totm nov 2023 SpaceX Discussion Thread

[RealKerbal3x]

Seems like Elon has moved away from all-up testing and onto super-fast iteration. It's probably a better way to go.

[kerbiloid]

Or they calculated that success chances are 1:25, so decided not build all 25 unhappy ones, and immediately make the lucky 26th.

[Wjolcz]

So the flaps got redesigned again? I wonder how much harder would it be to make them more like flush airbrakes than flappy wings.

Edited April 16, 2020 by Wjolcz

[RealKerbal3x]

2 minutes ago, Wjolcz said:

So the flaps got redesigned again? I wonder how much harder would it be to make them more like flush airbrakes than flappy wings.

Where did you get that from? We don't know whether the flaps have been redesigned, only that SN4 won't get them.

[RCgothic]

4 hours ago, kerbiloid said:

21 more skipped?

***
"Spaceballs", the final countdown. (from memory)

"9... 8... 6..."
"Where's 7?!"
"Just joking. 5... 4..."

SN26 is a raptor engine, and they built SN 1-25.

For starships, still on SN4, then going to SN 5 and 6 in sequence.

[Ultimate Steve]

51 minutes ago, RealKerbal3x said:

Where did you get that from? We don't know whether the flaps have been redesigned, only that SN4 won't get them.

 

Flaps being redesigned as of this week.

[RCgothic]

Anyone know what raptor's minimum throttle is? Trying to work out the max flight duration.

Takeoff TWR probably has to be at least 1.2

TWR has to average 1.

At the end of the flight you'll be in trouble if minimum TWR is greater than 1 because you won't be able to decelerate down to the pad (no significant downward velocity to arrest with a high terminal TWR).

 

Should be able to ballpark the average throttle setting, and therefore flight time and max altitude.

[tater]

 

[tater]

 

[mikegarrison]

8 hours ago, RCgothic said:

At the end of the flight you'll be in trouble if minimum TWR is greater than 1 because you won't be able to decelerate down to the pad (no significant downward velocity to arrest with a high terminal TWR).

It seems like you are confusing the engine's TWR with the entire ship's TWR. Or maybe you are just confusing me.

Anyway, I thought Falcon9's landing TWR was greater than 1, which is why it doesn't hover like New Shepard does. You can have an arbitrarily high TWR and still go for a 0/0 landing, but the required precision gets harder and harder as the TWR goes up.

[tater]

7 minutes ago, mikegarrison said:

It seems like you are confusing the engine's TWR with the entire ship's TWR. Or maybe you are just confusing me.

Anyway, I thought Falcon9's landing TWR was greater than 1, which is why it doesn't hover like New Shepard does. You can have an arbitrarily high TWR and still go for a 0/0 landing, but the required precision gets harder and harder as the TWR goes up.

The the TWR of the whole F9 booster is indeedn >1 at landing. Think they aim for zero velocity JUST above the pad (and it takes some small time for the thrust to spool down I think).

[RCgothic]

12 minutes ago, mikegarrison said:

It seems like you are confusing the engine's TWR with the entire ship's TWR. Or maybe you are just confusing me.

Anyway, I thought Falcon9's landing TWR was greater than 1, which is why it doesn't hover like New Shepard does. You can have an arbitrarily high TWR and still go for a 0/0 landing, but the required precision gets harder and harder as the TWR goes up.

I think we have a misunderstanding. A TWR of greater than 1 is required at landing or you go splat.

But you must also have negative velocity. If you are incapable of negative velocity because you can't throttle low enough you will not be landing today.

E.g: A notional craft has max TWR 1.4 at liftoff and 2 at burnout. But its minimum throttle setting gives a TWR of 1.2 at lowest. That craft is going up and up and up and not coming back down.

Starship without fins is incapable of controlled flight with engine off. It must burn continuously from take off to landing.

Edited April 16, 2020 by RCgothic

[CatastrophicFailure]

7 minutes ago, RCgothic said:

But you must also have negative velocity. If you are incapable of negative velocity because you can't throttle low enough you will not be landing today.

Well, I mean, you will, grabbity being what it is, just a bit later and, ironically, faster than you had planned... 

[mikegarrison]

1 hour ago, RCgothic said:

But its minimum throttle setting gives a TWR of 1.2 at lowest. That craft is going up and up and up and not coming back down.

Its minimum TWR ratio is 0, as long as the engines can be shut down.

Edited April 16, 2020 by mikegarrison

[tater]

33 minutes ago, RCgothic said:

I think we have a misunderstanding. A TWR of greater than 1 is required at landing or you go splat.

But you must also have negative velocity. If you are incapable of negative velocity because you can't throttle low enough you will not be landing today.

E.g: A notional craft has max TWR 1.4 at liftoff and 2 at burnout. But its minimum throttle setting gives a TWR of 1.2 at lowest. That craft is going up and up and up and not coming back down.

Starship without fins is incapable of controlled flight with engine off. It must burn continuously from take off to landing.

F9 TWR is never below 1 at all, even on 1 engine, throttled down. Dry mass is something like 22 tonnes? very little prop left.

"Hoverslam" is basically aiming for zero velocity just before impact.

[mikegarrison]

Everything that lands usually has a positive sink rate (that is to, it is going down) (If it is climbing it could possibly land, but only if the ground is rising to meet it, like an airplane landing on an upsloped runway.)

Airplanes and helicopters typically aim for a target sink rate at touchdown, and the landing gear are designed to cushion that blow. Parachutes are also designed to provide a specific sink rate at touchdown.

New Shepard seems to be designed to decelerate to a hover just above the ground, and then the thrust is reduced and it settles down. At the actual moment of touchdown, its lift <= weight and it is sinking.

Falcon9 is unusual because its lift > weight as it approaches the pad. So it relies on having built up an excess sink rate that it is scrubbing off. (This is functionally equivalent to "the ground rising to meet it".) The goal is to achieve something right about zero velocity very near the moment of touchdown, and then to shut off the engines.

Edited April 16, 2020 by mikegarrison

[kerbiloid]

This "Chronicles of Riddick" landing style won't finish happily (not) once, like any streetracers' shenanigans.

If it doesn't stop in several meters, hover, then slowly get down, like a lawful chopper, it's good for logs but not for pax.

[mikegarrison]

28 minutes ago, kerbiloid said:

This "Chronicles of Riddick" landing style won't finish happily (not) once, like any streetracers' shenanigans.

If it doesn't stop in several meters, hover, then slowly get down, like a lawful chopper, it's good for logs but not for pax.

Yes, we have already seen SpaceX divert Falcons to crash into the ocean when something wasn't nominal with the landing. That's not really an option with a crewed vehicle.

(Or rather, it is an option, just not a good one. It is occasionally done with airplanes.)

Edited April 16, 2020 by mikegarrison

[cubinator]

31 minutes ago, mikegarrison said:

Yes, we have already seen SpaceX divert Falcons to crash into the ocean when something wasn't nominal with the landing. That's not really an option with a crewed vehicle.

(Or rather, it is an option, just not a good one. It is occasionally done with airplanes.)

Definitely not a good option when the landing is happening on Mars, where there is no ocean to divert to...

[RCgothic]

3 hours ago, tater said:

F9 TWR is never below 1 at all, even on 1 engine, throttled down. Dry mass is something like 22 tonnes? very little prop left.

"Hoverslam" is basically aiming for zero velocity just before impact.

Yup, but Falcon 9 is coming in with substantial negative velocity that needs to be countered, and has fins for controlled flight before engine ignition.

Elon has said SN4 would be uncontrolled without the main engine(s) being constant on. So the TWR must be capable of going below 1 so that the craft can start to come back down again. 

Unless you're willing to do some serious cosine stunt flying.

[Nightside]

27 minutes ago, RCgothic said:

Yup, but Falcon 9 is coming in with substantial negative velocity that needs to be countered, and has fins for controlled flight before engine ignition.

Elon has said SN4 would be uncontrolled without the main engine(s) being constant on. So the TWR must be capable of going below 1 so that the craft can start to come back down again. 

Unless you're willing to do some serious cosine stunt flying.

Well maybe they are throttling the engine well below it's capacity for the test. the water tower was able to hover, right?

[tater]

26 minutes ago, RCgothic said:

Yup, but Falcon 9 is coming in with substantial negative velocity that needs to be countered, and has fins for controlled flight before engine ignition.

Elon has said SN4 would be uncontrolled without the main engine(s) being constant on. So the TWR must be capable of going below 1 so that the craft can start to come back down again. 

Unless you're willing to do some serious cosine stunt flying.

We don't know what the mass is on SN4, but north of 100t, right? Raptor throttles down to somewhere above 40% (he tweeted that at 40% it gets chuggy). So if it's 100-something tons with props, and throttles to 40-something, there you go.

I thought we were talking about a real entry, engines off, then they light up right before landing. Final SS might well be able to hover on Earth.

Just now, Nightside said:

Well maybe they are throttling the engine well below it's capacity for the test. the water tower was able to hover, right?

Same thing, super heavy build, engine throttles pretty deep.

[sevenperforce]

14 minutes ago, tater said:

We don't know what the mass is on SN4, but north of 100t, right? Raptor throttles down to somewhere above 40% (he tweeted that at 40% it gets chuggy). So if it's 100-something tons with props, and throttles to 40-something, there you go.

I thought we were talking about a real entry, engines off, then they light up right before landing. Final SS might well be able to hover on Earth.

Water tower was able to hover because it had a single engine. SN4 and all the rest of its line have three engines in a triad configuration so there's no way to hover on just one central engine.

[Meecrob]

35 minutes ago, RCgothic said:

Elon has said SN4 would be uncontrolled without the main engine(s) being constant on. So the TWR must be capable of going below 1 so that the craft can start to come back down again. 

Unless you're willing to do some serious cosine stunt flying.

I think there is a wording issue here with "uncontrolled."  Elon doesn't mean that if the engine is off, Starship is "unstable," he means that you cannot pitch or yaw without the engine/gimbal. With this in mind, a test flight will likely be a powered ascent actively controlled by engine/gimbal, coast to apogee and unpowered descent, then late burn to make final corrections (due to wind, etc) and arrest descent rate. In this scenario, the nose will be pointed the same direction in all phases of flight, so in a perfect world with no wind, you could in theory, not control Starship's pitch or yaw at all if it starts exactly 90* to the horizon

[tater]

8 minutes ago, sevenperforce said:

Water tower was able to hover because it had a single engine. SN4 and all the rest of its line have three engines in a triad configuration so there's no way to hover on just one central engine.

I totally spaced that. You are right.