bellyflop attempts.... but I never heard any discussion on it...

 

[dhanson865]

I highly doubt Falcon Heavy launching in 2024 or after.

I'm expecting those payloads to be reassigned to Starship by then.

 

[bxr140]

How is it the Starship does not have grid fins but is able to control precisely in the last minute and seconds of landing?

It's largely an aerodynamic thing. The grid fins need a good bit of velocity to work, and while they do scrub energy through aerodynamic drag, the engine firings do the heavy lifting on energy. The fins are really (mostly) for steering. It's a great solution for something that hasn't left the atmosphere.

Ship OTOH can't re-enter an atmosphere in a lawn dart orientation and definitely can't re-enter with something like grid fins sticking out; the practical solution is what they've settled on, which is a shuttle-like configuration with more traditional aerodynamic control surfaces. What that solution does is allow the whole thing to scrub energy (and, ultimately, velocity) ~all the way down in the aircraft-like orientation to the point where grid fins simply aren't necessary. By the time the thing rotates vertical it's going so slow that the fins would be largely useless, and that's before factoring in the downside of mass/complexity.

 

[Electroman]

By the time the thing rotates vertical it's going so slow that the fins would be largely useless,

Thanks, that what I figured. So the last minute lateral movements using just the engines and hot gas thrusters, is going to be a tricky task if say the rocket is in a position of having to move 1000 feet laterally at 2 km height, to land precisely on the target. Will be interesting to watch that.

 

[ICUDoc]

Will be interesting to watch that.

Understatement of the year!

 

[mongo]

Thanks, that what I figured. So the last minute lateral movements using just the engines and hot gas thrusters, is going to be a tricky task if say the rocket is in a position of having to move 1000 feet laterally at 2 km height, to land precisely on the target. Will be interesting to watch that.

Optomistic (simplified) opinion:
Control system is different, but path planning is similar to F9 booster. The ship would have been freefalling for tens of kilometers before landing. Velocity vector should be well aimed at the target by 2km up. A situation where it has to translate 0.3km with no inital horizontal velocity from 2km up should not happen unless planned. Which it may be to avoid Stage 0 damage on later ship failure.

If that point was a safety target, it's only a 9 degree angle and needs 15% lateral speed vs descent to correct. Should be achievable by Raptor (assuming sufficient fuel).
For the catch, it will end up basically hovering, so my concern is more cross winds, but mass and thrusters should minimize that impact; and arm movement can track drift (to some degree).

 

[Electroman]

This type of landing (catch by arms) is an order of magnitude complex then landing on legs. Margins are in low single digit feet. Scary..

 

[ecarfan]

This type of landing (catch by arms) is an order of magnitude complex then landing on legs. Margins are in low single digit feet. Scary..

SpaceX will practice booster landings over water multiple times to verify control capabilities and accuracy before attempting a Stage Zero landing. I have some confidence that they will get it right. They will do the same practice attempts with Starship but I expect that will take longer to get right, since Starship has to survive orbital re-entry without incurring damage to the TPMS and flap control mechanisms. The “hop” tests to date have obviousl not tested those systems at orbital re-entry speeds and temperatures.

On a different subject; there was more booster and ship cryo load testing yesterday. After it was completed, the transport stand was moved next to the OLM. Speculation is that S24 will soon be de-stacked so that booster static fire testing can begin!

 

[mongo]

This type of landing (catch by arms) is an order of magnitude complex then landing on legs. Margins are in low single digit feet. Scary..

Yes and no...
F9 can't hover.
SH can go to slow and zero vertical speed and the arms give a large capture range.

 

[ecarfan]

SH can go to slow and zero vertical speed and the arms give a large capture range.

Yes, and based on just eyeballing how accurately F9 boosters land and how wide the chopsticks can spread it appears that FH should be able achieve a sufficient degree of precision to allow for a successful “catch”. Obviously SpaceX thinks it is achievable and who am I to argue with them?

 

[Electroman]

Same launch tower to catch both booster and ship, with the ship landing right on top of the booster with another pair of arms?
Or will they be building another one to catch the ship?

 

[mongo]

Same launch tower to catch both booster and ship? Or will they be building another one to catch the ship? Or will the Ship land right on top of the booster with another pair of arms?

Same tower catches occur offset from mount.
Catch booster, place in mount
Catch ship, stack on booster
Can also reload with different ship from transporter.

 

[Electroman]

This is more wild than most science fiction I have read