Author Topic: Starship!  (Read 56916 times)

Offline cjameshuff

  • Mars
  • ***
  • Posts: 372
Re: Starship!
« Reply #135 on: February 02, 2021, 08:28:22 PM »
I think that the two flights are similar that they both lacked proper fuel to the engines.  I saw a definirte  green huele comino off the engine indicating it was consumming itself.

Are you sure you're looking at SN9?

https://youtu.be/_zZ7fIkpBgs?t=703

SN8's engines started up, but one showed clear, brilliant green in its exhaust. This was just a low-pressure jet of yellow flame...clearly subsonic, like maybe only the fuel turbopump was running. It could be that oxidizer pressure was low this time.

Some debris came out of the skirt, but the second engine continued to attempt to start and appeared to nearly succeed. I think it was just thermal blankets getting blown loose.

They previously did a series of static fires and engine swaps on SN9 that Musk described as something along the lines of "learning how to start Raptors". Looks like they're still tweaking that part of things.

Offline JayUtah

  • Neptune
  • ****
  • Posts: 3787
    • Clavius
Re: Starship!
« Reply #136 on: February 03, 2021, 04:01:59 PM »
Inflight restarts are hard.  Starship has several hurdles.  The Raptor is a pump-fed engine, and turbopumps need a sufficiently high, sufficiently stable head pressure.  Starship has highly exciting flight dynamics just when you need liquids not to slosh in various tanks and pipes.

Previously, inflight restarts were obviated by having a hypergolic upper stage and bladder-pressurized propellant tanks.  No pumps to cavitate, no igniters to fail.  But hypergolics bring their own engineering problems with them.  And the available chemistry limits power and efficiency.  So restartable pump-fed or pressure-fed engines, with chemical or electrical ignition, has to be part of the vocabulary.  As with Apollo, pump-fed engines can be restarted by an ullage burn to "settle" the tanks, and a spark plug that ignites the preburner using propellants fed to it only by inertia from the ullage.  This works best when the vehicle isn't otherwise maneuvering.  And sometimes it involves several stages of ignition (i.e., using small igniter torches) based on how much propellant flow is possible from the previous step.  And multi-stage ignition sequences generally have to be very precisely timed.

Now complicate the fluid slosh by having your vehicle do Olympic diving maneuvers.  Head pressure fluctuations, gas voids, pump cavitation -- so much can go wrong.  It looks to me like the turbopumps just weren't getting a bite on the propellants, for whatever ultimate reason.  When they say they're still working out the mechanics of Raptor restarts, I don't doubt it for a second.  What they're trying to do is very hard.  If I have some time later, I'll look into the Raptor restart sequence and the Starship propellant feed system in more detail.

The two debris sheds look like the insulation blankets around the upper landing struts.  The debris looks like it's coming from the perimeter of the skirt, not the center.  And it's not the struts or footpads themselves -- at least the first one -- because all six footpads are still in place after the first shed.  Plus, the ease with which the slipstream slows them down suggests lightweight film, not heavy structure.  Losing insulation in the last five seconds of the flight would be inconsequential.  It may be planned for in SpaceX's flight profile.  I would be interested to see the slipstream turbulence models for the interior of the skirt.
"Facts are stubborn things." --John Adams

Offline Peter B

  • Saturn
  • ****
  • Posts: 1268
Re: Starship!
« Reply #137 on: February 03, 2021, 06:07:17 PM »
Just a thought, but wouldn't it be easier to use the RCS to swing the vehicle upright, and then restart the engine? That way you have gravity on your side for draining propellant from the tanks.

Offline cjameshuff

  • Mars
  • ***
  • Posts: 372
Re: Starship!
« Reply #138 on: February 03, 2021, 06:41:38 PM »
Just a thought, but wouldn't it be easier to use the RCS to swing the vehicle upright, and then restart the engine? That way you have gravity on your side for draining propellant from the tanks.

Starship masses over a hundred metric tons. The current cold gas thrusters are nowhere near capable of doing that. They've talked about doing the flip using methox thrusters (the HLS landing thrusters might be a scaled-up relative of these), but if they can do it without dedicated thrusters, that's a whole thruster system they don't need...one capable of giving them comparable maneuverability to two gimbaled Raptors.

They're probably not that far from getting it working. SN8 had the engines light up and perform an essentially perfect flip, the problems came after that. SN9 had one engine not start, but there's many, many things that could cause that.

Offline cjameshuff

  • Mars
  • ***
  • Posts: 372
Re: Starship!
« Reply #139 on: February 03, 2021, 11:06:37 PM »
Inflight restarts are hard.  Starship has several hurdles.  The Raptor is a pump-fed engine, and turbopumps need a sufficiently high, sufficiently stable head pressure.  Starship has highly exciting flight dynamics just when you need liquids not to slosh in various tanks and pipes.

Even better, Raptor has two turbopumps to start, being a FFSC engine.


Now complicate the fluid slosh by having your vehicle do Olympic diving maneuvers.  Head pressure fluctuations, gas voids, pump cavitation -- so much can go wrong.  It looks to me like the turbopumps just weren't getting a bite on the propellants, for whatever ultimate reason.  When they say they're still working out the mechanics of Raptor restarts, I don't doubt it for a second.  What they're trying to do is very hard.  If I have some time later, I'll look into the Raptor restart sequence and the Starship propellant feed system in more detail.

They use smaller header tanks, one for LOX in the nose and one for LCH4 at the bulkhead between the LOX/LCH4 tanks. So they can basically have as little ullage space as they like. Too little ullage space might make pressure regulation difficult though.


The two debris sheds look like the insulation blankets around the upper landing struts.  The debris looks like it's coming from the perimeter of the skirt, not the center.  And it's not the struts or footpads themselves -- at least the first one -- because all six footpads are still in place after the first shed.  Plus, the ease with which the slipstream slows them down suggests lightweight film, not heavy structure.  Losing insulation in the last five seconds of the flight would be inconsequential.  It may be planned for in SpaceX's flight profile.  I would be interested to see the slipstream turbulence models for the interior of the skirt.

They also lost the skirt camera on the way up, so something might have come loose...but I'm thinking a cable harness or conduit, not part of the engine.

Offline Zakalwe

  • Uranus
  • ****
  • Posts: 1588
Re: Starship!
« Reply #140 on: February 04, 2021, 03:01:27 AM »
Perhaps the turbopumps don't like being rotated that fast? I would imagine that the gyroscopic precession effects on turbines spinning at the RPMs that turbopumps operate at would cause significant loads?
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov

Offline cjameshuff

  • Mars
  • ***
  • Posts: 372
Re: Starship!
« Reply #141 on: February 04, 2021, 08:15:42 AM »
Perhaps the turbopumps don't like being rotated that fast? I would imagine that the gyroscopic precession effects on turbines spinning at the RPMs that turbopumps operate at would cause significant loads?

The engines are what perform the rotation, SN9 wasn't rotating at the time the second engine failed to ignite.

Offline bknight

  • Neptune
  • ****
  • Posts: 3107
Re: Starship!
« Reply #142 on: February 04, 2021, 08:38:38 AM »
I think that the two flights are similar that they both lacked proper fuel to the engines.  I saw a definirte  green huele comino off the engine indicating it was consumming itself.

Are you sure you're looking at SN9?

https://youtu.be/_zZ7fIkpBgs?t=703

SN8's engines started up, but one showed clear, brilliant green in its exhaust. This was just a low-pressure jet of yellow flame...clearly subsonic, like maybe only the fuel turbopump was running. It could be that oxidizer pressure was low this time.

Some debris came out of the skirt, but the second engine continued to attempt to start and appeared to nearly succeed. I think it was just thermal blankets getting blown loose.

They previously did a series of static fires and engine swaps on SN9 that Musk described as something along the lines of "learning how to start Raptors". Looks like they're still tweaking that part of things.
Yes, I posted that after watching the launch.  After watching Scott Manley's slow mo video I correct my initial thoughts. I don't see any green engine consumption that occurred in SN-8.  It looked like the second engine just never ignited properly.  So they have to fix that.
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
Eugene Cernan

Offline JayUtah

  • Neptune
  • ****
  • Posts: 3787
    • Clavius
Re: Starship!
« Reply #143 on: February 07, 2021, 01:25:35 PM »
If I'm following social media correctly, it looks like the next idea is to light all three Raptors and shut down the least performant one, on the premise that this will improve the chances of getting two good engines for maneuvering and landing.  I'm interested to see whether it will work.
"Facts are stubborn things." --John Adams

Offline raven

  • Uranus
  • ****
  • Posts: 1637
Re: Starship!
« Reply #144 on: February 07, 2021, 06:26:16 PM »
Sounds like something of a bodge and a kludge, but I'd like to see them try all the same.

Offline Obviousman

  • Jupiter
  • ***
  • Posts: 735
Re: Starship!
« Reply #145 on: February 07, 2021, 06:31:00 PM »
If I'm following social media correctly, it looks like the next idea is to light all three Raptors and shut down the least performant one, on the premise that this will improve the chances of getting two good engines for maneuvering and landing.  I'm interested to see whether it will work.

I saw that was posted to Elon Musk, asking why they didn't do that. His reply?

"Because we were dumb"! LOL!

Offline smartcooky

  • Uranus
  • ****
  • Posts: 1959
Re: Starship!
« Reply #146 on: February 07, 2021, 06:47:13 PM »
Flying rockets is damned hard... how many times did NASA blow up rockets on the pad or just after launch in attempting to get the bloody things to fly? Landing rockets is double-damned hard.. how many times did SpaceX try to land a rocket before they got one to stick? Was it seven? Eight?

And landing Falcon 9 boosters comparatively easy - the Falcon booster comes in tail first at multi-mach speed, already lined up to land, and with a bit of tricky sideways translation and an accurately timed burn that starts about 8,000 m of altitude, they park it on a dime

What they are trying to do with Starship is at least a whole order magnitude harder - descend rapidly "skydiver" fashion, then restart, flip and land in quick succession, all at low altitude - just a few hundred feet.

I have no idea why they dont pull the flip manouver at a higher altitude and just run the landing burn longer
If you're not a scientist but you think you've destroyed the foundation of a vast scientific edifice with 10 minutes of Googling, you might want to consider the possibility that you're wrong.

Offline cjameshuff

  • Mars
  • ***
  • Posts: 372
Re: Starship!
« Reply #147 on: February 07, 2021, 07:33:24 PM »
Sounds like something of a bodge and a kludge, but I'd like to see them try all the same.

It's why they have three landing engines in the first place. They only want two for the actual landing and so they've only been starting two, but it sounds like having the redundancy implemented might have helped SN9. (Trying to start up a third engine when the fuel tank's pressure was already low would not have helped SN8.)

Offline cjameshuff

  • Mars
  • ***
  • Posts: 372
Re: Starship!
« Reply #148 on: February 07, 2021, 07:47:54 PM »
I have no idea why they dont pull the flip manouver at a higher altitude and just run the landing burn longer

It'd take a lot more propellant, and without two good engines to keep it straight, the vehicle would likely tumble out of control...it's not supposed to fly backwards at any significant airspeed. On SN8 the one landing engine was losing thrust near the end and on SN9 it couldn't even complete the flip without two engines...altitude would not have helped either.

Offline jfb

  • Mars
  • ***
  • Posts: 396
Re: Starship!
« Reply #149 on: February 09, 2021, 11:12:51 AM »
I have no idea why they dont pull the flip manouver at a higher altitude and just run the landing burn longer

The header tanks are only so big - I don't know what kind of margin they have right now, but I don't think they can start appreciably higher without increasing their size.

Which may ultimately be the answer - these are prototypes, they're meant to test out ideas under real-world conditions, and it may be their modeling was just plain wrong and they need to rethink the tankage or the plumbing or the engines.  They may use larger headers on 16/17/18/etc.  They may find out the current Raptor design just isn't robust enough to be swung around that violently and has to be tweaked. 

I would not expect a successful landing before SN15, and I would not be surprised if they had at least one kaboom after demonstrating a successful landing.  They are going to burn a lot of hardware before they get this figured out.