totm nov 2023 SpaceX Discussion Thread

[AVaughan]

On 10/2/2019 at 3:56 AM, sevenperforce said:

There's some math to be done here, I think.

We don't know what the TPS will weigh, yet, but let's say it comes in close to the shuttle high-temp reusable surface insulation (the black stuff) which was 9.2 kg per square meter. Covering half of a 45-meter-high, 9-meter-wide cylinder with this would come to 6.44 tonnes. A little more if the Starship TPS is heavier; a little less if it isn't. We'll assume that a returning aerobraking pass would be high-altitude enough to control with hot-gas thrusters alone; 2-3 passes are fine if needed.

Let's go with 80 tonnes and give it regular tanks. Some reference numbers (I'm using <> to indicate propulsive 1-way and >) to indicate aerobraking one-way):

• GTO-1 <> LEO = 2.27 km/s; GTO-1 >) LEO = 0.02 km/s
• EML-1 <> LEO = 3.77 km/s; EML-1 >) LEO = 0.77 km/s
• EML-2 <> LEO = 3.43 km/s; EML-2 >) LEO = 0.33 km/s
• LLO <> LEO = 4.04 km/s; LLO >) LEO = 1.31 km/s
• Earth escape <> LEO = 3.22 km/s; Earth escape >) LEO = 0.02 km/s
• GEO <> LEO = 4.33 km/s; GEO >) LEO = 2.06 km/s
• Jovian Transfer <> LEO = 8.8 km/s; Jovian Transfer >) LEO = 3.06 k/ms

Thus round-trip payload capacity is as follows for going propulsive only:

• GTO: 1232 tonnes
• EML-1: 479 tonnes
• EML-2: 592 tonnes
• LLO: 402 tonnes
• EE: 674 tonnes
• GEO: 330 tonnes
• Jovian transfer: -4 tonnes

And for using TPS (assuming 10 tonnes to be safe):

• GTO: 1268 tonnes
• EML-1: 532 tonnes
• EML-2: 645 tonnes
• LLO: 452 tonnes
• EE: 727 tonnes
• GEO: 373 tonnes
• Jovian transfer: 89 tonnes

So adding 10 tonnes of TPS offers a distinct advantage to any destination, with a bigger advantage for higher-energy orbits. Obviously these payloads are vastly bigger than what would ordinarily be delivered, even with LEO orbital assembly, so the tug would almost never need to be fully fueled. But TPS wins in every situation...moreso if it is not quite 10 tonnes.

The fact that the tug is SO overpowered for every single use inside the Earth-moon system is why I am not really so sure it is the best choice to begin with. It would be useful for BLEO payloads but those will be rarer (and anything going to Mars will need full TPS anyway) so why bother?

Personally I think that for any mission beyond about 4 years mission duration (eg Jupiter/Saturn) the vehicle should probably be considered expended, even if the mission requires returning the vehicle (or at least part of the vehicle, eg sample/crew return) to Earth. 

Why?  Because at the rate SpaceX iterates, after 4+ years the vehicle will probably be considered obsolescent anyway, and there is a reasonable chance SpaceX would decide to scrap it rather than modernise/refurbish it.  (Would it even be practical to modernise/refurbish a space only vehicle in space?  Given the difficulty of even doing a thorough inspection, my guess is they would just scrap it).

The default Starship design should be able to handle pretty much all our needs in Earth/Lunar space for the next few years, so no need to develop a custom "space only" vehicle derived from a full size Starship for those use cases (at least for the next few years).  For Mars, you want the TPS + flaps/wings/plasma deflectors anyway.   For Jupiter/Saturn or beyond, if the mission is one way, then just build an expendable third stage (possible based on either a Vac Raptor or an RL-10).  If it is sample return mission, then a return capsule is probably a simpler and safer choice than using a custom Starship that can only aerobrake into Earth orbit.  (Remember that the custom Starship won't see a lot of use, so it will have only limited testing compared to a regular Starship, so if you really need that much payload capacity for the return, just go with the regular and better tested Starship design).

So if the only remaining use case I can see is a one way mission somewhere that requires a payload big enough to justify expending a Starship.  If they want to do a customised expendable version of Starship without any TPS/wings/flaps/plasma defectors or landing legs, then that might make a small amount of sense.  (Replacing the SL raptors with a Vac Raptor might also make sense, but the savings might not be worth the cost of validating the new design).   But for the foreseeable future, I can't see a use case for a space only Starship that is intended to aerobrake into Earth orbit, and remain in space.   (Most likely scenario I can think of where it make sense is if we actually build a colony on the Moon.  Then there might be enough cargo to the Moon to justify a specialised version of Starship that shuttles between LEO and Lunar orbit).

 

[tater]

Cool fan renders:

 

[Nothalogh]

You know, I just realized something.

>great big two stage steel rocket

 

LOL, this is the revenge of the Sea Dragon, in many ways.

[Rakaydos]

3 minutes ago, Nothalogh said:

You know, I just realized something.

>great big two stage steel rocket

 

LOL, this is the revenge of the Sea Dragon, in many ways.

Slowpoke.jpg

Starship steals from ALL the best ideas. Sea Dragon was the classic "Big dumb rocket", but SpaceX made it a "Big Smart Rocket" by combining it with the DC-X, orbital fuel depots, and the idea behind MEMS Engine Superclusters. (albet at full engine scale, not microchip scale)

[Xd the great]

2 hours ago, Rakaydos said:

Slowpoke.jpg

Starship steals from ALL the best ideas. Sea Dragon was the classic "Big dumb rocket", but SpaceX made it a "Big Smart Rocket" by combining it with the DC-X, orbital fuel depots, and the idea behind MEMS Engine Superclusters. (albet at full engine scale, not microchip scale)

30 something engines. Sounds familiar.

Modular design is key here.

If a big dumb rocket can land itself, is it still dumb?

[Wjolcz]

Do we have any info on how well the heat shield tile did?

Edited October 6, 2019 by Wjolcz

[ThatGuyWithALongUsername]

1 hour ago, Wjolcz said:

Do we have any info on how well the heat shield tile did?

Yep

 

[Rakaydos]

8 hours ago, Xd the great said:

30 something engines. Sounds familiar.

https://www.technologyreview.com/s/401025/pocket-rockets-pack-a-punch/

[Xd the great]

1 hour ago, ThatGuyWithALongUsername said:

 

 

Hmm. Was it on the leeward or the windward side? It was shielded from the heat already by dragon's heat shield.

[magnemoe]

13 minutes ago, Xd the great said:

Hmm. Was it on the leeward or the windward side? It was shielded from the heat already by dragon's heat shield.

it was on on the heatshield, probably just added on top.  

[tater]

 

[sevenperforce]

On 10/4/2019 at 6:07 PM, mikegarrison said:

Kind of. Turbofan get efficiency in two ways. 1) They move more air, but slower. This increases propulsive efficiency. 2) The turbine makes use of heat energy that would otherwise be wasted. This increases thermodynamic efficiency.

This is actually an "eductor". It does try to improve the propulsive efficiency by using the same "move more air but slower" trick. However, it doesn't capture the lost heat energy.

I am fairly certain the ejector duct also improves propulsive heat energy by mixing the untapped heat of the exhaust with the additional working mass. This is a (lesser) limiting factor on air augmentation, in fact; at maximum speed, the intake air is bringing more heat than you can add with exhaust mixing.

The one thing an ejector duct does not accomplish is run a compressor turbine. For that, you'd need a turborocket.

EDIT: Okay, I see this is what you were saying all along.

On 10/5/2019 at 8:39 AM, AVaughan said:

Personally I think that for any mission beyond about 4 years mission duration (eg Jupiter/Saturn) the vehicle should probably be considered expended, even if the mission requires returning the vehicle (or at least part of the vehicle, eg sample/crew return) to Earth. 

Why?  Because at the rate SpaceX iterates, after 4+ years the vehicle will probably be considered obsolescent anyway, and there is a reasonable chance SpaceX would decide to scrap it rather than modernise/refurbish it.  (Would it even be practical to modernise/refurbish a space only vehicle in space?  Given the difficulty of even doing a thorough inspection, my guess is they would just scrap it).

So if the only remaining use case I can see is a one way mission somewhere that requires a payload big enough to justify expending a Starship.  If they want to do a customised expendable version of Starship without any TPS/wings/flaps/plasma defectors or landing legs, then that might make a small amount of sense.  (Replacing the SL raptors with a Vac Raptor might also make sense, but the savings might not be worth the cost of validating the new design).   But for the foreseeable future, I can't see a use case for a space only Starship that is intended to aerobrake into Earth orbit, and remain in space.

To be clear, none of the mission profiles I presented above were 4+ year missions. They were all missions of less than 2 weeks. I was analyzing how to use the vehicle for the Earth escape burn and then brake back into orbit after payload jettison.

On 10/5/2019 at 8:21 AM, Xd the great said:

On 10/5/2019 at 7:27 AM, AVaughan said:

Can a single Vac Raptor throttle low enough for that landing?  (I'm too short on time atm to do the math.  I'm only have enough time to skim through a weeks worth of posts in this thread).  They could do a hoverslam, (zero all velocity 0.5 m above the surface, cut the engine and just drop), but a controlled hover and landing is probably preferable.  They might also be able to build a smaller Raptor, but that would take up valuable design and testing time.  Alternatively would a cluster of hot gas (methane/oxygen) rcs engines have enough thrust for the actual touch down?

No. 1 raptor has a thrust of 2000kN.

1 starship mass is 110 tonnes.

Moon gravity is 1/6th of that of earth.

Weight of empty starship is about 180kN.

Raptor min throttle is about 50%. Or 1000kN. (Might be lower, I forgot).

You're right, it's 50%.

I have seen some nice ideas about using hot-gas thrusters as dedicated landing engines for the moon and perhaps Mars, damn the cosine losses. You don't need that many. If they are ten metric tonne thrusters then they are each more powerful than a SuperDraco. IIRC you need 6-8 to land a Starship on the moon with sufficient propellant for Earth return.

I'd really like to see hot-gas thrusters arranged thus double as launch abort motors.

Edited October 7, 2019 by sevenperforce
Understanding

[RCgothic]

Can it even land on one engine?

Also that's mass without payload. Starship would be delivering a payload to the moon, it wouldn't be landing empty.

But even on one engine and with a payload, the payload won't be 5x the dry mass. So yes, starship would have to hoverslam.

But at the same time the thrust requirement is approx 1/10 a raptor at full thrust even without payload, so I think it's pretty safe to say starship won't be hovering on RCS hot gas thrusters.

[Rakaydos]

15 minutes ago, RCgothic said:

Can it even land on one engine?

Also that's mass without payload. Starship would be delivering a payload to the moon, it wouldn't be landing empty.

But even on one engine and with a payload, the payload won't be 5x the dry mass. So yes, starship would have to hoverslam.

But at the same time the thrust requirement is approx 1/10 a raptor at full thrust even without payload, so I think it's pretty safe to say starship won't be hovering on RCS hot gas thrusters.

Once hoverslam is in the equation, lunar landings get much easier.

Hovering on the moon with the same engine as on earth requires 6 times the throttling capability.

Hoverslamming on the moon, for the exact same profile as on earth, requires a reduction in deceleration of less than 1 g. (5/6 of a g)

If it can do a 2 g hoverslam on earth, and hover (1g) on earth, it can do a 1 1/6g hoverslam on the moon. If it can do a 3g hoverslam on earth, it can do a 2g hoverslam on the moon.

Edited October 7, 2019 by Rakaydos

[sevenperforce]

12 minutes ago, RCgothic said:

Can it even land on one engine?

Also that's mass without payload. Starship would be delivering a payload to the moon, it wouldn't be landing empty.

But even on one engine and with a payload, the payload won't be 5x the dry mass. So yes, starship would have to hoverslam.

But at the same time the thrust requirement is approx 1/10 a raptor at full thrust even without payload, so I think it's pretty safe to say starship won't be hovering on RCS hot gas thrusters.

Notionally, it would do 99% of the burn on the three Vacuum Raptors at close to minimum throttle, then ignite one of the core SL engines for the actual landing. All three core engines can gimbal through the CoM and hot-gas thrusters would be used to correct attitude.

I calculated over in the horizontal-landing thread that you need nine and a half hot-gas thrusters to land, factoring in cosine losses. Completely doable.

[tater]

 

[sh1pman]

10 minutes ago, tater said:

Can’t wait until BO launches something to orbit. They’ve done a great job at being a “looming threat” for years, but if they keep advancing slowly but surely for another year or so, Starship will come and eat their lunch. And everyone else’s lunch too.

[tater]

Mk2 transport horizontally. Mass 135 tons. Shows a cylinder, but the length equals total height. Mass might or might not include fins, if we see any go on in FL, then clearly with fins. Still, they have the mass lower (no engines) than Mk1's 200 tons.

[CatastrophicFailure]

Buildup of the new Starship pad over the last couple weeks:

 

Edited October 7, 2019 by CatastrophicFailure

[mikegarrison]

37 minutes ago, sh1pman said:

Can’t wait until BO launches something to orbit. They’ve done a great job at being a “looming threat” for years, but if they keep advancing slowly but surely for another year or so, Starship will come and eat their lunch. And everyone else’s lunch too.

Maybe.

Giving yourself the freedom to fail can mean quick advances. But it can also mean failure.

[Barzon]

luka!

[sevenperforce]

51 minutes ago, sh1pman said:

Can’t wait until BO launches something to orbit. They’ve done a great job at being a “looming threat” for years, but if they keep advancing slowly but surely for another year or so, Starship will come and eat their lunch. And everyone else’s lunch too.

I suppose they could launch something to orbit now. If they fired the capsule escape motor at MECO that's another 625 m/s, and then New Shepard's capsule can carry up to 410 kg, which is plenty to get into a cubesat into orbit if they used their BE-2 engine.

[tater]

11 minutes ago, sevenperforce said:

I suppose they could launch something to orbit now. If they fired the capsule escape motor at MECO that's another 625 m/s, and then New Shepard's capsule can carry up to 410 kg, which is plenty to get into a cubesat into orbit if they used their BE-2 engine.

How much would they need to get to orbit from a NS? another 6-7 km/s? They could likely make a small US that would do the job, capsule is what, ~6t? Even if 1t was dry mass (stage plus small payload), they could make orbit with an I_sp of around 370 and 5t of props.

29 minutes ago, mikegarrison said:

Maybe.

Giving yourself the freedom to fail can mean quick advances. But it can also mean failure.

Totally different cultures at the two companies, that's for sure.

I agree with Koenigsmann that SpaceX is ahead, and I am hopeful about the prospects of SS, though it could see many setbacks.

BO will fly, and I think they are deeply serious about what they are up to. SS is certainly a short term threat to them, but Blue has the case to learn from NG, and move on, and if SpaceX proves true stage 2 operational reuse (land, refill, refly---like aircraft) then I think the path is much more straightforward for BO to follow in a fast way (once they have experience with NG).

[RealKerbal3x]

4 minutes ago, tater said:

How much would they need to get to orbit from a NS? another 6-7 km/s? They could likely make a small US that would do the job, capsule is what, ~6t? Even if 1t was dry mass (stage plus small payload), they could make orbit with an I_sp of around 370 and 5t of props.

Maybe if they expended the New Shepard. That would give them some more margin.

Edited October 7, 2019 by RealKerbal3x

[tater]

 

4 minutes ago, RealKerbal3x said:

Maybe if they expended the New Shepard. That would give them some more margin.

That would defeat the purpose, IMO. That said, I think they need more than 6-7, F9 stages at something like NS max alt in rough terms, but at 2.1+km/s. NS is effectively going 0 at that altitude. So they might need nearly 100% of orbital velocity for a stage 2.

770kg dry mass (6t wet), and a hydrolox engine would give them 9km/s.