totm nov 2023 SpaceX Discussion Thread

[Exoscientist]

11 hours ago, sevenperforce said:

So it doesn't have anything to do with engine reliability after all, then?

All engineering ideas are bad ideas until they are standard practice.

Your feelings about the term "Chief Engineer" are about as strange as your feelings about the term "full duration".

On the other hand, a Raptor still did explode on both booster landing attempts.

  Bob Clark

[RCgothic]

And for all you know a Merlin (most reliable rocket engine in flight hours ever by a long margin) would have exploded in those circumstances as well.

Pretty much every instance of engine-out so far on Starship/Superheavy appears to be some form of "rocket failed to provide the engine with acceptable/survivable operating conditions".

A rocket needs to be looked at holistically.

Is direct exhaust autogenous pressurisation turning out to be harder than expected? Possibly. But heat exchangers are also massive and complicated and need to move enormous quantities of liquid through a phase change, so we may not yet have reached the point where "stick more filters on it" isn't a better solution.

For context, each of the steam generators on a nuclear reactor are about half the size of the reactor vessel itself, and there are typically 2-4 of them.

Edited Wednesday at 12:29 PM by RCgothic

[RKunze]

14 hours ago, Brotoro said:

If you filter through this special tank, you are filtering out gas from liquids/solids,

No. At the point where the (hot) exhaust enters the tanks, everything ist gaseous. The water and CO2 freeze out when the exhaust gas mixes with the (very cold) LOX in the tank.

[sevenperforce]

7 hours ago, Exoscientist said:

19 hours ago, sevenperforce said:

So it doesn't have anything to do with engine reliability after all, then?

All engineering ideas are bad ideas until they are standard practice.

Your feelings about the term "Chief Engineer" are about as strange as your feelings about the term "full duration".

On the other hand, a Raptor still did explode on both booster landing attempts.

Engines that run on liquid do tend to explode when you feed them gas.

[RCgothic]

SpaceX appear to be preparing a simulated tower catch.

[Brotoro]

9 hours ago, RKunze said:

No. At the point where the (hot) exhaust enters the tanks, everything ist gaseous. The water and CO2 freeze out when the exhaust gas mixes with the (very cold) LOX in the tank.

I know. That's why you run the hot turbopump exhaust into this smaller tank of LOX and let the condensation and freezing of the water and carbon dioxide happen in this secondary tank (not in the main tank) Then let the gaseous oxygen out to the main tank pressurization line. 

[tater]

https://twitter.com/SawyerMerritt/status/1806072406793937215

 

[tater]

 

 

[RKunze]

1 hour ago, Brotoro said:

know. That's why you run the hot turbopump exhaust into this smaller tank of LOX and let the condensation and freezing of the water and carbon dioxide happen in this secondary tank (not in the main tank) Then let the gaseous oxygen out to the main tank pressurization line. 

That will just clog up your (now smaller) filters faster....

[RCgothic]

Also the gas will be colder by the time it reaches the main tank and less useful as pressurant.

[StrandedonEarth]

1 hour ago, tater said:

 

 

(Doc Brown voice) $843 million!

The pork must flow…

[tater]

59 minutes ago, StrandedonEarth said:

The pork must flow…

The trick is that if they wait until a deorbit burn is cheap (dv wise), they'll have less control I would imagine vs a very intentional deorbit over the SoPAC. Dv to move it to GEO requires something like 600 tons of props as a reality check on somehow saving it as a historical artifact.

Not sure if anyone else can plausibly come up with a vehicle capable of this any faster/cheaper.

[darthgently]

They could have sold it on the gov auction site with the stipulation that it must either be deorbited safely, or raised to GEO within some time frame or NASA gets it back, no refunds.  They could probably sell it many times, repeatedly, to starry-eyed multi-millionaires until they had enough to cover the actual SpaceX operation that gets it done.  J/k, mostly

Edited Thursday at 01:04 AM by darthgently

[StrandedonEarth]

1 hour ago, tater said:

The trick is that if they wait until a deorbit burn is cheap (dv wise), they'll have less control I would imagine vs a very intentional deorbit over the SoPAC. Dv to move it to GEO requires something like 600 tons of props as a reality check on somehow saving it as a historical artifact.

Not sure if anyone else can plausibly come up with a vehicle capable of this any faster/cheaper.

Yeah, I’m sure it’s still a lot cheaper than any other bids they had. Still seems a bit high, but WDIK. I don’t even know how much props it would take to do a controlled, targeted reentry.

One thing for sure, they better mount a few starlink transceivers and a zillion cameras on it for the grand finale…

Meanwhile, another FH launch goes off and I barely noticed, if I hadn’t read this thread. Even that beast is getting routinely common 

Edited Thursday at 01:33 AM by StrandedonEarth

[Terwin]

17 hours ago, Brotoro said:

I know. That's why you run the hot turbopump exhaust into this smaller tank of LOX and let the condensation and freezing of the water and carbon dioxide happen in this secondary tank (not in the main tank) Then let the gaseous oxygen out to the main tank pressurization line. 

You either need a large O2 tank for this, or you will or you will evaporate all the O2 and still need filters in the main O2 tank because you are no longer catching all of the H2O and CO2.

This is also a separate tank that needs filling and emptying, little better than a he tank.

 

A large enough O2 tank with adequate filters will always be heavier and more complex than the main O2 tank with the same filters .

(Filter size and mass is relative to the amounts of ice it needs to catch, not the size of the tank, so your solution will either clog or have more weight.  Possibly both)

[magnemoe]

17 hours ago, RCgothic said:

Also the gas will be colder by the time it reaches the main tank and less useful as pressurant.

This, now I wondered about the methane tank having the same issue as they have high pressure methane as the methane is used for cooling the engine and this shows in the engine diagram. 
But they need to provide pressurization even after engine cutoff as the gas is hot and cooling. My guess is that they uses burners for this. After shutdown I don't think they use the main tanks for anything than structure and cold gas thrusters. 

[CBase]

Nice detail from https://arstechnica.com/space/2024/06/nasa-will-pay-spacex-nearly-1-billion-to-deorbit-the-international-space-station/:

SpaceX did not bid while part of the contract was "cost plus", but severly undercut the NASA estimation once it was allowed fixed price offer

Sounds like someone is not keen to discuss internal costs and work allocation.

[Brotoro]

I don't think you can count on having hot gas in the cryogenic propellant tanks to maintain pressurization for long, since the gas will cool in contact with the liquid. You need to generate enough gas that there will still be sufficient pressure in the tanks after the gas cools and the tank approaches equilibrium… otherwise you won't have sufficient tank pressure when you need to restart your engines.

[RCgothic]

The header tanks also have helium COPVs for pressurisation. 

But also it's probably not safe for NASA to sell the ISS. It's a station that's getting gradually more decrepit and can't reasonably be fixed. If a private entity ended up getting someone killed, NASA could be liable.

[AckSed]

9 minutes ago, RCgothic said:

The header tanks also have helium COPVs for pressurisation. 

But also it's probably not safe for NASA to sell the ISS. It's a station that's getting gradually more decrepit and can't reasonably be fixed. If a private entity ended up getting someone killed, NASA could be liable.

Axiom Space has said they only want to have a two-module part before they detach from the ISS. I thought it was due to schedule delays, but I'm not so sure now.

[DAL59]

22 hours ago, tater said:

The trick is that if they wait until a deorbit burn is cheap (dv wise), they'll have less control I would imagine vs a very intentional deorbit over the SoPAC. Dv to move it to GEO requires something like 600 tons of props as a reality check on somehow saving it as a historical artifact.

Not sure if anyone else can plausibly come up with a vehicle capable of this any faster/cheaper.

Why not just attach a low thrust, high efficiency ion drive like Tiangong station has? It would only take ~40 tons of Xenon to send it to GEO, and less if you go to a lower orbit.

[Ultimate Steve]

2 minutes ago, DAL59 said:

Why not just attach a low thrust, high efficiency ion drive like Tiangong station has? It would only take ~40 tons of Xenon to send it to GEO, and less if you go to a lower orbit.

Tldr: It would probably take too long.

If the ISS is in direct sunlight it will generate 240kW. Average is a lot lower (84-120) but let's say 240 for the sake of a steel man. The NEXT ion thruster has 4170s of isp, consistent with 40 ish tons of prop. It consumes 6.9 kW for 237 millinewtons of thrust. If it's running nothing else (ISS power was expanded recently because what they had was barely enough for what they were doing), the ISS can power about 35 NEXT engines. This is about 8.3 Newtons of thrust. If the ISS is about 420 tons (it is heavier and with 40 tons of prop but steel man) it will accelerate at 0.00002 meters per second per second. About 1.7 meters per second per day. Putting it in GEO would take over six years not accounting for the losses from spiraling out, and drag.

In reality it would take far longer. Night becomes less of an issue the higher the orbit gets but at best you're only getting 120kW for the first while. The station takes a lot of power. Let's say you can dedicate half (which is a lot, especially if the station is still in operation) to the cause. Now its 24 years.

Idk how much they lose to drag per day but it might be close enough that they can't even counteract drag.

And then you have to built 9-35 NEXT ion thrusters when only one has flown I think. I think ions are lifetime limited by erosion unless they've fixed that recently. Idk how long they can burn continuously but it surely isn't 24 years.

The station also wasn't designed for that thermal and radiation environment. Also communications. Even assuming it's long been uninhibited by now you would still have to interface with the existing systems enough to steer the thing and point the solar panels.

[StrandedonEarth]

35 minutes ago, DAL59 said:

Why not just attach a low thrust, high efficiency ion drive like Tiangong station has? It would only take ~40 tons of Xenon to send it to GEO, and less if you go to a lower orbit.

I like it. With non-essential systems shut down there would be plenty of power for ion drives. 

I wonder how much dV they could get by carefully venting life support gasses in the proper direction. Not much, I’m sure, but something. Or it could be used for RCS if there’s enough vents. 

E: And then I read Steve’s post, hmm. 

Bear in mind , I’m not thinking to raise ot to use it, just until the tech arrives that can recycle that mass on orbit. Or more likely it becomes a museum, since by the time we could recycle it we probably wouldn’t need to. Well, hello I enjoy the smell of rubber in my hair. How high does it need to be so that orbital decay becomes negligible?

Hmm, and how much would it cost, with what return when? Yeah, probably a technically do-able but pointless exercise…

EDIT 2:  Omg, lol. I’m going to leave that non-sequitur in there, because that struck me as hilarious. I guess the last iPhone update included a “tap to insert dictation here” feature that I accidentally tapped while I talked to the cat who was sniffing my hair. I didn’t notice it until after I posted the edit 

Edited Thursday at 11:21 PM by StrandedonEarth

[darthgently]

Love it.

Paraphrasing:  At SpaceX we specialize in changing the "impossible" into "merely late"

 

[StrandedonEarth]

Gotta keep doing something new or even SSSH flights will be in danger of becoming routine. Amazing that they’ve already launched four times and the fifth won’t be too far in the future…

Edited yesterday at 06:44 PM by StrandedonEarth
Typo