totm nov 2023 SpaceX Discussion Thread

[magnemoe]

9 hours ago, Beccab said:

We don't know that for now, just that as far as Artemis IV is concerned for NASA the mission ends when the crew is back to Orion. What they do next with it is mainly up to spacex

You are correct, refueling in moon orbit makes most sense. Retire the first and use as an base in moon orbit is another one. 
SpaceX might want an second one in case the first has problems.

Now Marcus House had some baffling news of the first human landing. NASA plan to launch 4 astronaut on Orion. Dock with moonship, then 2 astronauts will enter moonship and land on the moon while two stays behind on Orion. 
https://www.youtube.com/watch?v=mhB_9nQedU4
Why only 2? it makes no sense at all, leaving one might make some sort of sense but I assume Orion is fully fly by wire and could be remote controlled just as well from moonship if they have to do an manual docking. Yes having one in Orion could let you do an spacewalk to Orion this was an Apollo fallback option. 

But if you are stuck in moonship in lunar orbit you are not in immediately danger unlike the LEM who had very limited life support. 

 

[tater]

The top, cargo crew section of a LSS is maybe 9t of steel at 3mm thickness. Each ring is ~1.25t @ 3mm, and each ring roughs out to ~92t of prop capacity (total props/tank rings). Each Raptor is ~2t. I'm using the I_sp of 378, so these numbers could go up.

What if they made a stubby LSS? It has the full crew volume, but just enough tank to do the NRHO<—>surface round trip with some margin?

Call the total fitted out crew area ~20t (11t above steel mass), how many Rvacs—remember, only has to work on the Moon—1? Thrust is 185t, R2 is 230t. Assume 1 Rvac.

Assume a Raptor 2, Rvac. 230t thrust. (Regular Rvac would work, TWR would be low at start of trip from NRHO, but would get to 1:1 after it burns 45t of props).

OK, so 20t of crew volume, and we need 150t of props That's 1.6 rings. We need 2 more rings for skirt. Call it 5 rings for slop. 6.25t. Dry mass is now 26.25t. Slop it up to 30t. We have 150t of props.

Our stubby LSS can land with itself, plus 13t of cargo (under the skirt for easy unloading) and make the RT. It has 6800 m/s of dv with no cargo. It is 31m tall (vs 50). The crew volume mass is likely higher than it needs to be. 15t is probably more reasonable, so maybe 5t of cargo is included, and the prop requirements also drop).

We have commonality with other versions (same crew volume). Ah, but how does it get to the Moon? Well, it could fly there from LEO, but then it lacks the props for landing, etc. But if refilling is done at Gateway, that doesn't matter, and taking it all the way up is not a big deal with only 150t total props, and it only needs 105t to do a RT (no cargo).

A regular, 100t dry mass SS can arrive at NRHO with 330t of props remaining. It can leave 125t with the LSS (any LSS version), and still propulsively return to LEO (so could be depot). If it can do direct EDL (regular SS), it can deliver 294t of props to a LSS.

Edited November 21, 2021 by tater

[Rakaydos]

2 hours ago, magnemoe said:

Still assume they will use the end to end for refueling. The quick disconnect port is not very suitable for docking and is designed for pretty high pressure transfer.
Rear to rear makes it easy to get good mechanical connection, is save and make it easy to use RCS to settle fuel and then use pressure difference to transfer. 

Using RCS to settle fuel here will be very complex and will put a lots of stress on the quick disconnect connect port. 

I suspect it will be an asymetric approch.  Tanker has arms in the skirt that can connect and brace against standard quick disconnect ports.

Edited November 20, 2021 by Rakaydos

[grungar3x7]

6 hours ago, magnemoe said:

Why only 2? it makes no sense at all, leaving one might make some sort of sense but I assume Orion is fully fly by wire and could be remote controlled just as well from moonship if they have to do an manual docking. Yes having one in Orion could let you do an spacewalk to Orion this was an Apollo fallback option. 

But if you are stuck in moonship in lunar orbit you are not in immediately danger unlike the LEM who had very limited life support. 

 

To use the Buddy System, probably.

[Beccab]

9 hours ago, magnemoe said:

But if you are stuck in moonship in lunar orbit you are not in immediately danger unlike the LEM who had very limited life support. 

True, but the other part of the architecture is the problem here. Crew stuck on LSS = no rescue mission for more than a year with SLS being SLS. If FH was crew rated and Gray Dragon prepared before the first mission, though...

[hms_warrior]

you could allways just refuel lunar Starship, bring it back to LEO and use any normal dragon to bring crew home... 

[sevenperforce]

Placement of the depot is anticipated to be LEO, right? It’s a little tricky, I think. It’s a shame to burn propellant to move a  heat shield and flaps and such into lunar orbit when you don’t need them there. But a heat shield and flaps and such are the best way to get back into LEO. You want to spend your propellant in as efficient a way as possible.

In the long-term, what if they put the fuel depot into an Earth-Moon cycler trajectory? Each tanker mission would need to boost to TLI in order to match the fuel depot’s trajectory, but then could return for EDL easily. The depot would perform the NRHO insertion, which it can do more efficiently than the tankers because it has less mass to contend with. And then once (nearly) empty, it could perform Earth Interface Injection, returning it to the cycler trajectory but never needing to brake back into LEO propulsively.

[StrandedonEarth]

1 hour ago, sevenperforce said:

heat shield

How much of a heat shield is necessary for a stainless steel ship to aerobrake down to LEO? Heat tolerance was one of the reasons for the material, after all. Granted, it could aerobrake more aggressively with shielding, but it should still be able to take a fair bit, or so I would think.

[tater]

16 minutes ago, StrandedonEarth said:

How much of a heat shield is necessary for a stainless steel ship to aerobrake down to LEO? Heat tolerance was one of the reasons for the material, after all. Granted, it could aerobrake more aggressively with shielding, but it should still be able to take a fair bit, or so I would think.

I was looking into this, and I'm unsure. I think if you are open to longer time periods (multiple passes) it can probably be done. The trick I think is that something like LSS can return with a decent amount of residual props, just not enough to propulsively circularize in LEO without aerobraking. So while at some point with almost no props left the problem might become orbital period vs heat dissipation (load up with heat at perigee during aerobraking, then need time to radiate that heat away before the next pass), if you can scrub off just enough the remaining props can do the job, then it's fine.

Also:

 

[Beccab]

Starship QD extension

Booster QD hood (woah)

Also, the S20 missing heat tiles have now been mostly replaced
 

Edited November 21, 2021 by Beccab
added word

[sevenperforce]

8 hours ago, tater said:

I was looking into this, and I'm unsure. I think if you are open to longer time periods (multiple passes) it can probably be done. The trick I think is that something like LSS can return with a decent amount of residual props, just not enough to propulsively circularize in LEO without aerobraking. So while at some point with almost no props left the problem might become orbital period vs heat dissipation (load up with heat at perigee during aerobraking, then need time to radiate that heat away before the next pass), if you can scrub off just enough the remaining props can do the job, then it's fine.

I think the challenge is not re-radiation of absorbed heat, but peak heating during the passes. The highest-velocity passes produce the highest dV savings but also have the highest peak heating. 

[magnemoe]

18 hours ago, Beccab said:

True, but the other part of the architecture is the problem here. Crew stuck on LSS = no rescue mission for more than a year with SLS being SLS. If FH was crew rated and Gray Dragon prepared before the first mission, though...

FH does not need to be crew rated if the dragon was unmanned at launch. 
But unless they had an FH ready making some Frankenstage with two header tanks and a raptor is probably faster. 
Bonus for using 4 or more tanks and make this look as ugly as possible. 

[tater]

2 hours ago, sevenperforce said:

I think the challenge is not re-radiation of absorbed heat, but peak heating during the passes. The highest-velocity passes produce the highest dV savings but also have the highest peak heating. 

Yeah, and shallow passes barely scrub off velocity. You can shave a handful of m/s off, and do hundreds of passes.

[StrandedonEarth]

22 minutes ago, tater said:

Yeah, and shallow passes barely scrub off velocity. You can shave a handful of m/s off, and do hundreds of passes.

Yeah, true, now I’m getting flashbacks of trying to slowly land a beast of a rocket on Eve…

But  if it could handle just one good deeper dive…

Edited November 22, 2021 by StrandedonEarth

[tater]

It depends on the required mission cadence, etc. If the crew is delivered with Orion, they have almost a year, after all

 

[Beccab]

It's alive!

[kerbiloid]

17 hours ago, StrandedonEarth said:

How much of a heat shield is necessary for a stainless steel ship to aerobrake down to LEO?

In the glorious early days of the spacenautics allof them required a niobium shield at some distance from the hull itself.

Niobium was very expensive. If they could, they would never want it.

It would be understandable if the Starrship was made of a post-industrial nanosilicarbon, not of same steel which needed niobium a half-century earlier...

[sevenperforce]

4 hours ago, kerbiloid said:

21 hours ago, StrandedonEarth said:

How much of a heat shield is necessary for a stainless steel ship to aerobrake down to LEO?

In the glorious early days of the spacenautics allof them required a niobium shield at some distance from the hull itself.

Niobium was very expensive. If they could, they would never want it.

It would be understandable if the Starrship was made of a post-industrial nanosilicarbon, not of same steel which needed niobium a half-century earlier...

What on god's green earth are you on about?

No vehicles have ever performed multi-pass aerobrake circularization into LEO. There has never been any niobium-shielded stainless steel re-entry vehicle.

The only vehicle to perform multi-bass aerobrake circularization was the Mars Reconnaissance Orbiter, which did it at Mars, and quite famously had no heat shield at all. 

6 hours ago, Beccab said:

It's alive!

Well @tater it looks like you were right and I was wrong. I couldn't think of any reason that the OLT would need to have QDs to the individual engines, since the Raptor igniter is so small, but I forgot about the need for spin-up gas. 

[kerbiloid]

1 hour ago, sevenperforce said:

There has never been any niobium-shielded stainless steel re-entry vehicle.

All spaceplane projects of 1960s were requiring a niobium shield in front.

The tiles have appeared much later, in the late Space Shuttle version.

[magnemoe]

1 hour ago, kerbiloid said:

All spaceplane projects of 1960s were requiring a niobium shield in front.

The tiles have appeared much later, in the late Space Shuttle version.

Wonder if it would be practical? I imagine you would have an outer niobium skin then a gap before the steel tank. using pins to connect the skin to tanks. 
Think its more an question if it manages the heat. Its used on the cone of the falcon 9 vacuum engines but reentry is even more demanding. 

Now I thought spaceX has thought about it they use it in engines after all, don't think it would be heavier than the tiles as it would not be structural. 
And its so much simpler than the tiles. 
 

[sevenperforce]

2 hours ago, kerbiloid said:

All spaceplane projects of 1960s were requiring a niobium shield in front.

And exactly zero of those projects (none of which were ever constructed, mind you) were intended to perform aerobrake circularization. The heat shields were for re-entry.

This is, to date, the only vehicle to have ever performed multi-pass aerobrake circularization:

You will note the conspicuous lack of a niobium heat shield anywhere.

[kerbiloid]

9 minutes ago, magnemoe said:

Wonder if it would be practical? 

It would be too expensive to raise this question. One of the reasons, why all of them have been cancelled.

2 minutes ago, sevenperforce said:

And exactly zero of those projects (none of which were ever constructed, mind you) were intended to perform aerobrake circularization.

Yes, they were for the <= 8 km/s re-entry, not for 8...11 km/s.
If they were, the things would be even worse.

[sevenperforce]

10 minutes ago, magnemoe said:

Wonder if it would be practical? I imagine you would have an outer niobium skin then a gap before the steel tank. using pins to connect the skin to tanks. 
Think its more an question if it manages the heat. Its used on the cone of the falcon 9 vacuum engines but reentry is even more demanding. 

Now I thought spaceX has thought about it they use it in engines after all, don't think it would be heavier than the tiles as it would not be structural. 
And its so much simpler than the tiles. 

A niobium heat shield for Starship would be vastly heavier than the tiles. The tiles are extremely lightweight. Also, heat management would still be a problem.

The limiting factor for Starship is the temperature on the back of the heat shield, where it radiates to the steel. Niobium radiates and conducts heat very well, which makes it great for radiatively-cooled engine nozzles in a vacuum but not so great for a thin heat shield attached to steel. 

[kerbiloid]

1 minute ago, sevenperforce said:

A niobium heat shield for Starship would be vastly heavier than the tiles.

Indeed.

But if the constructors of 1960s could use bare steel without asking for the overexpensive niobium, they would definitely do this.

Since then, the steel unlikely has changed so much.

This tells about the steel hull, not about the niobium.

[magnemoe]

7 hours ago, kerbiloid said:

In the glorious early days of the spacenautics allof them required a niobium shield at some distance from the hull itself.

Niobium was very expensive. If they could, they would never want it.

It would be understandable if the Starrship was made of a post-industrial nanosilicarbon, not of same steel which needed niobium a half-century earlier...

It was expensive back in the 60's, having problem like not working in in air, falcon 9 vacuum engine nozzle is made from it. 
https://en.wikipedia.org/wiki/Niobium#Superalloys

Again steel can handle much higher temperatures than aluminium or composites.