totm nov 2023 SpaceX Discussion Thread

[RCgothic]

1 hour ago, sevenperforce said:

It definitely has a heat shield, but the question is whether the intact section would have had passive aerodynamic stability. I think the answer is almost certainly no. Starship achieves shield-first aerodynamic stability during re-entry using active control of all four drag flaps, but without only two flaps and no active control I don't see how any orientation provides passive stability. The center of mass and the center of drag will be very close to each other which would likely induce either a tumble or a flat spin, either of which would result in the vehicle being ripped apart by aerodynamic forces.

^{(This was the same fate for both Challenger and Columbia. Although Challenger is often said to have "exploded" and Columbia is said to have "burned up", both orbiters were ultimately destroyed by aerodynamic forces while tumbling: Challenger when the loss of active engine control could no longer keep the stack oriented and Columbia when differential drag on the half-melted wing caused roll and yaw deviations too great for the control surfaces to counteract.)}

It is very unlikely that the intact-but-tumbling payload bay of Starship would achieve ANY passively-stable orientation, let alone one in which the heat shield was properly pointed into the airstream.

And even if it did achieve passive heat-shield-first stability, that wouldn't last for long because aero forces would shred the open base of the fairing.

[sevenperforce]

22 minutes ago, RCgothic said:

And even if it did achieve passive heat-shield-first stability, that wouldn't last for long because aero forces would shred the open base of the fairing.

Most likely not, but I (personally) wouldn't be too dogmatic. When Skylab fell on Australia, surviving fragments included a 1600-lb fuel tank bulkhead from the attached stage as well as a 5100-lb airlock shroud 22 feet in length. These early Starships are relatively over-engineered so the steel fairing rings could conceivably be strong enough to handle the base drag.

Of course nothing would survive hitting the water at half the speed of sound.

[Exoscientist]

 This observer noted the booster reached far lower speed than expected:

 

To get all engines to fire without leaking or otherwise failing I was wondering if they were  fired at partial thrust.

  Robert Clark

Edited November 25, 2023 by Exoscientist

[sevenperforce]

8 minutes ago, Exoscientist said:

This observer noted the booster reached far lower speed than expected:

https://twitter.com/phrankensteyn/status/1726033391605211547?s=61

To get all engines to fire without leaking or otherwise failing I was wondering if they were  fired at partial thrust.

In the video @tater attached above, the Starship+Superheavy stack clears its own height in under eight seconds. Very sporty coming off the pad. I don't see any indication that the engines were being downthrottled.

Remember that the engine bay leaks in OFT-1 were associated primarily with hydraulic fluid. Full replacement with ETVC means no hydraulic fluid to leak this time.

I'm not sure where that particular dude on Twitter is getting the notion that the booster should have dumped 3.2 km/s at staging. A nominal Falcon 9 mission with boostback, like Crew-7, stages at 1.72 km/s airspeed; that's only slightly higher than Starship+Superheavy which is itself a completely different vehicle with a different launch profile and different boostback characteristics (not to mention that this was a test).

[Mikki]

On 11/19/2023 at 9:09 PM, Deddly said:

Don't/can't they have booster rotation on the same axis as the pump rotors? 

Flipping rotating uniform, narrow, straight shafts in a housing is not much of an issue, tangential forces proportional to the rotational energy (!) build up immediatly when fast rotating disc-like shapes are suddenly accelerated no matter the direction except  for the longitude axis of rotation of said shaft. And not along the new vector, but tangential, which is is the problem...

A simple live test for you at home in your workshop/ garage, whatever, is a anglegrinder without a tool attached and doing 3500 rpm, you can move it with one hand around in all directions, the rotor inside is quite light. Then attach a heavy disk (I.e. made of steel and diamond tipped) for concrete or stone and try to move the tool with one hand around, and you can experience the very unintuitive forces acting on the anglegrinder yourself... and this is only few hundred grams at 3500 rpm.

The kinetic force of a say 80 kg turbopump rotor at say 30`000 rpm excerts sideloads in the region of tons for sure, tangential to the new force vector (Right hand rule) when flipped around 180 degrees... I can imagine SpaceX is well aware of this issue, because the pumphousings of the raptor look narrow compared to other pumps used in other types of rocketmotors... But fans and impellers must go somewhere... 

The direction of the pumprotors along the flip maneuver axis would solve this issue to 100%(see above), but as we all can see the pumpshafts are mounted straight down the rocket long axis... 

Edit: Apparently the two raptor turbopumps develop between 30 and 36 megawatt kinetic energy at full speed and this equals to 30- and 36`000`000 Nm/s potential energy... This a lot of torque depending on the weight of the shafts, which i don`t know actually

Edited November 20, 2023 by Mikki
Typos:) and stuff, spelling sorry

[magnemoe]

4 hours ago, mikegarrison said:

I assume he was confusing km/s with km/h.

Yes it was an sprelling mistake   or rather more used to using m/s here, 24000 km/s would be kind of pointless for an icbm but would be very nice interplanetary missions, 

[magnemoe]

19 hours ago, darthgently said:

How aggressive the pitch over is can be a big factor also.  IFT-2 seemed to pitch fairly conservatively early on and achieved a high AP fairly quickly, but did seem close to horizontal at stage sep (within 10 or 15 degrees of zero pitch?  Just guessing obviously)

Agree position of AP decide it, but unlike KSP and because orbital velocity is so much higher AP tend to be more towards the other side of earth, perhaps closer on Starhip than on an Atlas 5 as the second stage has plenty of trust and Starship stages pretty early, earlier than falcon 9 as I understand. 

[tater]

The vehicle was at ~1573 m/s at sep. Looks like hot stage was 5-6 seconds late vs the published timeline before launch, MECO slightly late as well, but the vehicle I assume is driving itself, so any number of issues could alter that timeline by a few seconds (winds aloft, whatever). We also don't know what the planned velocity for sep was—or if a suborbital test flight like this one has a different value for that than a future, operational mission.

All the calls were nominal, so I assume it was nominal.

[Ultimate Steve]

4 minutes ago, tater said:

The vehicle was at ~1573 m/s at sep. Looks like hot stage was 5-6 seconds late vs the published timeline before launch, MECO slightly late as well, but the vehicle I assume is driving itself, so any number of issues could alter that timeline by a few seconds (winds aloft, whatever). We also don't know what the planned velocity for sep was—or if a suborbital test flight like this one has a different value for that than a future, operational mission.

All the calls were nominal, so I assume it was nominal.

It is also possible that the published timeline accounted for some number of expected engine outs. I don't think this would be the case, but it is possible.

[tater]

22 minutes ago, Ultimate Steve said:

It is also possible that the published timeline accounted for some number of expected engine outs. I don't think this would be the case, but it is possible.

The sep occurred slightly late, not early.

[CBase]

I am still struggeling with the details Scott Manley found about starships end. Everything he concluded seems right, but why would after 5 minutes the LOX plumbing fail ? At the end of stage 2 burn the G forces are at maximum. For sure they expected this G forces, so some material failed due to the stress. However throttling starship to limit g force might be a quick fix. Some added fuel should be able to compensate gravitational loss. Although no permanent solution it might be enough to reach target trajectory until plumbing is improved.

And there were some talks about FAA rating this flight as mishap since both FTS were used. If this is true a december launch does not seem believable to me.

[Royalswissarmyknife]

23 minutes ago, CBase said:

And there were some talks about FAA rating this flight as mishap since both FTS were used. If this is true a december launch does not seem believable to me.

There wont be a December launch but I have heard they are aiming for January.

There will probably be a lot of testing in December though.

 

[tater]

1 hour ago, CBase said:

I am still struggeling with the details Scott Manley found about starships end. Everything he concluded seems right, but why would after 5 minutes the LOX plumbing fail ? At the end of stage 2 burn the G forces are at maximum. For sure they expected this G forces, so some material failed due to the stress. However throttling starship to limit g force might be a quick fix. Some added fuel should be able to compensate gravitational loss. Although no permanent solution it might be enough to reach target trajectory until plumbing is improved.

And there were some talks about FAA rating this flight as mishap since both FTS were used. If this is true a december launch does not seem believable to me.

Was FTS actually used?

Regarding Starship's demise, one plausible thought is that the enclosed engine bay resulted in overheating (hence the end of burn—when it had the maximum time to heat).

[Exoscientist]

6 hours ago, sevenperforce said:

In the video @tater attached above, the Starship+Superheavy stack clears its own height in under eight seconds. Very sporty coming off the pad. I don't see any indication that the engines were being downthrottled.

Remember that the engine bay leaks in OFT-1 were associated primarily with hydraulic fluid. Full replacement with ETVC means no hydraulic fluid to leak this time.

I'm not sure where that particular dude on Twitter is getting the notion that the booster should have dumped 3.2 km/s at staging. A nominal Falcon 9 mission with boostback, like Crew-7, stages at 1.72 km/s airspeed; that's only slightly higher than Starship+Superheavy which is itself a completely different vehicle with a different launch profile and different boostback characteristics (not to mention that this was a test).

We  may be able to estimate the proportion of full thrust is being produced by estimating propellant burn rate from the graphic displayed on screen during launch of how much propellant is remaining:



[Kerwood Floyd]

2 hours ago, tater said:

Was FTS actually used?

Yeah, to me, this is the big question. I really want to know, for both vehicles, but especially for SH, whether they were destroyed by FTS, or suffered some catastrophic failure before FTS had a chance to activate.

[Meecrob]

Maybe both vehicles were supposed to be FTS'd? It was the biggest failure of the first launch. Launchpad was a second, but the fact that they could not reliably control when the rocket terminated is the main problem. Lets be real. We all love the "Little John" test. Elon does too. We all saw him giddy as poop when Grasshopper was FTS'd. The Crew Dragon test also ended in an explosion. I think he blows up his extra stuff. We are all trying to figure out what went wrong, when maybe nothing went wrong. It was maybe a test of the FTS system. That could be why it "failed" so far into the burn. And why Elon looked so chilled when they showed shots of him in the live feed?

Edited November 21, 2023 by Meecrob

[tater]

1 hour ago, Meecrob said:

Maybe both vehicles were supposed to be FTS'd? It was the biggest failure of the first launch. Launchpad was a second, but the fact that they could not reliably control when the rocket terminated is the main problem. Lets be real. We all love the "Little John" test. Elon does too. We all saw him giddy as poop when Grasshopper was FTS'd. The Crew Dragon test also ended in an explosion. I think he blows up his extra stuff. We are all trying to figure out what went wrong, when maybe nothing went wrong. It was maybe a test of the FTS system. That could be why it "failed" so far into the burn. And why Elon looked so chilled when they showed shots of him in the live feed?

Wasn't an FTS test, and I am unsure FTS was used at all, honestly. The booster was a RUD (within danger area), and SS was within seconds of SECO—nominally within the corridor, so no reason for FTs at all.

[Meecrob]

3 hours ago, tater said:

Wasn't an FTS test, and I am unsure FTS was used at all, honestly. The booster was a RUD (within danger area), and SS was within seconds of SECO—nominally within the corridor, so no reason for FTs at all.

I get it, but think of "what is one test that they have the  hardware to toss at?" The FAA was correct in asking "How can you make sure Starship rain does not fall on civilians? We don't care about superheavy really...it'll hit the Atlantic...we want a 100% positive demonstration of your ability to toss Starship up, and show us you can bring it down safely if something messes up before orbit."

 

Edit: Wait, wait..the booster was totally an FTS...c'mon, it detonated too perfectly! Just because they did not declare it excplicitly as an FTS test means nothing. SpaceX is under no obligation to make sense to us. The booster had some problem I am still having fun trying to figure out, but they let it go a bit to gather data past when they knew it was done, as witnessed by the shutdown of engines on one "side." It was not aerodynamic forces that ripped it apart, it was an FTS detonation, then aerodynamic forces that took out the booster. It was more figuratively a "Challenger," as opposed to a " Columbia" to be crude.

Edited November 21, 2023 by Meecrob

[Deddly]

13 hours ago, Mikki said:

Flipping rotating uniform, narrow, straight shafts in a housing is not much of an issue, tangential forces proportional to the rotational energy (!) build up immediatly when fast rotating disc-like shapes are suddenly accelerated no matter the direction except  for the longitude axis of rotation of said shaft. And not along the new vector, but tangential, which is is the problem...

It's basically the same as gyroscopic procession, right? So my point was, why not position the turbo pump so that rotation of the ship is along that longitudinal axis? But that would require a special version of Raptor just for those engines that need to be fired up during hot staging. 

If they're ready to go in 3-4 weeks, we can probably assume this wasn't the issue, or that they have a few ideas for dealing with it. 

[Meecrob]

6 hours ago, Deddly said:

It's basically the same as gyroscopic procession, right? So my point was, why not position the turbo pump so that rotation of the ship is along that longitudinal axis? But that would require a special version of Raptor just for those engines that need to be fired up during hot staging. 

If they're ready to go in 3-4 weeks, we can probably assume this wasn't the issue, or that they have a few ideas for dealing with it. 

Everyone is trying to say that the ship itself is not traveling along the longitudinal axis, but a tangential one with regards to the thought that the turbopumps could be put on a certain axis. The engines are all mounted in different axes. The solution is not putting that amount of force on the pumps to begin with.

I suspect that if SpaceX was planning to FTS the booster, they pushed the limits to see what happens to the turbopumps. Those engines weren't coming back. Might as well gain data. This is "Test Flight"!! Everyone should remember that. They will maximize the amount of data they get back per vehicle/launch. They don't give two craps what the media thinks. They are playing the long game.

Others upthread commented on how the CofG is probably in the thrust puck somewhere, or even lower at that point in flight. Not many have tried to maneuver such an unbalanced vehicle before, aside from Starship...SN15 rings a bell to me. Give them a try or two.

Edited November 21, 2023 by Meecrob

[darthgently]

1 hour ago, Meecrob said:

The solution is not putting that amount of force on the pumps to begin with.

Yes.  Given it is still traveling down range, the center of mass is in the tail, and the grid fins are at the "nose" I think it would eventually weathervane/weathercock to retrograde.  But would likely take too long to be satisfactory and perhaps not necessarily do so stably enough.   But perhaps this could be built upon and augmented with hot gas-gas thrusters that I think @sevenpercentforce mentioned. 

Only 1/3 serious: what if the slotted hot stage ring were in six sections that could open outward like flower petals creating more windage at that end?  I already know it is too complicated, but would look dang cool popping open at stage sep

[Superluminal Gremlin]

Based of Wikipedia

OFT-1 Milestones Passed

7/21 Passed

3/21 Semi-Passed

OFT-2 Milestones

11/22 Passed (The new one is Water Deluge)

1/22 Partial

 

For OFT-3, There is probably a good chance that all systems will work. Their main problem seems to be fuel leaks, and after 3 OFT they will probably have all under control. After all, these flights are to gather data, the Milestones are just bonus points

[darthgently]

Interesting thread

 

 

[sevenperforce]

35 minutes ago, darthgently said:

Interesting thread

 

 

Ooooooh.

It would seem, perhaps, that six Raptors blasting Superheavy at close range produce more “negative thrust” than the prograde thrust of three Raptors running at the other end. To the point that the propellant would continue forward while Superheavy started to slow. 

[Deddly]

5 hours ago, Meecrob said:

if SpaceX was planning to FTS the booster, they pushed the limits to see what happens to the turbopumps

They did say on the stream that they were planning for a water landing