### Author Topic: Starship hot staging  (Read 344242 times)

#### Slarty1080

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##### Re: Starship hot staging
« Reply #1220 on: 12/07/2023 11:17 am »
Re Starship hot staging
Given the events of ITF2, the rapid turn around of Superheavy and it's subsequent destruction, I think it would be beneficial to do some calculations around the staging process.

For starters there has been a lot of speculation concerning over pressure in the interstage an it's effects. This should be calculable given a few specifics such as Raptor vac end nozzle diameter and interstage height. I might try pixel counting the interstage vent holes to calculate the total vent area (unless anyone has already done this) and then can compare the area of the nozzle exit v the area of the vent exit, and the separation between Starship an Superheavy required to balance the areas.
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#### eriblo

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##### Re: Starship hot staging
« Reply #1221 on: 12/07/2023 11:28 am »
Re Starship hot staging
Given the events of ITF2, the rapid turn around of Superheavy and it's subsequent destruction, I think it would be beneficial to do some calculations around the staging process.

For starters there has been a lot of speculation concerning over pressure in the interstage an it's effects. This should be calculable given a few specifics such as Raptor vac end nozzle diameter and interstage height. I might try pixel counting the interstage vent holes to calculate the total vent area (unless anyone has already done this) and then can compare the area of the nozzle exit v the area of the vent exit, and the separation between Starship an Superheavy required to balance the areas.
Since I know where to look
I was trying to avoid any complicated math and choked flow in non ideal gas rocket exhaust sounds like complicated math But it does sound like the kind of thing RPA might answer:

1 bar chamber pressure 3.6 OF ratio LOX/LCH4 through a 40 m2 throat: 2.4 t/s. This would be all Raptors at 58% throttle. Chamber Temperature estimated as 3013 K.

Scaling up/down gives about 1.7 bar/3080 K for all Raptors 100% and about 0.4 bar/2910 K at 3 Raptors at 50%
The above treats the Starship Raptors as injectors into the interstage main chamber. When just igniting the RVacs a large portion of the flow would likely be deflected out through the vents with a minimum loss in speed so the pressure will be lower.

#### OTV Booster

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##### Re: Starship hot staging
« Reply #1222 on: 12/11/2023 06:10 pm »
Re Starship hot staging
Given the events of ITF2, the rapid turn around of Superheavy and it's subsequent destruction, I think it would be beneficial to do some calculations around the staging process.

For starters there has been a lot of speculation concerning over pressure in the interstage an it's effects. This should be calculable given a few specifics such as Raptor vac end nozzle diameter and interstage height. I might try pixel counting the interstage vent holes to calculate the total vent area (unless anyone has already done this) and then can compare the area of the nozzle exit v the area of the vent exit, and the separation between Starship an Superheavy required to balance the areas.
Since I know where to look
I was trying to avoid any complicated math and choked flow in non ideal gas rocket exhaust sounds like complicated math But it does sound like the kind of thing RPA might answer:

1 bar chamber pressure 3.6 OF ratio LOX/LCH4 through a 40 m2 throat: 2.4 t/s. This would be all Raptors at 58% throttle. Chamber Temperature estimated as 3013 K.

Scaling up/down gives about 1.7 bar/3080 K for all Raptors 100% and about 0.4 bar/2910 K at 3 Raptors at 50%
The above treats the Starship Raptors as injectors into the interstage main chamber. When just igniting the RVacs a large portion of the flow would likely be deflected out through the vents with a minimum loss in speed so the pressure will be lower.
Can RPA accurately model a radically non steady state with supersonic flow? Two RapVs and a third chiming in. Three RapSL kicking in then vectoring. And a constantly changing distance between stages. All startup sequences starting with a an unignited non supersonic flow followed by ignition and supersonic flow. All this without any real world data points other than the expected engine performance.

Without any experience in this area I'd guess the error bars to be too big to be much use but I respect your knowledge and experience enough to take a reply at face value.

Now, after IFT2, the error bars should be smaller.
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#### BT52

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##### Re: Starship hot staging
« Reply #1223 on: 12/11/2023 07:23 pm »
I think this is exactly point of "hardware rich" development. Hot stating could be solved in many ways as i can see:

1. Shut 1 stage engines split second before separation (at least for first tests) and grab PENALTY in extra fuel because increased downrange distance. That could be problem if "hammer effect" damaged turbopumps. Then scrap this idea.

2. Abuse heat shield more and keep thrusting first stage harder in order to counteract "SS raptor KICK". Thats increased risk and weight of heatshield (thicker or usage of doublers on hot zones).

3. Engineer tanks and maybe redesign suction side (if needed) to be more ROUBUST, even when they get some counter KICK. For example use advantage of tipping side of booster and maybe shape baffles too work in PITCH direction. And engineer some damping solution into baffle system and plumbing too dampen hammer effect.

3 option is most efficient because u get best of both worlds. Booster pitch and separates fastest from SS. Hence  starts boostback very very early almost seconds after SS separation.

Other 1 and 2 are variation but just different way to do same thing. Arguably use combination of one those and develop more evolved, efficient version later.

#### Slarty1080

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##### Re: Starship hot staging
« Reply #1224 on: 12/11/2023 09:42 pm »
I think this is exactly point of "hardware rich" development. Hot stating could be solved in many ways as i can see:

1. Shut 1 stage engines split second before separation (at least for first tests) and grab PENALTY in extra fuel because increased downrange distance. That could be problem if "hammer effect" damaged turbopumps. Then scrap this idea.

2. Abuse heat shield more and keep thrusting first stage harder in order to counteract "SS raptor KICK". Thats increased risk and weight of heatshield (thicker or usage of doublers on hot zones).

3. Engineer tanks and maybe redesign suction side (if needed) to be more ROUBUST, even when they get some counter KICK. For example use advantage of tipping side of booster and maybe shape baffles too work in PITCH direction. And engineer some damping solution into baffle system and plumbing too dampen hammer effect.

3 option is most efficient because u get best of both worlds. Booster pitch and separates fastest from SS. Hence  starts boostback very very early almost seconds after SS separation.

Other 1 and 2 are variation but just different way to do same thing. Arguably use combination of one those and develop more evolved, efficient version later.
The problem with 1 (as far as I can see) is that it will cause ullage collapse. With the engines off the stage is no longer under compression and will expand a little projecting a spray of cryogenic liquid props into the hot autogenously pressurized head space causing loss of pressure and no appropriate means of repressurizing it.

Option 3 might have some merit, but I fear that any large movement turning the thing back to front in just a few seconds will be impossible to deal with using baffles or dampers and would also result in ullage collapse.

So I suggest option 2 followed by a relatively slow (12 seconds?) looping turn perhaps using baffles to limit slosh. Although I'm up to be persuaded otherwise.
My optimistic hope is that it will become cool to really think about things... rather than just doing reactive bullsh*t based on no knowledge (Brian Cox)

#### edzieba

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##### Re: Starship hot staging
« Reply #1225 on: 12/12/2023 12:43 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).

#### wannamoonbase

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##### Re: Starship hot staging
« Reply #1226 on: 12/12/2023 02:31 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).

Because IFT-3 is scheduled so aggressively, I believe you are correct, they are going to work on timing and engine settings.

Starship engine levels at hot staging, booster throttle down (not as deep) and waiting longer before starting the flip, keep it more controlled and maybe stay out of the SS wake hitting the side of SH like a giant sail.

Edit: I think they'll tinker with that and take their current design as far as they can with each successive IFT.  Eventually they will refine the design and timing to optimize fractions of seconds here and there.
« Last Edit: 12/12/2023 02:33 pm by wannamoonbase »
Wildly optimistic prediction, Superheavy recovery on IFT-4 or IFT-5

#### DanClemmensen

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##### Re: Starship hot staging
« Reply #1227 on: 12/12/2023 02:36 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
If the three center engines never faltered, then the "obvious" solution is to wait a little longer before trying to start more engines. What problems does this cause, other than loss of precious time and the resulting fuel needed to boost back further?

Do the additional engines need to restart quickly for thermal reasons?

#### steveleach

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##### Re: Starship hot staging
« Reply #1228 on: 12/12/2023 03:13 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
If the three center engines never faltered, then the "obvious" solution is to wait a little longer before trying to start more engines. What problems does this cause, other than loss of precious time and the resulting fuel needed to boost back further?

Do the additional engines need to restart quickly for thermal reasons?
Didn't one of the three fail shortly after staging?

#### daveglo

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##### Re: Starship hot staging
« Reply #1229 on: 12/12/2023 05:09 pm »

Didn't one of the three fail shortly after staging?

Your memory is correct.  Also noted that the engine status graphic on the webcast led the video a bit.  So you never see the center engine shutdown.

« Last Edit: 12/12/2023 05:11 pm by daveglo »

#### wannamoonbase

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##### Re: Starship hot staging
« Reply #1230 on: 12/12/2023 05:13 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
If the three center engines never faltered, then the "obvious" solution is to wait a little longer before trying to start more engines. What problems does this cause, other than loss of precious time and the resulting fuel needed to boost back further?

Do the additional engines need to restart quickly for thermal reasons?

Agreed, stay on the flight path and wait for SS to pull away, then begin the boostback flip.  Once flipped, and fuel settles then spark up the other 10 engines, drop the hammer and boostback like a crazy person.
Wildly optimistic prediction, Superheavy recovery on IFT-4 or IFT-5

#### Brigantine

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##### Re: Starship hot staging
« Reply #1231 on: 12/12/2023 08:04 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
Flip more gently how? It wasn't a controlled maneuver, it got (allegedly) rag dolled by "SS Raptor KICK"
(also initially by 2 RVacs so off-centre)

How to mitigate or counter those forces so that you have more control over the flip?

1. Quickly increase distance from "SS Raptor KICK" to mitigate forces

2. Increase SH control authority (i.e. thrust) to sufficiently counteract torque from "SS Raptor KICK"

3. Change design of vented interstage such that interaction with "SS Raptor KICK" results in lower net torque
(within existing control authority)

My preferred option is to use lower throttle on SS for longer, in conjunction with Option 2.
« Last Edit: 12/12/2023 08:12 pm by Brigantine »

#### rsnellenberger

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##### Re: Starship hot staging
« Reply #1232 on: 12/12/2023 08:09 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
If the three center engines never faltered, then the "obvious" solution is to wait a little longer before trying to start more engines. What problems does this cause, other than loss of precious time and the resulting fuel needed to boost back further?

Do the additional engines need to restart quickly for thermal reasons?

Agreed, stay on the flight path and wait for SS to pull away, then begin the boostback flip.  Once flipped, and fuel settles then spark up the other 10 engines, drop the hammer and boostback like a crazy person.
Starship is 4-8 times heavier than SH at separation due to fuel loads (assuming SH has 5-10% fuel remaining), and their thrust/weight ratios are nearly the same if the three center SH engines were throttled to 50% for separation. Having SH follow Starship "in trail" at separation seems a trifle risky for re-contact if there is any kind of Starship startup glitch (possibly due to the hot staging ring) upon startup.  I suspect that's one reason the IFT-2 booster performed that "get out of Dodge" flip — the booster surviving through the boostback burn being a nice-to-have, but Starship orbit(ish) flight having higher priority.

With the hot staging experience gained in IFT-2, the booster should still maneuver to the new flight path but more gradually to allow any separation transients to damp out before restarting the remaining engines for the boostback.

(aside: I seem to remember that 50% thrust level being mentioned in the run-ups to one of the flights, but has it been confirmed?)

#### wannamoonbase

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##### Re: Starship hot staging
« Reply #1233 on: 12/12/2023 08:16 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
If the three center engines never faltered, then the "obvious" solution is to wait a little longer before trying to start more engines. What problems does this cause, other than loss of precious time and the resulting fuel needed to boost back further?

Do the additional engines need to restart quickly for thermal reasons?

Agreed, stay on the flight path and wait for SS to pull away, then begin the boostback flip.  Once flipped, and fuel settles then spark up the other 10 engines, drop the hammer and boostback like a crazy person.
Starship is 4-8 times heavier than SH at separation due to fuel loads (assuming SH has 5-10% fuel remaining), and their thrust/weight ratios are nearly the same if the three center SH engines were throttled to 50% for separation. Having SH follow Starship "in trail" at separation seems a trifle risky for re-contact if there is any kind of Starship startup glitch (possibly due to the hot staging ring) upon startup.  I suspect that's one reason the IFT-2 booster performed that "get out of Dodge" flip — the booster surviving through the boostback burn being a nice-to-have, but Starship orbit(ish) flight having higher priority.

With the hot staging experience gained in IFT-2, the booster should still maneuver to the new flight path but more gradually to allow any separation transients to damp out before restarting the remaining engines for the boostback.

(aside: I seem to remember that 50% thrust level being mentioned in the run-ups to one of the flights, but has it been confirmed?)

Wikipedia says 40% throttle https://en.wikipedia.org/wiki/SpaceX_Raptor

I recall seeing a claim somewhere of it being even lower (20%), but that may have been forum talk.

Agreed, the mass and thrust of each vehicle makes the maneuver very challenging.  They will figure out how to thread the needle.

They could also drop another 1 or 2 of the center engines as well.
Wildly optimistic prediction, Superheavy recovery on IFT-4 or IFT-5

#### matthewkantar

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##### Re: Starship hot staging
« Reply #1234 on: 12/12/2023 08:27 pm »
Two things:
One, the three lit Raptors on the booster have more authority over the booster’s attitude than the Raptor impingement. SpaceX needs a few more tries to suss out what the booster should be doing with that authority.
Second, I think future hot staging gear will be a lot lighter. A slightly thicker top dome, some armor for he grid fin actuators, and a coat of SPAM should do it. The saved height can then be used to add a ring to the ship.

#### sdsds

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##### Re: Starship hot staging
« Reply #1235 on: 12/12/2023 08:31 pm »
Agreed, the mass and thrust of each vehicle makes the maneuver very challenging.  They will figure out how to thread the needle.

Concur. Carry the added risks of recontact and heat shield failure by following along the SS flight path longer. Take the hit to propellant spent doing so. Conduct the flip further from SS and more slowly. Take the hit to propellant spent on a longer boost-back burn.
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#### DanClemmensen

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##### Re: Starship hot staging
« Reply #1236 on: 12/12/2023 08:50 pm »
One, the three lit Raptors on the booster have more authority over the booster’s attitude than the Raptor impingement.
This may be true but it is not instantly obvious. Have you seen an analysis? Sure, the booster Raptors have high thrust and some gimballing, but the impingement is further away from the CoM and it acts on a lot of surface area including the grid fins.

#### matthewkantar

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##### Re: Starship hot staging
« Reply #1237 on: 12/12/2023 09:19 pm »
One, the three lit Raptors on the booster have more authority over the booster’s attitude than the Raptor impingement.
This may be true but it is not instantly obvious. Have you seen an analysis? Sure, the booster Raptors have high thrust and some gimballing, but the impingement is further away from the CoM and it acts on a lot of surface area including the grid fins.

I have not done an analysis, and I thought about the longer moment arm, but the gimbals are very sporty and they are coupled to the booster. Even throttled down with limited trust vectoring, I think the Raptors win. Or at least have the potential to win unthrottled.

#### eriblo

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##### Re: Starship hot staging
« Reply #1238 on: 12/12/2023 10:25 pm »
One, the three lit Raptors on the booster have more authority over the booster’s attitude than the Raptor impingement.
This may be true but it is not instantly obvious. Have you seen an analysis? Sure, the booster Raptors have high thrust and some gimballing, but the impingement is further away from the CoM and it acts on a lot of surface area including the grid fins.

I have not done an analysis, and I thought about the longer moment arm, but the gimbals are very sporty and they are coupled to the booster. Even throttled down with limited trust vectoring, I think the Raptors win. Or at least have the potential to win unthrottled.
The Raptors on SH obviously win for small deflections between SH and Starship while the impingement from Starship obviously win for large deflections. Where the cross over is and whether it occurs before the distance becomes large enough to compensate for the extra moment arm is the question.

#### OTV Booster

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##### Re: Starship hot staging
« Reply #1239 on: 12/13/2023 08:26 pm »
Option 4: do what IFT-2 did, but flip more gently.

Whatever actual-or-illusory 'negative acceleration' occurred during hot-staging, it neither disrupted operation of the three centre engines nor prevented re-ignition of 9 of the 10 inner ring engines, with the first failed ignition occurring well into the flip (past 45°, made it to around 140° before a second engine failed).
Flip more gently how? It wasn't a controlled maneuver, it got (allegedly) rag dolled by "SS Raptor KICK"
(also initially by 2 RVacs so off-centre)

How to mitigate or counter those forces so that you have more control over the flip?

1. Quickly increase distance from "SS Raptor KICK" to mitigate forces

2. Increase SH control authority (i.e. thrust) to sufficiently counteract torque from "SS Raptor KICK"

3. Change design of vented interstage such that interaction with "SS Raptor KICK" results in lower net torque
(within existing control authority)

My preferred option is to use lower throttle on SS for longer, in conjunction with Option 2.
Or 4. Immediately yaw SS slightly on separation to retarget the plume away from the booster. It already has the RapSL's splayed out. Center up one to induce yaw and accept/mitigate the beating it gives it ring/shield for a moment.
We are on the cusp of revolutionary access to space. One hallmark of a revolution is that there is a disjuncture through which projections do not work. The thread must be picked up anew and the tapestry of history woven with a fresh pattern.

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