Author Topic: IFT-2 Pacific Recovery  (Read 8884 times)

Offline edzieba

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Re: IFT-2 Pacific Recovery
« Reply #20 on: 11/08/2023 01:03 pm »
Worst case, the engines light, starship goes vertical but then the engines somehow lose the ability to kick it back to horizontal.
No, worst case is engines ignite, Starship remains horizontal under positive thrust (IIP starts moving and Starship accelerates towards the edge of its cleared airspace), and the FTS system does not fire or is ineffective resulting in Starship heading towards Kauai effectively uncontrolled.
"Vehicle out of control under thrust with FTS ineffective" is sadly a demonstrably possible outcome, so precluding it by not even attempting engine relight is prudent. Once it has been demonstrated that Starship can remain within a designated corridor for entry and descent, and SpaceX can recover telemetry for the vehicle and confirm no avionics issues as a result of entry (thermal damage to engine bay avionics has killed more than one vehicle, so whether by local fires or by radiant heat from entry plasma this is a non-zero possibility), then it is reasonable to start experimenting with the terminal EDL stages after entry.
It is easy to make sure that the vehicle does not have enough propellant to get out of the exclusion zone before attempting a (simulated or real) landing burn.
Only by either shaving your propellant margins to near zero, or greatly expanding the exclusion zone (and thus moving its target centre out of the range of the tracking assets you are trying to drop it next to). Remember that prior to entry, one of the passive safety measures is to inject into the desired suborbital trajectory and then safe the vehicle, with no deorbit burn. That means if you aim for an absolute-minimum prop load for the flip & burn, you must accomplish that through propellant management during ascent, whilst still hitting the desired suborbital trajectory (not running short or running long). If you instead remove the propellant mass at entry constraint, you can more easily target the desired suborbital trajectory with generous propellant margins for any engine-outs during ascent.

Offline eriblo

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Re: IFT-2 Pacific Recovery
« Reply #21 on: 11/08/2023 01:23 pm »
Worst case, the engines light, starship goes vertical but then the engines somehow lose the ability to kick it back to horizontal.
No, worst case is engines ignite, Starship remains horizontal under positive thrust (IIP starts moving and Starship accelerates towards the edge of its cleared airspace), and the FTS system does not fire or is ineffective resulting in Starship heading towards Kauai effectively uncontrolled.
"Vehicle out of control under thrust with FTS ineffective" is sadly a demonstrably possible outcome, so precluding it by not even attempting engine relight is prudent. Once it has been demonstrated that Starship can remain within a designated corridor for entry and descent, and SpaceX can recover telemetry for the vehicle and confirm no avionics issues as a result of entry (thermal damage to engine bay avionics has killed more than one vehicle, so whether by local fires or by radiant heat from entry plasma this is a non-zero possibility), then it is reasonable to start experimenting with the terminal EDL stages after entry.
It is easy to make sure that the vehicle does not have enough propellant to get out of the exclusion zone before attempting a (simulated or real) landing burn.
Only by either shaving your propellant margins to near zero, or greatly expanding the exclusion zone (and thus moving its target centre out of the range of the tracking assets you are trying to drop it next to). Remember that prior to entry, one of the passive safety measures is to inject into the desired suborbital trajectory and then safe the vehicle, with no deorbit burn. That means if you aim for an absolute-minimum prop load for the flip & burn, you must accomplish that through propellant management during ascent, whilst still hitting the desired suborbital trajectory (not running short or running long). If you instead remove the propellant mass at entry constraint, you can more easily target the desired suborbital trajectory with generous propellant margins for any engine-outs during ascent.
I am not sure I get what you are saying. The dv to get out of the exclusion zone is higher than that required for landing unless you start the simulated landing burn much higher or closer to the edge than what you would plan for. If the ascent is nominal there sould be plenty of excess propellant (no payload) but we already know they will be venting it during the coast phase...

Offline Slothman

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Re: IFT-2 Pacific Recovery
« Reply #22 on: 11/08/2023 07:11 pm »
Worst case, the engines light, starship goes vertical but then the engines somehow lose the ability to kick it back to horizontal.
No, worst case is engines ignite, Starship remains horizontal under positive thrust (IIP starts moving and Starship accelerates towards the edge of its cleared airspace), and the FTS system does not fire or is ineffective resulting in Starship heading towards Kauai effectively uncontrolled.
"Vehicle out of control under thrust with FTS ineffective" is sadly a demonstrably possible outcome, so precluding it by not even attempting engine relight is prudent. Once it has been demonstrated that Starship can remain within a designated corridor for entry and descent, and SpaceX can recover telemetry for the vehicle and confirm no avionics issues as a result of entry (thermal damage to engine bay avionics has killed more than one vehicle, so whether by local fires or by radiant heat from entry plasma this is a non-zero possibility), then it is reasonable to start experimenting with the terminal EDL stages after entry.

So.. rotate the potential horizontal thrust vector... Away from hawaii? There really is not much to hit anywhere close to the designated splashdown zone, especially north of it as far as I know.

Offline edzieba

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Re: IFT-2 Pacific Recovery
« Reply #23 on: 11/08/2023 07:45 pm »
Worst case, the engines light, starship goes vertical but then the engines somehow lose the ability to kick it back to horizontal.
No, worst case is engines ignite, Starship remains horizontal under positive thrust (IIP starts moving and Starship accelerates towards the edge of its cleared airspace), and the FTS system does not fire or is ineffective resulting in Starship heading towards Kauai effectively uncontrolled.
"Vehicle out of control under thrust with FTS ineffective" is sadly a demonstrably possible outcome, so precluding it by not even attempting engine relight is prudent. Once it has been demonstrated that Starship can remain within a designated corridor for entry and descent, and SpaceX can recover telemetry for the vehicle and confirm no avionics issues as a result of entry (thermal damage to engine bay avionics has killed more than one vehicle, so whether by local fires or by radiant heat from entry plasma this is a non-zero possibility), then it is reasonable to start experimenting with the terminal EDL stages after entry.

So.. rotate the potential horizontal thrust vector... Away from hawaii? There really is not much to hit anywhere close to the designated splashdown zone, especially north of it as far as I know.
If your proposed solution for a "vehicle is not under control" scenario requires the vehicle to be under control, that doesn't really work.

Offline aperh1988

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Re: IFT-2 Pacific Recovery
« Reply #24 on: 11/08/2023 08:03 pm »
Worst case, the engines light, starship goes vertical but then the engines somehow lose the ability to kick it back to horizontal.
No, worst case is engines ignite, Starship remains horizontal under positive thrust (IIP starts moving and Starship accelerates towards the edge of its cleared airspace), and the FTS system does not fire or is ineffective resulting in Starship heading towards Kauai effectively uncontrolled.
"Vehicle out of control under thrust with FTS ineffective" is sadly a demonstrably possible outcome, so precluding it by not even attempting engine relight is prudent. Once it has been demonstrated that Starship can remain within a designated corridor for entry and descent, and SpaceX can recover telemetry for the vehicle and confirm no avionics issues as a result of entry (thermal damage to engine bay avionics has killed more than one vehicle, so whether by local fires or by radiant heat from entry plasma this is a non-zero possibility), then it is reasonable to start experimenting with the terminal EDL stages after entry.

So.. rotate the potential horizontal thrust vector... Away from hawaii? There really is not much to hit anywhere close to the designated splashdown zone, especially north of it as far as I know.
If your proposed solution for a "vehicle is not under control" scenario requires the vehicle to be under control, that doesn't really work.

I think what Slothman is saying is while the vehicle is still under control (because attempting to go vertical requires vehicle to be under control) the rotation to vertical should be attempted towards a vector pointing away from the island such that if a control failure develops the vehicle is already pointing away.

That being said, it wouldn’t mitigate the risk of the vehicle pointing towards the island due to some aerodynamic buffeting or whatever once control is lost. Or other scenarios such as control is lost with thrust positive before attempt to go vertical and vehicle ends up pointing towards land because *waves hands* reasons.
« Last Edit: 11/08/2023 08:13 pm by aperh1988 »

Online tgr9898

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Re: IFT-2 Pacific Recovery
« Reply #25 on: 11/09/2023 01:36 am »
Worst case, the engines light, starship goes vertical but then the engines somehow lose the ability to kick it back to horizontal.
No, worst case is engines ignite, Starship remains horizontal under positive thrust (IIP starts moving and Starship accelerates towards the edge of its cleared airspace), and the FTS system does not fire or is ineffective resulting in Starship heading towards Kauai effectively uncontrolled.
"Vehicle out of control under thrust with FTS ineffective" is sadly a demonstrably possible outcome, so precluding it by not even attempting engine relight is prudent. Once it has been demonstrated that Starship can remain within a designated corridor for entry and descent, and SpaceX can recover telemetry for the vehicle and confirm no avionics issues as a result of entry (thermal damage to engine bay avionics has killed more than one vehicle, so whether by local fires or by radiant heat from entry plasma this is a non-zero possibility), then it is reasonable to start experimenting with the terminal EDL stages after entry.

So.. rotate the potential horizontal thrust vector... Away from hawaii? There really is not much to hit anywhere close to the designated splashdown zone, especially north of it as far as I know.
If your proposed solution for a "vehicle is not under control" scenario requires the vehicle to be under control, that doesn't really work.

I think what Slothman is saying is while the vehicle is still under control (because attempting to go vertical requires vehicle to be under control) the rotation to vertical should be attempted towards a vector pointing away from the island such that if a control failure develops the vehicle is already pointing away.

That being said, it wouldn’t mitigate the risk of the vehicle pointing towards the island due to some aerodynamic buffeting or whatever once control is lost. Or other scenarios such as control is lost with thrust positive before attempt to go vertical and vehicle ends up pointing towards land because *waves hands* reasons.

In a hardware-rich environment like the Starship development environment, I think they're more inclined to focus on the hot staging test, separation, and the Starship startup.  If they can achieve all that, the best bet is to leave Sir Issac Newton in the driver's seat for the rest of the flight - he's an excellent driver, BTW - and prep for the next test campaign since newer hardware is sitting at Boca Chica

Offline CorvusCorax

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Re: IFT-2 Pacific Recovery
« Reply #26 on: 11/10/2023 07:45 am »

In a hardware-rich environment like the Starship development environment, I think they're more inclined to focus on the hot staging test, separation, and the Starship startup.  If they can achieve all that, the best bet is to leave Sir Issac Newton in the driver's seat for the rest of the flight - he's an excellent driver, BTW - and prep for the next test campaign since newer hardware is sitting at Boca Chica

In theory I agree. I practice, Starship hardware rich development is currently hampered by slow regulatory progress, which forces SpaceX to advance in much slower, larger steps instead of many small ones when it comes to flight tests. The result of this is only every nth Starship or booster actually gets launched, while the in-betweens get built, used for ground tests, then scrapped.

Offline Barley

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Re: IFT-2 Pacific Recovery
« Reply #27 on: 11/10/2023 03:43 pm »
So.. rotate the potential horizontal thrust vector... Away from hawaii? There really is not much to hit anywhere close to the designated splashdown zone, especially north of it as far as I know.
If your proposed solution for a "vehicle is not under control" scenario requires the vehicle to be under control, that doesn't really work.
You can't design the hardware so it is intrinsically incapable of harm in all phases of flight.  The stack on the pad could hit Hawaii -- as an orbital rocket it could hit anywhere on the planet.  This is of course why there is an independent flight termination system.  This could be extended to any phase of flight if the game is worth the candle.

"Under control" is not a binary condition.  It is possible to not have enough control to simulate a landing but have enough control to miss Hawaii.  It doesn't take much control to miss Hawaii.  You can design different parts of the system with different complexity and robustness. 

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