totm nov 2023 SpaceX Discussion Thread

[wumpus]

30 minutes ago, Codraroll said:

Not so sure about that. In real life, somebody will probably tap you on the shoulder and give you a reminder if you design a parachute-dependent landing capsule without a parachute.

Vladimir Komarov (Soyuz 1) had to heroically deal with life threatening failure after life threatening failure, solving them all to return from orbit.  Unfortunately, there was nothing he could do about failed parachutes...

[tater]

(the Orion capsule + LES is ~16t, and the LES is entirely external, so very costly from a mass standpoint)

[mikegarrison]

My preference would be wings >> parachutes >> powered thrust. If for no other reason than that the atmosphere never runs out of fuel.

(This obviously would not apply on the moon, or even on Mars.)

Edited June 18, 2020 by mikegarrison

[sevenperforce]

32 minutes ago, tater said:

I still think that it has enough margin for a LES in any Earth-Moon use case, or even NASA style Mars exploration (for liftoff or EDL at Earth). Simply put there is no reason for more than some smallish number of passengers (10?). "Wasting" a decent % of payload mass on a LES is not a big deal when you can take 150t to LEO. Seems like they could make a 10-20 ton crew capsule on top, including LES motors and chutes that could hold as many people as they would need for space use short of their colonization goal.

The placement of the forward LOX tank makes this complicated, and I understand why they want the same vehicle configuration for every variant (if nothing else, it adds to reliability when you are flying the same design over and over). But yeah, it would be a good thing. The problem is that this doesn't allow for P2P, which is something they seem to desperately want. Not enough margins for P2P LES.

33 minutes ago, tater said:

(the Orion capsule + LES is ~16t, and the LES is entirely external, so very costly from a mass standpoint)

You mean not very costly?

33 minutes ago, mikegarrison said:

My preference would be wings >> parachutes >> powered thrust. If for no other reason than that the atmosphere never runs out of fuel.

(This obviously would not apply on the moon, or even on Mars.)

I like giving a vehicle enough wing space (preferably wet wings) to safely belly-glide to a landing, but also giving it retropropulsive horizontal landing. That way you don't have to deal with the weight of wheels but you have a backup in the event of a propulsion system problem.

[sevenperforce]

The trouble is that even in KSP's simplified universe, I really can't come up with a good reusable crewed upper stage design that hits all the design requirements. Ideally, a 21st-century spaceship/shuttle would have the following attributes:

• Required
  • Auxiliary, 0-0 abort system from launch to landing
  • Contingency landing modes
  • Two-system fault tolerance
  • Passive aerodynamic stability from re-entry to touchdown
• Preferred
  • Parallel-staged
  • Horizontal-attitude touchdown
  • Drop-in cargo variant

You would think that it would be possible to design Starship such that the wings/flaps could be "locked" in a position on orbit, such that you'd gradually transition from a lifting re-entry to a tail-first orientation as your airspeed and the air density changed. Then you'd just need to feather the wing position to different places depending on your entry angle, entry speed, and the world you were landing on.

Edited June 18, 2020 by sevenperforce

[tater]

21 minutes ago, sevenperforce said:

You mean not very costly?

No, I mean that the external LES on Orion/SLS is very costly from a mass standpoint, since it included the tractor, AND the shroud/structure/etc. For Orion the LES alone is about like the actual capsule mass.

I understand their header tank placement for now, but they are still showing art of a crew version with glass to the nose, unsure how the 2 things square.

I suppose they could do a sideways LES like Bono had on some of his designs.

Edited June 18, 2020 by tater

[kerbiloid]

Why nobody mentions Rogallo?

Spoiler

 

 

https://www.apogeerockets.com/Advanced_Construction_Videos/Rocketry_Video_59

 

[sevenperforce]

1 minute ago, kerbiloid said:

Why nobody mentions Rogallo?

Not rapidly reusable, mebbe.

[mikegarrison]

10 minutes ago, sevenperforce said:

Passive aerodynamic stability from re-entry to touchdown

This is pretty much impossible, so I wouldn't be using it as a criterion.

(Unless you consider "touchdown" to allow for a smoking crater in the ground, capsules can't touch down passively.)

Edited June 18, 2020 by mikegarrison

[sevenperforce]

50 minutes ago, mikegarrison said:

This is pretty much impossible, so I wouldn't be using it as a criterion.

(Unless you consider "touchdown" to allow for a smoking crater in the ground, capsules can't touch down passively.)

Doesn't mean it needs to land passively, just that it doesn't tumble or enter an unrecoverable attitude without active stabilization. 

[Nightside]

46 minutes ago, sevenperforce said:

Doesn't mean it needs to land passively, just that it doesn't tumble or enter an unrecoverable attitude without active stabilization. 

Would passive stability inhibit maneuverability? Would the shuttle be passively stable?

 

 

[sevenperforce]

11 minutes ago, Nightside said:

Would passive stability inhibit maneuverability? Would the shuttle be passively stable?

The Shuttle was more passively stable than Starship will be, but less passively stable than the X-37, which is much less passively stable than a capsule.

The Shuttle really was NOT that passively stable, all things considered, because it needed S-turns to control its descent rate. 

[RealKerbal3x]

4 hours ago, sevenperforce said:

The trouble is that even in KSP's simplified universe, I really can't come up with a good reusable crewed upper stage design that hits all the design requirements. Ideally, a 21st-century spaceship/shuttle would have the following attributes:

• Required
  • Auxiliary, 0-0 abort system from launch to landing
  • Contingency landing modes
  • Two-system fault tolerance
  • Passive aerodynamic stability from re-entry to touchdown
• Preferred
  • Parallel-staged
  • Horizontal-attitude touchdown
  • Drop-in cargo variant

You would think that it would be possible to design Starship such that the wings/flaps could be "locked" in a position on orbit, such that you'd gradually transition from a lifting re-entry to a tail-first orientation as your airspeed and the air density changed. Then you'd just need to feather the wing position to different places depending on your entry angle, entry speed, and the world you were landing on.

A couple of detachable escape pods (similar to some early Shuttle proposals) could probably be used as an abort system. They'd probably be pretty heavy but since an LES is essentially useless on the Moon or Mars, they could just be used on a dedicated LEO crew-transfer variant, with the Mars/Moon bound ship launched separately and uncrewed. In order to work for pad aborts they might have to be launched from the ship at an angle, which could pose a problem.

If done right, those pods could also be used as a last ditch escape system during Earth EDL, but any failure during EDL is most likely to be the flaperons, which (unless I'm mistaken) can't be too different to aircraft control surfaces. In that case they can reduce the chance of that kind of sketchy abort mode by just having redundancies upon redundancies, just like most aircraft do. If an aircraft's control surfaces lock up it's probably going to crash no matter what anyway, just as Starship would.

I'm still not sure how a passively stable system would work without having aerodynamics that would be undesirable during launch, or having to pump a lot of propellant around. But then again, I'm just an armchair engineer and don't really know what I'm talking about. Plus, Starship's design is very fluid and we could see this landing system change very soon

[Nightside]

46 minutes ago, RealKerbal3x said:

Plus, Starship's design is very fluid and we could see this landing system change very soon

If their fin/control surface design is as good as their pressure vessel design, we may see LOT of changes.

[sevenperforce]

18 minutes ago, RealKerbal3x said:

In that case they can reduce the chance of that kind of sketchy abort mode by just having redundancies upon redundancies, just like most aircraft do. If an aircraft's control surfaces lock up it's probably going to crash no matter what anyway, just as Starship would.

A passenger airliner has multi-fault tolerance and multiple landing modes (runway landing, belly landing, splashdown). If the rudder becomes stuck, for example, the pilot can use differential engine thrust to counteract it. If an aileron locks up, the pilot can use leading edge flaps, slats, and elevators to balance and provide supplementary control. If the landing gear locks up they can belly land. If they lose both engines on ascent they have an aux power unit to provide enough power to operate the control surfaces, and on and on. There are contingency modes for practically any set of failures. 

Starship can ONLY land safely if all of its flaps work with sufficient precision to successfully execute the tail-flip AND if the tanks have sufficient pressure AND if the legs all lock out. There are no other options. It cannot glide. It cannot splash down. If a flaperon gets locked, it might be able to feather the others and survive re-entry but the tail-flip is going to be extra dicey.

[Guest]

2 hours ago, sevenperforce said:

The Shuttle was more passively stable than Starship will be, but less passively stable than the X-37, which is much less passively stable than a capsule.

The Shuttle really was NOT that passively stable, all things considered, because it needed S-turns to control its descent rate. 

S-turns had nothing to do with stability. It could have done a reentry without them, but it would have ended up over the ocean. S-turns were to bleed energy in a controlled manner, so it'd arrive over the runway at the right altitude. IRL, atmospheric reentry is not quite as predictable as in KSP, and consider how difficult it's to accurately reenter with a glider so that you can get to the runway. The Shuttle actually flew an elaborate pattern based around ground-based radio navigation beacons.

Capsules do the same thing, BTW. An offset COM gives them some lift, allowing them to control their touchdown point by rolling, and reduce G-loads while at it. Unpowered, they'll typically enter a spin around their roll axis and this effect will cancel out. This results in the landing site being much more difficult to pin down, though.

[CatastrophicFailure]

4 hours ago, sevenperforce said:

A passenger airliner has multi-fault tolerance and multiple landing modes (runway landing, belly landing, splashdown). If the rudder becomes stuck, for example, the pilot can use differential engine thrust to counteract it. If an aileron locks up, the pilot can use leading edge flaps, slats, and elevators to balance and provide supplementary control...

Up to a point. Eventually there has to be some acceptance of an “acceptable” probability of failure. In a small plane like a Cessna, for example, if the elevator gets stuck, you’re pretty much boned, and there are examples of such failures happening, yet the design hasn’t changed much. Now, for examples on the airliner side, you’ve got Alaska 261, where the jackscrew controlling the tailplane (elevator trim) failed, leading to a catastrophic loss of control. There was no contingency for this, since it just “wasn’t supposed to happen” with proper maintenance. Then there’s American 587, the plane that crashed just after 9/11 when the vertical fin broke off. That’s just “not supposed to happen,” and there is no contingency if it does. And the infamous United 232, where an engine failure just so led to a complete loss of hydraulic control despite three redundant systems. The fact that 181 people survived at all is a bona-fide miracle, no air crew since has even made to the airport in the simulator with similar failures. 

Anyways, point is, sooner or later, a system has to be just “good enough.” We don’t generally build airplanes with contingencies if a major piece just falls off, because it’s extremely rare. Wasn’t so in the early days of aviation, until they learned how to git gud at not having important bits fall off. One very simple way to give Starship some redundancy is simply to use split control surfaces like most airliners already do. You don’t have just four flapperons, you have eight, four pairs moving in tandem, but to the layman watching from a distance it’s just four. With sufficient redundancies in the motion systems (direct electric actuation works in that favor here, it’s much simpler than hydraulics), I think it’s quite reasonable that Starship reaches that level of “good enough” to operate without traditional abort systems. Having that high flight rate to reveal the bugs so they can be squished is central to that. 

Factoid: the Space Shuttle used the same jackscrew system as the MD-80. NASA was actually involved in improving it after the crash. 

[kerbiloid]

Dreamchaser/Spiral design looks more passively stable thn others.

[CatastrophicFailure]

It appears there really shall be a rise of the un-poofed. 

@Just Jim poke... pokepoke...

[kerbiloid]

If it ever flies, this will be the real birthday of private spacenautics. A second-hand patched rocket for sale...

Spoiler

 

 

[sevenperforce]

14 hours ago, Dragon01 said:

S-turns had nothing to do with stability. It could have done a reentry without them, but it would have ended up over the ocean. S-turns were to bleed energy in a controlled manner, so it'd arrive over the runway at the right altitude.

Perhaps that is why I said that S-turns were required to control descent rate and not something to do with stability.

Quote

IRL, atmospheric reentry is not quite as predictable as in KSP, and consider how difficult it's to accurately reenter with a glider so that you can get to the runway. The Shuttle actually flew an elaborate pattern based around ground-based radio navigation beacons.

It was more than re-entry accuracy. If it was just a matter of landing in the right spot, the Shuttle simply could have begun re-entry earlier.

The Shuttle had to maintain a strict 40-degree angle of attack to shield vulnerable portions of the airframe from direct re-entry heat. However, at this airspeed and angle of attack, the Shuttle had a tremendous amount of hypersonic compressive body lift due to its high wing plan area (necessary in order to allow a reasonable, somewhere-better-than-brick subsonic glide ratio). It had so much lift that if the 40-degree AoA was maintained through the re-entry interface, the orbiter would have actually begun to climb, trading velocity for altitude. This would have been bad, because both speed and atmospheric pressure would be decreasing, leading to less control authority on aerodynamic surfaces. The only way to maintain control authority while losing airspeed is to descend into thicker air, but the Shuttle couldn't descend with a 40-degree AoA.

The way to maintain descent rate without decreasing AoA was to turn the lift vector sideways with a bank/roll maneuver. This pulled the Shuttle off-track, but induced cosine loss in the lift vector that allowed a better descent rate. The craft would then need to bank in the opposite direction to stay on track, and repeat through the re-entry interface.

Quote

Capsules do the same thing, BTW. An offset COM gives them some lift, allowing them to control their touchdown point by rolling, and reduce G-loads while at it. Unpowered, they'll typically enter a spin around their roll axis and this effect will cancel out. This results in the landing site being much more difficult to pin down, though.

Indeed they do. However, a loss of roll authority (as happened in TMA-10 and TMA-11) or a steeper-than-planned entry trajectory (as with MS-10) resulting in a ballistic re-entry is still aerodynamically stable. It's just very unpleasant. TMA-10 actually re-entered nose-first, with the orbital module still attached, and it melted off and then the craft tumbled into its aerodynamically-stable re-entry orientation. The crew was banged up but the chutes deployed properly and they survived. That is what I mean by passive aerodynamic stability.

9 hours ago, kerbiloid said:

Dreamchaser/Spiral design looks more passively stable thn others.

Yes, Dream Chaser and the MiG-105 (as well as the X-37B to a lesser extent) have better passive aerodynamic stability. With that sort of shape, a failed control surface may result in an off-target entry but you will at least survive to an altitude where you could conceivably bail out.

12 hours ago, CatastrophicFailure said:

We don’t generally build airplanes with contingencies if a major piece just falls off, because it’s extremely rare.

Yep. That's where "what if you lose tank pressure" comes in. Some things like major structural failure are never going to be recoverable. 

Crew Dragon can obviously save the crew if there is a major structural failure of the F9 booster, but if there is a major structural failure of Dragon itself there's simply nothing you can do. 

Edited June 19, 2020 by sevenperforce

[StrandedonEarth]

1 hour ago, sevenperforce said:

The Shuttle had to maintain a strict 40-degree angle of attack to shield vulnerable portions of the airframe from direct re-entry heat. However, at this airspeed and angle of attack, the Shuttle had a tremendous amount of hypersonic compressive body lift due to its high wing plan area (necessary in order to allow a reasonable, somewhere-better-than-brick subsonic glide ratio). It had so much lift that if the 40-degree AoA was maintained through the re-entry interface, the orbiter would have actually begun to climb, trading velocity for altitude. This would have been bad, because both speed and atmospheric pressure would be decreasing, leading to less control authority on aerodynamic surfaces. The only way to maintain control authority while losing airspeed is to descend into thicker air, but the Shuttle couldn't descend with a 40-degree AoA.

The way to maintain descent rate without decreasing AoA was to turn the lift vector sideways with a bank/roll maneuver.

Thanks for that. I always wondered why they didn’t just re-enter farther back, but figured there must be a reason  Now I know 

[CatastrophicFailure]

2 hours ago, sevenperforce said:

TMA-10 actually re-entered nose-first, with the orbital module still attached, and it melted off and then the craft tumbled into its aerodynamically-stable re-entry orientation.

Don’t you mean the service module? The same thing happened on the next flight too. 

[wumpus]

4 minutes ago, CatastrophicFailure said:

Don’t you mean the service module? The same thing happened on the next flight too. 

You mean that real rocket scientists share one of my most annoying KSP design failures?

[sevenperforce]

9 minutes ago, CatastrophicFailure said:

Don’t you mean the service module? The same thing happened on the next flight too. 

Good catch -- yes, I meant the service module.

Totally unpolitical: