totm nov 2023 SpaceX Discussion Thread

[RCgothic]

I thought the most interesting details:

-Raptor2's a 240mt engine.

-Superheavy with 33 engines is 2.2-2.3x more powerful than Saturn V.

-Depot isn't just a loitering tanker, it's heavily insulated to reduce boiloff.

-Orbital launch Jan/Feb.

-A dozen orbital launches next year potentially!

-Proper payloads from 2023.

 

I thought he could have done better on suggesting potential payloads however. This blog has some pretty good suggestions (it's a long read from a former member of JPL):

https://caseyhandmer.wordpress.com/2021/11/17/science-upside-for-starship/

https://caseyhandmer.wordpress.com/2021/10/28/starship-is-still-not-understood/

 

[Rakaydos]

13 hours ago, sevenperforce said:

 

They have a very good oxygen-rich preburner so they should have no trouble scaling it up to make it an ORSC engine, and ORSC gives you a slightly denser fuel mix to boot. Perhaps using only a single preburner and turbopump would make the engine lighter.

But then you lose three main advantages of FFSC: lack of ox/fuel hot seals, variable mixtures and efficiently-sized preburners, and gas-gas combustion efficiency.

Not ORSC.

FFSC, but on the oxygen rich side of schtochimetric. 

Higher thrust, lower ISP, cheaper and denser propellant mix.

Edited November 18, 2021 by Rakaydos

[tater]

 

 

 

[kerbiloid]

That strange moment when you realise that you're too short to lift and close the opened thing,

[AtomicTech]

1 hour ago, tater said:

 

 

 

I actually haven't seen an image of F9 getting serviced on the barge, thanks so much for sharing!

[sevenperforce]

6 hours ago, Rakaydos said:

Not ORSC.

FFSC, but on the oxygen rich side of schtochimetric. 

Higher thrust, lower ISP, cheaper and denser propellant mix.

Raptor already has such a deep throttle range, so the turbopumps must have a pretty large allowable speed range as well. And they run independently. So I don't see why you couldn't vary the mixture ratio already, like the F-1.

[sevenperforce]

On 11/17/2021 at 1:58 PM, CatastrophicFailure said:

Hmm... someone mentioned nuclear... could you run a NTR on methane? Or even methane/LOX? Sure, it wouldn't be as efficient as hydrogen, but would surely be better ISP than Raptor could ever do, with the simplicity of using the same propellant(s).

I was going to do the math on this, casually, but it looks like someone already has:

http://toughsf.blogspot.com/2021/10/nuclear-conversion-for-starship.html

They go through the possibility of (a) adding a second bulkhead and using liquid hydrogen with NTRs in place of the RVacs, (b) the same as before, but filling the cargo space with liquid hydrogen too, (c) moving the common bulkhead to increase the methane proportion and pushing methane through simple NTRs, and (d) filling the whole thing (except header tanks) with methane and pushing that through advanced NTRs.

Hydrogen is so fluffy and the mass ratio plunges so low that the hydrogen nuclear Starship can't even reach orbit. Filling the whole cargo space with hydrogen allows it to just barely reach orbit, without payload.

Simple methane NTRs can approximately meet the performance of the current Starship.

Advanced methane NTRs can approximately double the payload to LEO, but payloads beyond LEO are only slightly improved. They conclude that it's not worth it, and I'm inclined to agree.

[CatastrophicFailure]

November twenty-Ford... 

[tater]

 

 

[Rakaydos]

15 hours ago, sevenperforce said:

Raptor already has such a deep throttle range, so the turbopumps must have a pretty large allowable speed range as well. And they run independently. So I don't see why you couldn't vary the mixture ratio already, like the F-1.

First things first- if you perfectly mix the fuel and oxidiser, combustion will be too complete and you'll melt any material in existance. They have to be off-balance to have enough unburnt cool propellant to keep the temperature to reasonable levels, for rocket science levels of reasonable.

Second- metals that dont combust in hot oxygen rich enviroments are difficult. So it's normally easier to run the exhausts fuel rich, and also offers better ISP with most fuels that are lighter than their oxidiser.

 

[RCgothic]

14 minutes ago, Rakaydos said:

First things first- if you perfectly mix the fuel and oxidiser, combustion will be too complete and you'll melt any material in existance. They have to be off-balance to have enough unburnt cool propellant to keep the temperature to reasonable levels, for rocket science levels of reasonable.

Second- metals that dont combust in hot oxygen rich enviroments are difficult. So it's normally easier to run the exhausts fuel rich, and also offers better ISP with most fuels that are lighter than their oxidiser.

 

Sevenperforce already knows this. What they're saying is that Raptor as a FFSC engine with independent fuel and oxidiser pumps and preburners should already be capable of utilising variable fuel to oxidiser ratios to some degree.

Running the main combustion chamber over onto the oxygen-rich side of stoichiometric may indeed require a different construction. And switching between fuel rich and Oxygen rich may indeed not be healthy within a firing unless the changeover is swift.

Edited November 19, 2021 by RCgothic

[Rakaydos]

5 minutes ago, RCgothic said:

Sevenperforce already knows this. What they're saying is that Raptor as a FFSC engine with independent fuel and oxidiser pumps and preburners should already be capable of utilising variable fuel to oxidiser ratios.

Given that the intermediate zone is RUD territory, and that rocket engines are incredibly optimized, I doubt an off the shelf engine could cover both regimes. Certainty not without including the same exotic materials from the OR turbopump in the main combustion chamber and nozzle!

[sevenperforce]

Moving this discussion with @tater to the correct thread….

Suppose we have an 84-tonne lunar Starship that leaves LEO, picks up crew from Orion in NRHO, goes to the lunar surface, returns crew to Orion, and then returns propulsively to GTO (that's the max burnout mass we can achieve without a tank stretch). If a Starship tanker (which we will notionally say has a dry mass of 85 tonnes and carries 30 tonnes of landing propellant) can deliver 150 tonnes of propellant to LEO, then it can deliver 33 tonnes of propellant to the lunar Starship (assuming a 2.27 km/s GTO burn at 380 s isp).

This gives our lunar Starship, in turn, 1.2 km/s of dV, which it can use to lower its apogee significantly. The next tanker will only need to burn 1.07 km/s out of LEO to meet it, meaning it arrives with 86 tonnes of propellant, giving the lunar Starship enough propellant to reach LEO with 43 tonnes of remaining residuals.

It will need just under eight tanker missions to refill its tanks for the next sortie.

On its initial launch, it would have reached LEO with 171 tonnes of residuals, requiring less than 7 tanker missions to refill.

So you need a total of 10 launches for the first mission (1 lunar Starship + 7 tanker missions for the outbound journey + 2 tanker missions for the return journey) and you need 10 launches for each subsequent mission (8 tanker missions for the outbound journey + 2 tanker missions for the return journey).

On the other hand, if you are only going to make it a one-way trip (LEO to NRHO to the lunar surface to NRHO), that same 84-tonne lunar Starship will need only 7.8 km/s of dV, meaning it needs only 598 tonnes of propellant in LEO. Accounting for the 171 tonnes of residuals that lunar Starship will have when it reaches LEO on its initial launch, you only need 3 tanker missions and you'll have 127 m/s of extra margin...hopefully enough to get the now-derelict spacecraft clear of the Lunar Gateway.

So that's 10 launches per mission for a reusable lunar Starship or 4 launches per mission for an expendable lunar Starship. This all assumes the minimum dry mass of a naked Starship is something like 45 tonnes (based on Elon saying that a naked Starship without SL Raptors is 40 tonnes) and that ~39 additional tonnes is enough mass budget for landing legs, paint, solar panels, landing engines, landing propellant, crew capsule, fairing, and cargo.

A landing demonstration mission with no NRHO stopover and no lunar ascent (except perhaps a hop) could probably get away with only 1-2 tanker missions, though I haven’t run the numbers. 

[sevenperforce]

EDIT: I ran the numbers. An 84-tonne lunar Starship requires 5.79 km/s for a landing demonstration mission if it skips NRHO. That’s 333 tonnes of props. Lunar Starship’s LEO residuals plus a single tanker mission will get it to 321 tonnes of props, so if it can shave off some added mass (perhaps by only carrying a mock-up crew capsule and no cargo) it’s doable. 

[tater]

The 84t LSS in GEO needs 151t of props for a direct propulsive trip to LEO. That 85t tanker (full) can deliver the 151t and still come home to LEO with plenty of margin. Or it could deliver 225t, and the LSS has residuals in LEO vs the next flight.

 

18 minutes ago, sevenperforce said:

EDIT: I ran the numbers. An 84-tonne lunar Starship requires 5.79 km/s for a landing demonstration mission if it skips NRHO. That’s 333 tonnes of props. Lunar Starship’s LEO residuals plus a single tanker mission will get it to 321 tonnes of props, so if it can shave off some added mass (perhaps by only carrying a mock-up crew capsule and no cargo) it’s doable. 

I still think lower mass is entirely possible. Every 1mm of steel thickness reduction reduces the mass by ~10t. Engines are 12t. For engines+4mm steel mass is ~44t, for 3mm ~34t, and for 2mm, 24t. Then whatever the crew mass is... and a depot tanker can be thin, but added insulation mass. Depot tanker can be the thing that flies to whatever non-LEO is desirable.

Edited November 19, 2021 by tater

[StrandedonEarth]

30 minutes ago, sevenperforce said:

On the other hand, if you are only going to make it a one-way trip (LEO to NRHO to the lunar surface to NRHO), that same 84-tonne lunar Starship will need only 7.8 km/s of dV, meaning it needs only 598 tonnes of propellant in LEO. Accounting for the 171 tonnes of residuals that lunar Starship will have when it reaches LEO on its initial launch, you only need 3 tanker missions and you'll have 127 m/s of extra margin...hopefully enough to get the now-derelict spacecraft clear of the Lunar Gateway.

And how many more tankers to take the derelict back down to the base for salvage? As opposed to salvaging a crushed beer can stainless steel beer keg.

Edited November 19, 2021 by StrandedonEarth

[tater]

 

 

[tater]

 

 

[mikegarrison]

6 hours ago, tater said:

 

 

Any high school physics student should be able to tell you what the result of an inelastic collision is.

[tater]

3 minutes ago, mikegarrison said:

Any high school physics student should be able to tell you what the result of an inelastic collision is.

DART is set to impact at something like 6.6 km/s. Recent asteroid missions have shown them to be perhaps loosely bound conglomerates, so the result of the impact in terms of dv delivered might be expected to be whatever it is (cm/s?), the impact can also send a bunch of material out into a cloud of ejecta... should be interesting.

[sevenperforce]

1 hour ago, tater said:

1 hour ago, mikegarrison said:

Any high school physics student should be able to tell you what the result of an inelastic collision is.

DART is set to impact at something like 6.6 km/s. Recent asteroid missions have shown them to be perhaps loosely bound conglomerates, so the result of the impact in terms of dv delivered might be expected to be whatever it is (cm/s?), the impact can also send a bunch of material out into a cloud of ejecta... should be interesting.

Indeed.

An inelastic collision may be simple enough…as long as it remains a two-body problem. But a three-body problem has only discrete solutions. And a 999,999-body problem? Well that’s a new bird entirely. 

[magnemoe]

14 hours ago, sevenperforce said:

Moving this discussion with @tater to the correct thread….

Suppose we have an 84-tonne lunar Starship that leaves LEO, picks up crew from Orion in NRHO, goes to the lunar surface, returns crew to Orion, and then returns propulsively to GTO (that's the max burnout mass we can achieve without a tank stretch). If a Starship tanker (which we will notionally say has a dry mass of 85 tonnes and carries 30 tonnes of landing propellant) can deliver 150 tonnes of propellant to LEO, then it can deliver 33 tonnes of propellant to the lunar Starship (assuming a 2.27 km/s GTO burn at 380 s isp).

This gives our lunar Starship, in turn, 1.2 km/s of dV, which it can use to lower its apogee significantly. The next tanker will only need to burn 1.07 km/s out of LEO to meet it, meaning it arrives with 86 tonnes of propellant, giving the lunar Starship enough propellant to reach LEO with 43 tonnes of remaining residuals.

It will need just under eight tanker missions to refill its tanks for the next sortie.

On its initial launch, it would have reached LEO with 171 tonnes of residuals, requiring less than 7 tanker missions to refill.

So you need a total of 10 launches for the first mission (1 lunar Starship + 7 tanker missions for the outbound journey + 2 tanker missions for the return journey) and you need 10 launches for each subsequent mission (8 tanker missions for the outbound journey + 2 tanker missions for the return journey).

On the other hand, if you are only going to make it a one-way trip (LEO to NRHO to the lunar surface to NRHO), that same 84-tonne lunar Starship will need only 7.8 km/s of dV, meaning it needs only 598 tonnes of propellant in LEO. Accounting for the 171 tonnes of residuals that lunar Starship will have when it reaches LEO on its initial launch, you only need 3 tanker missions and you'll have 127 m/s of extra margin...hopefully enough to get the now-derelict spacecraft clear of the Lunar Gateway.

So that's 10 launches per mission for a reusable lunar Starship or 4 launches per mission for an expendable lunar Starship. This all assumes the minimum dry mass of a naked Starship is something like 45 tonnes (based on Elon saying that a naked Starship without SL Raptors is 40 tonnes) and that ~39 additional tonnes is enough mass budget for landing legs, paint, solar panels, landing engines, landing propellant, crew capsule, fairing, and cargo.

A landing demonstration mission with no NRHO stopover and no lunar ascent (except perhaps a hop) could probably get away with only 1-2 tanker missions, though I haven’t run the numbers. 

As I understand moonship stay in moon orbit, its refilled by tankers who can aerobrake returning from the moon, even if that fails you just lost an tanker. 

[Beccab]

8 minutes ago, magnemoe said:

As I understand moonship stay in moon orbit, its refilled by tankers who can aerobrake returning from the moon, even if that fails you just lost an tanker. 

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

[RealKerbal3x]

Great render:

[magnemoe]

5 minutes ago, RealKerbal3x said:

Great render:

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.