totm nov 2023 SpaceX Discussion Thread

[tater]

 

[RCgothic]

1 hour ago, YNM said:

Is it correct though that SN8 went to 12.5 km and SN9 only went up to 10 km ? I know that SN9's altitude was announced in the livestream but for SN8 there wasn't any.

Flight comparison:

Looks like SN9 went a little over 10km, and SN8 was a little under 12.5km. SN9 had a slower ascent. SN8 started descending before pitch and flip, whereas SN9 flipped straight from apogee.

Interesting that descent was much faster than Ascent, which it doesn't really look like.

SN9 also had a higher peak velocity, which may have been a result of it going a bit nose-down after flip.

 

[tater]

 

[sevenperforce]

15 minutes ago, RCgothic said:

Looks like SN9 went a little over 10km, and SN8 was a little under 12.5km. SN9 had a slower ascent. SN8 started descending before pitch and flip, whereas SN9 flipped straight from apogee.

Interesting that descent was much faster than Ascent, which it doesn't really look like.

SN9 also had a higher peak velocity, which may have been a result of it going a bit nose-down after flip.

I think one of the risk-retiring issues for SN8 was controllability at the flip. Starship isn't aerodynamically stable in unpowered prograde so they were worried a bit about getting into that nice smooth bellyflop. Once they proved it was no trouble for SN8, there was no reason to worry about doing it gently for SN9.

I also saw the more aggressive nose-down maneuver. Guessing they wanted to experiment more with the range of attitudes and the type of body lift that Starship generates.

[tater]

SN10 is likely very similar to 9... I wonder what could be done to make more redundancy in engines.

I think it's clear engine redundancy is more of the nature, "We lost engine 2 from a regolith strike on Mars, so we can still land on Earth with the remaining 2" vs any failure while actually landing, there is simply not time to light an extra engine in the handful of seconds before landing.

Payload is not an concern, like other LVs, Starship will be volume limited, except for props. They should consider a pair of header tanks per engine. They can always immediately shut down an engine if they have too many, there is not time to start a new one. Light 3, drop immediately to 2, then 1, where the software picks the engine based on nominal function.

[JoeSchmuckatelli]

46 minutes ago, tater said:

 

Interesting - I did not know that they did faring recovery.  I wonder how that works. 

... 

On another note: why do they not light up all three engines for the landing?  Presumably they could shut one down if they did not need it easier than trying to light the third if the second failed, as happened. 

[sh1pman]

8 minutes ago, JoeSchmuckatelli said:

On another note: why do they not light up all three engines for the landing?  Presumably they could shut one down if they did not need it easier than trying to light the third if the second failed, as happened. 

Probably depends on how many G's can a near-empty Starship tolerate. But theoretically  it should be fine with 3 Raptors because orbital Starships will use 3 R-Vacs to reach orbit.

[RyanRising]

At 5:59 in the video (T+00:33.2) this ring appears for one frame. Anyone have any ideas what's going on there? Is that just a drop of something forming on the camera, then getting immediately vapourised after?

Edited February 3, 2021 by RyanRising

[magnemoe]

18 minutes ago, sh1pman said:

Probably depends on how many G's can a near-empty Starship tolerate. But theoretically  it should be fine with 3 Raptors because orbital Starships will use 3 R-Vacs to reach orbit.

Guess its more how much yaw they can manage to get upright and the last engine will not help much here. given that its on the wrong side of CoM and gimbal is restricted by the other engines. 

[RealKerbal3x]

2 minutes ago, RyanRising said:

At 5:59 in the video (T+00:33.2) this ring appears for one frame. Anyone have any ideas what's going on there? Is that just a drop of something forming on the camera, then getting immediately vapourised after?

I'm leaning towards a droplet of water forming on the camera lens and then being immediately vaporised. The relative scales make it look like a large component (since it would be taking up a significant portion of the plume's diameter) and if it was actually an engine part of that size I'd think we'd see one of the engines fail or shut down or something. The engines continue working perfectly up until shutdown anyway, which doesn't seem consistent with one of them shedding a giant part during ascent.

[tater]

41 minutes ago, JoeSchmuckatelli said:

Interesting - I did not know that they did faring recovery.  I wonder how that works. 

They've been doing that for a while.

Most of the time they end up in the water, then they pick them up and wash  them off.

[tater]

Second Starlink launch the 4th slipped to Friday.

[cubinator]

I still think they should split one package 30/30 and launch two Falcon 9s at the same time.

[YNM]

2 hours ago, RCgothic said:

Flight comparison:

Thanks !

Lack of any noticeable form of downrange movement is also interesting... That means that most of the time they were really going at 90 degrees AoA. So yeah I guess the priority here is to fully test the aerodynamic envelope rather than the landing sequence / method.

I'd still take that doing it a mere 5 km from another country isn't the best place to do it however. Honestly if there's any problem from the FAA it might very well be that - for orbital/suborbital launches you'd quickly move downrange and any off-center tendencies would be helped by the downrange movement - but here it all takes place in that one rough spot, so if anything goes wrongly off-plan it'll be amplified much more quickly.

Edited February 3, 2021 by YNM

[xebx]

1. they need to move the header Lox Tank inside the main Lox tank like the header Lch4 is in the main Lch4.

From a CoM point of view, the Lox header Tank at the top is a good idea, but when you consider these two small tanks are deep cryo tank designed for use 1h (leo to earth sl) or 5 months (leo or lunar to mars injection and landing) after the launch, you want both of them well insulated in the middle of Starship tanks.

And there is another reason, like said before, during the final maneuver, the quick rotation will mess with fuel in the pipes (ullage/vapors), it's better to have these pipes shorter and well insulated to reduce liquid instability/vapor generation.

Also you don't want to split these header tanks into one header tank per engine, why ? because they are deep cryo tank and the bigger a cryo tank is the better it is : volume law is ^3 and surface is ^2.

 

2. they need to have more margin for this final maneuver.

First, make the rotation slower, it will decrease the risk of problems in the header tanks pipes. (ullage/vapors)

Second, start the maneuver a little higher because you never know if an engine will fail, and have the third engine ready to fire or fired on with throttle at 20%  if one of the other fails.

Btw when i say one engine is enough (two messages before) to land an empty starship : if they are aiming for a short 2 or 3 engines landing burn, one engine will never be enough; because the landing burn start altitude is a lot higher x3? for 1 engine compared to 2 or 3 engines. (approx twr sl for 1,2,3 engines : 1.8, 3.6, 5.4)

 

In the end i can understand if they want to stick with a short and very efficient landing burn, and focus on it until they have 99.9% success, but i still think that they should do 1. and 2.

 

Edited February 4, 2021 by xebx

[tater]

[mikegarrison]

22 minutes ago, YNM said:

That means that most of the time they were really going at 90 degrees AoA.

That is not how angle of attack works.

AoA is the angle relative to the air velocity vector. If they point their nose in the direction of their velocity vector, they have zero angle of attack (leaving aside wind gusts and such effects).

Pitch angle is the direction relative to the horizon, and I think that must be what you meant.

[kerbiloid]

2 hours ago, RyanRising said:

At 5:59 in the video (T+00:33.2) this ring appears for one frame. Anyone have any ideas what's going on there? Is that just a drop of something forming on the camera, then getting immediately vapourised after?

Some obturator has gone.

Imho, of course.

Edited February 3, 2021 by kerbiloid

[JoeSchmuckatelli]

@tater

That's some hinkey - jinky work there.  Someone should have wrapped it up in gold foil for the more professional space look 

Edited February 3, 2021 by JoeSchmuckatelli

[tater]

Or maybe both sill go...

 

[YNM]

1 hour ago, mikegarrison said:

If they point their nose in the direction of their velocity vector, they have zero angle of attack (leaving aside wind gusts and such effects).

I mean most of the time they were falling... Launching is easy for them already.

[mikegarrison]

18 minutes ago, YNM said:

I mean most of the time they were falling... Launching is easy for them already.

Oh, I see. Well, that's a matter of definition, really. Reference body angle compared to velocity vector.

Angle of attack is not really well-defined with something like a sphere, for instance. In the case of something that is simply falling rather than trying to generate lift, I don't think angle of attack is particularly meaningful.

One way to look at it is the difference between a traditional parachute and a parawing. Angle of attack really means nothing with a traditional parachute, but it's meaningful with a parawing because there is meaningful lift generated by forward velocity.

Edited February 3, 2021 by mikegarrison

[tater]

And back to the earlier report of a delay to Friday for the second launch:

 

[tater]

T-6hrs:

 

[JoeSchmuckatelli]

I gotta ask this: given that Sn8 did not land.  And Sn9 was another real 'let's toss this up and see what happens' launch (such is the fate of prototypes)...

 

Why put Sn10 anywhere near where Sn9 might FOD the heck out of it?