totm nov 2023 SpaceX Discussion Thread

[sevenperforce]

2 hours ago, Exoscientist said:

The comparison has been made of the SpaceX SuperHeavy/Starship approach to the Soviet multiple failed N-1 rocket in that they both wanted to test by actually flying the full rocket until it works. 

Do you have evidence that this was, in fact, the Soviet approach during the N-1 launch program? The N-1 program was underfunded, rushed, and certainly not hardware-rich. The first two and final launches were all sent up with functioning spacecraft intended for either lunar flyby or orbit. Only with the third launch did they use dummy payloads, likely out of desperation.

Just because the Soviets repeatedly spammed N-1s at the sky without success doesn't mean that they were intentionally using these launches as their initial non-operational test campaign. If those launches had succeeded, we wouldn't have characterized it as a launch vehicle test, but as a successful mission with an actual payload. In contrast, SpaceX's launch (and future planned launches) of Starship in its test campaign have been explicitly just that: tests, with no possibility of an actual orbital payload delivery.

2 hours ago, Exoscientist said:

here’s the major failing of the Raptor: even if the engine is tested successfully there is still a quite high chance the engine will still fail when used on a flight.

The sample size of 1 single flight (for Superheavy, which is the configuration we are talking about) isn't enough to reach any positive conclusions about "a quite high chance" of anything.

2 hours ago, Exoscientist said:

 Because of the numerous failures of the Raptor both on the test stand...

Do you have inside information identifying when Raptor has suffered test stand RUDs, as opposed to rapid scheduled disassemblies in intentionally destructive tests to failure? 

2 hours ago, Exoscientist said:

...in short test hops of the Starship landing methods...

Different engine, different vehicle, different configuration, different flight envelope. 

2 hours ago, Exoscientist said:

I estimated the chance of engine failures of the SuperHeavy/Starship test flight  was 1 out of 3.

Where?

2 hours ago, Exoscientist said:

The result? Only 1 in 4 of the engines failed.

You're moving the goalposts here. Above, you were talking about in-flight failures, but now you are including startup failures, which can be commanded by the vehicle due to sensor issues when the engine is just fine. We also have a brand new environment including the possibility of debris impact from the concrete pad, which is not an engine reliability issue either.

37 minutes ago, Kartoffelkuchen said:

perhaps the test feels a bit rushed, and if they had spent a couple more months proving their engines work halfway reliable it would have greatly increased the chance of a successful first test flight.

Well, it was a successful first test flight. It cleared the tower; that was the primary objective.

And more to the point, we have no specific evidence that engine reliability was a contributing factor to the outcome of OFT-1. There's circumstantial evidence to suggest it might have been engine reliability, but it also might have been overly-sensitive sensors, debris impacts, or issues with other parts of the vehicle. Presuming to know it is one thing or the other is a little silly.

[Exoscientist]

14 hours ago, darthgently said:

Maybe from vibrations, from deluge flow and thrust.  Maybe laying the Fondag in hex tile pattern would alleviate.  Either way, Fondag is surface only and quite patchable

Some deeper thoughts...

 

 

 Nice video by CSI Starbase. It corrects a mistaken impression of mine that there was a large hole beneath the area where the water was shooting up from. Actually there are only small holes like an upside down shower head. But starting at about the 6:50 point the video begins discussing that the water pressure is intended to be above the force of the exhaust coming to a halt impinging on the metal plate, where the water jets emanate. SpaceX wants this so that the exhaust gases don’t force their way down into the piping beneath the plate.

 But this raises the possibility this excess pressure could reach the engines also.

 

   Robert Clark

[darthgently]

1 hour ago, Exoscientist said:

But this raises the possibility this excess pressure could reach the engines also.

Not really as the holes are angled at 30 degrees from level outward as noted in the video narration and visibly obvious in the videos of the deluge system in operation for some time

[darthgently]

Dammit Jim, no powered landing!  Still cool

 

 

[Kerb24]

4 hours ago, darthgently said:

Dammit Jim, no powered landing!  Still cool

Someone actually did it.

[darthgently]

26 minutes ago, Kerb24 said:

Someone actually did it.

Thanks for posting, I watched a vid of his years back when he was sorting out the "pincher" engine throttling then lost track.  Great to see he persisted and won. I look forward to what he does next

Edited August 15, 2023 by darthgently

[sevenperforce]

17 hours ago, Exoscientist said:

But starting at about the 6:50 point the video begins discussing that the water pressure is intended to be above the force of the exhaust coming to a halt impinging on the metal plate, where the water jets emanate. SpaceX wants this so that the exhaust gases don’t force their way down into the piping beneath the plate.

 But this raises the possibility this excess pressure could reach the engines also.

The engine exhaust gases are in collimated, laminar flow; the water jets are not in laminar flow. Thus the pressure drop is exponential. 

[sevenperforce]

57 minutes ago, Kerb24 said:

5 hours ago, darthgently said:

Dammit Jim, no powered landing!  Still cool

Someone actually did it.

I had an idea for a much simpler self-landing rocket a while back.

The idea was to do a two-stage solid-fueled rocket, but use a cluster of motors on the first stage. Using clustered motors can be dicey with a hobby rocket because of inconsistent startup transients, but it can be remedied as long as you have a sufficiently large central motor to provide the bulk of liftoff thrust. Basically you'd use a short, stubby, high-powered central motor ringed with four additional smaller, longer motors to act as sustainers, each angled slightly to point through the center of mass and thus reduce the impact of off-axis thrust during startup and shutdown transients. The second stage would pop off and fire normally.

At separation, the first stage would have "grid fins" that pop out, releasing long transparent streamers to provide drag and keep the vehicle oriented tail-first.

The landing would be achieved using a CO2 cartridge leading to a pair of cold-gas nozzles tucked in with the sustainers. A cold-gas blow-down thruster has the advantage of starting with high thrust that drops off quickly, which is just what you need. The idea would be for the grid fins and streamers to provide pointing and give it a reasonable terminal velocity, then trigger the CO2 burst disc at about 30 feet above the ground. The blow-down would kill maybe 80% of terminal velocity, at which point the shock-absorbing landing legs would handle the rest. Any remaining pressure in the CO2 cartridge would be lost quickly enough that the residual T/W ratio would be less than 1. Cold gas also has the important advantage of not starting gas fires, which is helpful on a test range.

[Exoscientist]

18 hours ago, darthgently said:

Not really as the holes are angled at 30 degrees from level outward as noted in the video narration and visibly obvious in the videos of the deluge system in operation for some time

 

 This @Erdayastronaut clip shows quite alot of water reaching the level of the engines even though the water is angled outwards at the base. Then depending on the pressure of the water at this height, the exhaust flow impinging on it can cause reverberating pressure waves back on the engines.

 

 In the 5+ second Booster 7 static fire in February, only two engines failed. In the test flight in April with a longer time on the pad to liftoff with a veritable concrete tornado throwing up chunck’s of concrete, only 3 engines failed in the initial liftoff. In this latest test, 4 engines failed after only 2.7 seconds.
Cause:  the water deluge.

  Robert Clark

Edited August 15, 2023 by Exoscientist

[darthgently]

39 minutes ago, Exoscientist said:

This @Erdayastronaut clip shows quite alot of water reaching the level of the engines even though the water is angled outwards at the base. Then depending on the pressure of the water at this height, the exhaust flow impinging on it can cause reverberating pressure waves back on the engines.

But by the time it reaches the height of the engines it is outside the ring of outer engines.  As designed I presume.  Add to that the countering thrust of the engines, as clearly seen in the fire test video, and the massive conversion to steam, as clearly seen in the fire test video, it would be seriously difficult for a jet of water to reach, much less damage, an engine.  But that is just my take.

Chaotic pressure waves of thrust and expanding steam may be a factor, but I'm doubting it

Maybe if there are really strong pressure resonances it might interfere with spin starting?  I just don't see it causing actual damage to the engines that are the designed sources nearly all the energy involved

[StrandedonEarth]

5 hours ago, Exoscientist said:

In this latest test, 4 engines failed after only 2.7 seconds.

The engines did not exactly fail. They were prematurely shut down, presumably for out-of-parameter sensor readings. It’s quite possible those engines could still fly after relatively minor repairs (if any).

Edit: after further thought, if they all shutdown at the same moment, it's possible it was more of a plumbing issue, trying to feed so many engines. This is why they test. The ultimate test is flight, and in a hardware rich environment flying can be less expensive and provide more useful data than building a full-duration test stand (which would need to be stupidly robust to withstand the fury of a full-duration SH static fire). 

Edited August 16, 2023 by StrandedonEarth

[sevenperforce]

5 hours ago, Exoscientist said:

 This @Erdayastronaut clip shows quite alot of water reaching the level of the engines even though the water is angled outwards at the base.

Water, like everything else, travels in a parabolic arc. So if it is angled outward at the base, it will be traveling at a shallower angle once it reaches the level of the engines, meaning that it will miss the engines.

5 hours ago, Exoscientist said:

Then depending on the pressure of the water at this height, the exhaust flow impinging on it can cause reverberating pressure waves back on the engines.

Exhaust flow is supersonic, so pressure waves can't travel backwards.

5 hours ago, Exoscientist said:

 In the 5+ second Booster 7 static fire in February, only two engines failed. In the test flight in April with a longer time on the pad to liftoff with a veritable concrete tornado throwing up chunck’s of concrete, only 3 engines failed in the initial liftoff. In this latest test, 4 engines failed after only 2.7 seconds.
Cause:  the water deluge.

Purely speculative and highly implausible.

Also, a commanded shutdown is not the same as a failure. CRS-1 had an engine failure on ascent. A commanded shutdown during a static fire doesn't tell us anything.

[darthgently]

31 minutes ago, sevenperforce said:

Exhaust flow is supersonic, so pressure waves can't travel backwards

Excellent point

[Exoscientist]

1 hour ago, darthgently said:

Excellent point

 When impinging on another object liquid, gas, or solid they can travel in any direction, like a ball headed backwards after hitting a wall. Explosion shockwaves for example can bounce off of atmosphere layers and bounce back to Earth known as atmospheric focusing:

Atmospheric focusing is a type of wave interaction causing shock waves to affect areas at a greater distance than otherwise expected. Variations in the atmosphere create distortions in the wavefront by refracting a segment, allowing it to converge at certain points and constructively interfere. In the case of destructive shock waves, this may result in areas of damage far beyond the theoretical extent of its blast effect. Examples of this are seen during supersonic booms, large extraterrestrial impacts from objects like meteors, and nuclear explosions.
https://en.wikipedia.org/wiki/Atmospheric_focusing

 Robert Clark

[magnemoe]

6 hours ago, StrandedonEarth said:

The engines did not exactly fail. They were prematurely shut down, presumably for out-of-parameter sensor readings. It’s quite possible those engines could still fly after relatively minor repairs (if any).

Edit: after further thought, if they all shutdown at the same moment, it's possible it was more of a plumbing issue, trying to feed so many engines. This is why they test. The ultimate test is flight, and in a hardware rich environment flying can be less expensive and provide more useful data than building a full-duration test stand (which would need to be stupidly robust to withstand the fury of a full-duration SH static fire). 

Even if not shut down at the same time it could still be plumbing issues or more accurate pressure drop into the methane turbopump as the LOX is directly from the tank. 
And as you say it could just be an sensor, engine could run fine with an head pressure of 4 bar but engine is cut if below 5. 

[darthgently]

5 hours ago, Exoscientist said:

 When impinging on another object liquid, gas, or solid they can travel in any direction, like a ball headed backwards after hitting a wall. Explosion shockwaves for example can bounce off of atmosphere layers and bounce back to Earth known as atmospheric focusing:

Atmospheric focusing is a type of wave interaction causing shock waves to affect areas at a greater distance than otherwise expected. Variations in the atmosphere create distortions in the wavefront by refracting a segment, allowing it to converge at certain points and constructively interfere. In the case of destructive shock waves, this may result in areas of damage far beyond the theoretical extent of its blast effect. Examples of this are seen during supersonic booms, large extraterrestrial impacts from objects like meteors, and nuclear explosions.
https://en.wikipedia.org/wiki/Atmospheric_focusing

 Robert Clark

Ok, but as excellently pointed out by sevenpercentforce they can't travel backwards up the plume

[sevenperforce]

8 hours ago, Exoscientist said:

10 hours ago, sevenperforce said:

16 hours ago, Exoscientist said:

Then depending on the pressure of the water at this height, the exhaust flow impinging on it can cause reverberating pressure waves back on the engines.

Exhaust flow is supersonic, so pressure waves can't travel backwards.

Explosion shockwaves for example can bounce off of atmosphere layers and bounce back to Earth known as atmospheric focusing:

Atmospheric focusing is a type of wave interaction causing shock waves to affect areas at a greater distance than otherwise expected. Variations in the atmosphere create distortions in the wavefront by refracting a segment, allowing it to converge at certain points and constructively interfere. In the case of destructive shock waves, this may result in areas of damage far beyond the theoretical extent of its blast effect. Examples of this are seen during supersonic booms, large extraterrestrial impacts from objects like meteors, and nuclear explosions.

Atmospheric focusing can take place when a shockwave interacts with a discrete atmospheric boundary later, similar to the way that light will be partially refracted and partially reflected at the boundary layer between water and air or between glass and water. This requires, however, that the boundary layer exist in the same medium through which the wave is propagating. Shock waves are pressure waves that travel through the air, and since the air is the thing that has the boundary layer, the boundary layer can refract the shock wave.

The exhaust flow from the business end of a Raptor engine is not a pressure wave in a static medium; it's a supersonic flow of exhaust. Supersonic flows are not shock waves, and the mass of chaotically-moving water spray is not a propagation medium. 

Even if this was an apt analogy, which it isn't, a pressure wave experiencing atmospheric focusing is still going to be traveling away from the source, not back toward it.

8 hours ago, Exoscientist said:

When impinging on another object liquid, gas, or solid they can travel in any direction, like a ball headed backwards after hitting a wall.

When a near-laminar supersonic flow impinges on a surface, it doesn't bounce. Rather, the collimation is destroyed and the flow experiences a transition normal to the original direction of travel, carrying the energy away in the normal plane.

Sound pressure is an issue, of course, but the water significantly damps the sound pressure. There's no reflection off the water spray.

[Exoscientist]

4 hours ago, darthgently said:

Ok, but as excellently pointed out by sevenpercentforce they can't travel backwards up the plume

Sorry, but this is incorrect. Shockwaves or any kind of wave can be reflected any direction.

This is a key point. Take a look at this video by Everyday Astronaut:

 

 The reverberating shockwaves visible at the 1:57 point in the @Erdayastronaut video suggest the SpaceX approach to water deluge can damage the engines. SpaceX should stop dismissing the lessons of Apollo and learn from them. Use a flame trench.

 

  Robert Clark

Edited August 16, 2023 by Exoscientist

[sevenperforce]

17 minutes ago, Exoscientist said:

4 hours ago, darthgently said:

Ok, but as excellently pointed out by sevenpercentforce they can't travel backwards up the plume

Sorry, but this is incorrect. Shockwaves or any kind of wave can be reflected any direction.

Shockwaves cannot travel upstream in a supersonic flow. If they could, then supersonic jets would be destroyed by shockwave reflections.

But more to the point, it's now become unclear what you're talking about here, exactly. Are you saying that the exhaust plumes could be reflected off of the water deluge and impinge on the booster? Or are you saying that the sonic shockwaves produced by engine sound pressure could be reflected off of the water deluge and cause more harm to the booster than sonic reflection off bare concrete?

If the former, the answer is simply no. Exhaust plumes don't bounce.

If the latter, what you're suggesting is at least physically possible, but since the water is not in a steady-state flow that is both planar and laminar, it's not possible here.

17 minutes ago, Exoscientist said:

This is a key point. Take a look at this video by Everyday Astronaut:

<snip>

 The reverberating shockwaves visible at the 1:57 point in the @Erdayastronaut video suggest the SpaceX approach to water deluge can damage the engines.

The fact that rocket engines inevitably produce reverberating sound waves -- something we have known for a long long time -- does not lead to the unsupported conclusion that this specific water deluge system would cause damage to the engines. There is neither a mechanism of action for such damage nor evidence that such a mechanism is in play.

17 minutes ago, Exoscientist said:

SpaceX should stop dismissing the lessons of Apollo and learn from them. Use a flame trench.

Which Apollo test taught the "lesson" of using a flame trench? The Apollo launch site was built from the ground up with a flame trench. So you're not really advocating that we learn from "the lessons of Apollo"; you're just saying "they should do it like Apollo did it" without any reference to lessons.

The pads that launched the Saturn V never had a sound suppression deluge at all; their meager water spray was for cooling and fire prevention. A sound suppression deluge was not added until after the first few Shuttle flights, to reduce sonic shockwave tile damage.

And SpaceX does have a flame trench. It has six of them, in fact. The Saturn V's flame trench was 13 meters deep; the Orbital Launch Platform for Superheavy is 25 meters deep. 

[Terwin]

37 minutes ago, Exoscientist said:

Use a flame trench.

You do realize that the height of that boca chica launch pad is the same as greater than the depth of the flame trenches at the NASA launch pads, correct?  Now that there is a deluge system, the only real difference is the flame diverter, which may or may not be counter-productive as it would need to interfere with the exhaust closer to the rocket than the current setup 

Edited August 16, 2023 by Terwin
Fixed depth comparison

[magnemoe]

1 hour ago, Terwin said:

You do realize that the height of that boca chica launch pad is the same as greater than the depth of the flame trenches at the NASA launch pads, correct?  Now that there is a deluge system, the only real difference is the flame diverter, which may or may not be counter-productive as it would need to interfere with the exhaust closer to the rocket than the current setup 

This, the flame trench is set into an man made hill who is massive so the crawler can climb it with an rocket, the pad itself and the service tower. 
Here I say SpacX has an more elegant system, simply lift the rocket up on the pad. 
Granted the KSC system is more flexible, used by saturn 5 and 1b, the shuttle and falcon heavy but later don't use the crawlers. 

And I don't see the issue with the water coming from below as long as it don't spray into the engines. If the engines are running you are not getting anything into them anyway. 
Only weakness might be that you get to little cooling at the center of the pad, This could be solved by having an spiral of tilted holes there, yes this will hit the other spray but all this would be defeated by the engines anyway. 

[RCgothic]

The engine plumes won't directly impinge on the centre of the spray pattern.

The water does get to full height before engine ignition, but if it isn't a problem before ignition it won't be after a supersonic flow is acting like a repulsor.

[darthgently]

Interesting comments on this post:

 

[RCgothic]

1 hour ago, darthgently said:

Interesting comments on this post:

 

Care to enlighten those of us without X?

[darthgently]

58 minutes ago, RCgothic said:

Care to enlighten those of us without X?

There are a lot of varied comments.  In a nutshell no engine swaps have occurred nor appear to be necessary, apparently testing of stage 0 engine start GSE, monitoring, etc.

You realize you don't have to install X app to look at the posts in a browser, right?