totm nov 2023 SpaceX Discussion Thread

[RCgothic]

If they can stick the landing with accuracy, maybe they'll be able to build a freshwater /de-ionised landing pool?

[Nibb31]

6 hours ago, DDE said:

I was unaware that they've completely ditched the use of the trunk for fuel carriage.

The trunk was never used for carrying propellant.

4 hours ago, tater said:

SpaceX commercial crew has always been a parachute landing at sea. Nothing changed.

The dracos were always the LES system. Landing was secondary for them. Right now they will still offer a backup should the parachute fail, D2 can land, it just needs a soft pad since there are no feet. They judged making the feet safe harder than it was worth it since they have to demonstrate very low %s for LOC incidents and any break in the TPS is a possible failure mode.

Are we really sure that the feet are the problem ? That seems like a pretty trivial problem to fix. The Shuttle had hatches in its heat shield with no problem, and even if they fail to deploy, you only get a scratched heatshield and a slightly harder landing, not a LOM/LOC situation.

My understanding of Musk's statement is that the problem is certification, which means that they have trouble ensuring the propulsive landing is reliable enough to meet NASA's standards. If the engines fail to fire, or if there is a glitch in the guidance or attitude control, then the crew is toast and there is not enough time to do anything about it. Maybe there was nothing they could do to get the mathematical probabilities down to an acceptable level for NASA.

I'm curious about their new Mars landing concept. I never believed in the ITS propulsive landing on unprepared terrain. The debris problem is nearly impossible to fix, especially as the renderings showed the engine bells dangerously close to the ground. I also have trouble with the horizontal reentry/vertical landing concept. And parachutes are simply not practical for something as big as the ITS.

Edited July 20, 2017 by Nibb31

[tater]

I agree about why Musk stated, that it was a cert issue. Boeing and SpaceX are both having issues with the LOC requirements (1:270?), and propulsive adds another failure mode that is less well understood.

They had said that they would not do propulsive for crew a long time ago---until they had tested it with cargo returns enough to certify it. This gets rid of that possibility. The feet seem like a failure mode that would be added to engine start issues, making that certification even harder, right?

BTW, the total dv of D2 is pretty unclear right now. I have seen a couple numbers for prop mass.

[_Augustus_]

11 minutes ago, Nibb31 said:

they have trouble ensuring the propulsive landing is reliable enough to meet NASA's standards. 

Which are of course, insane. If NASA would loosen up on the safety ratings I think D2 and maybe Starliner would be flying by now.

Edited July 20, 2017 by _Augustus_

[Nibb31]

8 minutes ago, _Augustus_ said:

Which are of course, insane. If NASA would loosen up on the safety ratings I think D2 and maybe Starliner would be flying by now.

Maybe. Maybe not. Maybe they would, but a LOC would end NASA's existence. NASA is the customer here, they are paying for the development, so they are pretty much entitled to set the requirements that they deem necessary.

[_Augustus_]

1 minute ago, Nibb31 said:

Maybe. Maybe not. Maybe they would, but a LOC would end NASA's existence. NASA is the customer here, they are paying for the development, so they are pretty much entitled to set the requirements that they deem necessary.

But one of the big certification issues with D2, Starliner, and flying Orion with the ICPS is micrometeoroid protection, despite the fact that micrometeoroids have never even come CLOSE to LOC or LOM.

Edited July 20, 2017 by _Augustus_

[tater]

BTW, the Isp of Draco is 300s, so using those thrusters for OMS makes a difference. With the higher Isp I get 525 m/s dv. The dv is better on return as it is lighter, as well I would assume.

525-150-100 leaves 275 at 300s Isp. Looks like closer to 180 on the super dracos. I honesty don't know what the dv budget is for a D2 to ISS in terms of what is required for rendezvous and return though.

[Nibb31]

7 minutes ago, _Augustus_ said:

But one of the big certification issues with D2, Starliner, and flying Orion with the ICPS is micrometeoroid protection, despite the fact that micrometeoroids have never even come CLOSE to LOC or LOM.

Actually they have. Both Endeavour and Atlantis were hit by MMOD that came a few millimeters to impacting the coolant loop. If it had punctured the freon circuit, the vehicle would have survived, but they would have had a LOM and deorbited immediately. If it had gone through a window, they were LOC.

Edited July 20, 2017 by Nibb31

[softweir]

1 hour ago, tater said:

Hypergolic's are nasty, we all know that. That incident didn't stop them from using it on Shuttle. If NASA was concerned, it would not be there. Ditto Orion. And CST-100, I presume, they're using hypergolic thrusters as well, right?

NASA are concerned about hypergolics - the safety procedures surrounding them are complex, detailed, and rigorously documented and adhered to. But there are no reliable alternatives.

[tater]

Just now, softweir said:

NASA are concerned about hypergolics - the safety procedures surrounding them are complex, detailed, and rigorously documented and adhered to. But there are no reliable alternatives.

That was my point. They are not concerned enough to stop using them because the failure modes for hypergolic are worth the lower failure rate of engine operations.

The thrusters would be hypergolic regardless, which means they have tanks aboard anyway. A couple hundred kg thruster tank blowing up or leaking inside in flight is pretty much just as deadly as a bigger tank, I bet.

[_Augustus_]

37 minutes ago, Nibb31 said:

Actually they have. Both Endeavour and Atlantis were hit by MMOD that came a few millimeters to impacting the coolant loop. If it had punctured the freon circuit, the vehicle would have survived, but they would have had a LOM and deorbited immediately. If it had gone through a window, they were LOC.

But aren't the shuttles a much bigger target for MMOD than the capsules?

[DerekL1963]

20 minutes ago, _Augustus_ said:

But aren't the shuttles a much bigger target for MMOD than the capsules?

That means "shuttles get hit more often" not "capsules don't get hit".

[tater]

I'd imagine that risk assessment for orbital collisions includes interaction cross section.

[kerbiloid]

1 hour ago, tater said:

BTW, the Isp of Draco is 300s, so using those thrusters for OMS makes a difference. With the higher Isp I get 525 m/s dv. The dv is better on return as it is lighter, as well I would assume.

525-150-100 leaves 275 at 300s Isp. Looks like closer to 180 on the super dracos. I honesty don't know what the dv budget is for a D2 to ISS in terms of what is required for rendezvous and return though.

Fuel mass and ISP stay pretty unknown, of course.
I just proceed on the assumption that: if that test was with full tank, we have more or less exact duration and acceleration which gives us an estimation of dV.
But if the tank was half-empty, and they can more, then (as we can see in a combined movie) a fully fueled (and thus, more heavy) Dragon would not escape from the fireball.

So, in my understanding, either its total dV always was too low, or its LES cannot save it at all.
In any case what I hear differs from what I see.

Edited July 20, 2017 by kerbiloid

[tater]

Why do you say it only has one tank? I haven't seen a plumbing diagram for it.

[kerbiloid]

I mean all its inner tanks.

If they have more, they couldn't fly away with T/W=6.

Both cases look not nice,

[tater]

What I mean is that the super draco tanks for the LES test were filled (and burned to depletion), but the thrusters likely have separate tanks altogether. So the trusters could have the same dv as Dragon, and the superdraco tanks are in addition to that, and are only for LES or landing.

Edited July 20, 2017 by tater

[magnemoe]

12 minutes ago, tater said:

What I mean is that the super draco tanks for the LES test were filled (and burned to depletion), but the thrusters likely have separate tanks altogether. So the trusters could have the same dv as Dragon, and the superdraco tanks are in addition to that, and are only for LES or landing.

Easy to use an valve and an small extra tank, LES burn out and you still got enough fuel for orientation, however they does not have their own fuel supply just an emergency tank. 
For correction and orbit matching burns it would use the common fuel tank .
This would anyway mostly be needed for final part of first stage and upper stage abort as the trusters are to weak to work well in atmosphere. 
The reason they abort with trunk is for aerodynamic stability during abort, afterward the trunk is dropped and the dragon turn bottom first 

[Racescort666]

For the launch abort test, it's possible that they ran the test on a partial test and ballasted the vehicle to be the appropriate weight. In fact, they almost certainly needed ballast to get the vehicle to the flight weight. We don't know if they burned a full tank of fuel for the test whether it was partially filled, smaller than production intent, or shut down early. The dV estimate for the LES is basically a minimum value without a solid maximum.

Also, I wouldn't be surprised to see them attempt a propulsive landing on a cargo mission although that would probably require NASA approval since SpaceX returns science cargo that presumably has to survive the landing.

[KSK]

I can't really think of a tank of rocket fuel that I'd like to hard land on. Seems like I've got an enticing choice of being poisoned, flash fried, flash frozen, asploded or a happy fun combination of any of the above.

 

[CatastrophicFailure]

7 minutes ago, KSK said:

I can't really think of a tank of rocket fuel that I'd like to hard land on. Seems like I've got an enticing choice of being poisoned, flash fried, flash frozen, asploded or a happy fun combination of any of the above.

 

How bout nitrous oxide? The landing might suck, but after a few breaths you wouldn't care. 

[sevenperforce]

8 hours ago, Elthy said:

8 hours ago, KSK said:

 

Quote

And it ended up being way harder to do Falcon Heavy than we thought. Because at first it sounds really easy to just stick to first stages on as strap-on side boosters. But then everything changes. The loads change, the air dynamics totally change. You triple the vibration and acoustics. So you break the qualification levels and so much of the hardware. The amount of load you’re putting through that center core is crazy because you have two super powerful boosters also shoving that center core. So we had to redesign the whole center-core airframe on the Falcon 9 because it’s going to take so much load. And then you’ve got the separation systems... and, yeah, it just ended up being way way more difficult than we originally thought. We were pretty naive about that.
 

I think I may keep a copy of that second paragraph around somewhere for use in the more off-the-wall armchair rocket design discussions that seem to happen round this way.

Yeah, KSP makes rockets seem way, way to easy.

Heyyyyyy, I resemble that remark!

[kerbiloid]

Usually LES weights ~ 1/4..1/2 of the capsule mass and gets dropped after the 1st stage burnout.

Delivering a reusable LES to orbit one must deliver:

• additional mass of hull to hide the LES in;
• additional fuel to accelerate the LES mass to to 1..2 km/s, but to 8 km/s;
• additional mass of the last stage to keep the additional fuel in.

So, instead of launching a single-use traditional LES weighting, say, 3 t, they would spend 3*5 = 15 t of fuel more to get it to orbit and add ~1 t of tthe rocket fuel tanks to keep this fuel inside.
So, a reusable LES means spending 15 t of fuel and hundreds kg of tanks to save 3 t of powder and still several hundred kg of metal.
I.e inner LES = burn 10..15 t of liquid fuel to save 3 t of powder.
Not a great choice unless suppose that LES option happily appears for the already being planned landing engine (which needs almost the same dV as LES).

There is another option, though, which was supposed to be used in Clipper project.
8 solid motors below the ship, on the last stage top.
If abort - they separate from the stage and throw up th ship.
If success - they were supposed to be ignited before the speed gets orbital, and spend their fuel to give the required delta-V to get into orbit.
I.e. LES not spent and not reusable, but used just as an additional stage burning in every flight.

[Kryten]

4 hours ago, DerekL1963 said:

That means "shuttles get hit more often" not "capsules don't get hit".

And there's much more to get hit with nowadays than there was during most of the shuttle era.

[Vanamonde]

Overlapping threads have been merged.