totm nov 2023 SpaceX Discussion Thread

[tater]

 

Edited October 3, 2019 by tater

[tater]

On 10/1/2019 at 11:56 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?

Definitely not enough thrust with a 3-atm pressure-fed engine.

If you add another Raptor up there, you're golden. Or you could put all the RCS bottles up there and use repurposed 10-tonne RCS thrusters for escape engines.

Powered landing would have too many variables and require too much dV. Better to do a splashdown.

Partially wanted this up front where I can find it easier, partially... likes work!

[tater]

 

NET date is Nov 23, FWIW.

[RealKerbal3x]

It doesn’t look like that booster has recovery hardware...there’s little chance it will survive anyway.

[magnemoe]

1 hour ago, tater said:

 

use an large mold I guess. 
The issue is that casing titanium is not very common. its more used for low weight and you roil or forge it to increase that
Spacex use it for heat resistance and casting is easy. 
We make cast much much  larger items in cast iron and has done for an long time. 
 

[tater]

^^^dunno if that one got posted.

Also:

 

[Guest]

Seems like an aggressive testing progression to go to orbit without first doing some lower sideways boost back and coast to test the control surfaces... unless they plan on testing them in the 20km flight, which would mean it’s not just going to be a hop but some kind of belly glide for some of it. 

[sevenperforce]

12 minutes ago, Dale Christopher said:

Seems like an aggressive testing progression to go to orbit without first doing some lower sideways boost back and coast to test the control surfaces... unless they plan on testing them in the 20km flight, which would mean it’s not just going to be a hop but some kind of belly glide for some of it. 

The 20 km test is not a hop. Starship doesn't have enough propellant to go to 20 km and back under constant thrust. The 20 km test will be a high-gee vertical launch, apogee 20 km, kill the engines, and skydive back to the landing site for the turn-and-burn to land.

1 hour ago, magnemoe said:

use an large mold I guess. 
The issue is that casing titanium is not very common. its more used for low weight and you roil or forge it to increase that
Spacex use it for heat resistance and casting is easy. 
We make cast much much  larger items in cast iron and has done for an long time. 

Elon is saying they will use welded steel rather than titanium (F9B5) or Al (F9B2-3).

1 hour ago, RealKerbal3x said:

It doesn’t look like that booster has recovery hardware...there’s little chance it will survive anyway.

It likely will not. An abort at Max-Q is almost definitely not survivable for the booster, as the sudden aerodynamic shear will either frag it directly or cause it to tumble and break up. 

If it turns out that they have some aluminum grid fins, I suppose they will program a landing test routine in case by some miracle it survives.

[CatastrophicFailure]

5 minutes ago, sevenperforce said:

The 20 km test is not a hop. Starship doesn't have enough propellant to go to 20 km and back under constant thrust. The 20 km test will be a high-gee vertical launch, apogee 20 km, kill the engines, and skydive back to the landing site for the turn-and-burn to land.

Even so, I’d be pretty darned surprised if they go directly for orbit even after a fully successful 20km flight. They’ve got two ships right now, after all, and are prepping to build two more before they even start the first one that would go to orbit, according to Musk. Tho I’m sure they’re expecting to lose some along the way. Sadly, I don’t think Mk 1 that we saw the other night in all its shiny glory will make it to the Smithsonian or anything.

8 minutes ago, sevenperforce said:

Elon is saying they will use welded steel rather than titanium (F9B5) or Al (F9B2-3).

Quick reference: titanium has a melting point just below 1700c, 301 stainless is “only” around 1400.

[sevenperforce]

4 hours ago, Rakaydos said:

If I recall the EDA video right, the problem isn't the nozzle, the problem is the combustion chamber.  Aerospikes simply have a harder time getting the same combustion efficiencies and still feed an aerospike's angled nozzle, without having a combustion chamber hanging off to the side penalizing the thrust-to-area ratio.

Right, you need independent chambers so you have a nice well-understood spherical or cylindrical combustion zone.

As far as thrust-to-area ratio is concerned...well, let's see. SL Raptor is 1.3 m at the bell but 0.47 meters just below the throat. They are 1.2 meters long from the top of the turbopump assembly to just below the throat. If truncated-bell Raptors were arranged in an inward-facing circle as I showed above, inscribing an outer diameter of 10 meters (the size of Superheavy's skirt), the throats would form a circle 23.9 meters in circumference, more than enough space to fit fifty Raptors. Granted, there's going to need to be some necessary spacing, but that's significant. Plus, if they were angled/canted down as would most likely be the case, you'd have even more space.

1 minute ago, CatastrophicFailure said:

Even so, I’d be pretty darned surprised if they go directly for orbit even after a fully successful 20km flight. They’ve got two ships right now, after all, and are prepping to build two more before they even start the first one that would go to orbit, according to Musk. Tho I’m sure they’re expecting to lose some along the way. Sadly, I don’t think Mk 1 that we saw the other night in all its shiny glory will make it to the Smithsonian or anything.

I find it hard to believe only because I think they will fly Mk1 well before they have the first Superheavy built.

[Guest]

That was some really good journalism from Marina Koren, the questions were relatively hard hitting, I’m not used to seeing legitimate reporting these days but that interview was top notch 

Thanks for the link.

also crazy that they are going to 20km and skydiving back on the first test. That’s going to be an amazing launch to watch! 

[sevenperforce]

3 hours ago, tater said:

I tend to agree with Musk that it's an SSTO solution, though. What's the best they'd get on I_sp improvements as a function of altitude when the booster is only going to take them to 2.X km/s? I suppose a decent chunk of the orbital velocity component due to the booster is at higher alt, but that moves the I_sp towards 380 from 330, and I think Musk already said that the vac I_sp of SL Raptor is 350. So it's basically to make a plug engine to buy some fraction of 30s of I_sp for a few tens of seconds, right?

Well he basically said there would need to be a reason. This reduces mass, allows fitting more engines (not fewer) into the same space, and also allows for some potential air augmentation flow, which would raise I_sp during early boost as well. Good reasons, even if not SSTO reasons.

[Guest]

Was also really impressed by Elon’s thoughts on engineering in that Tim Dodd interview. (The one where he was asked about aerospikes). 

[sevenperforce]

2 minutes ago, Dale Christopher said:

Was also really impressed by Elon’s thoughts on engineering in that Tim Dodd interview. (The one where he was asked about aerospikes). 

Agreed. Wished I'd heard more about the Starship itself but the geekspeak was great.

[tater]

9 minutes ago, sevenperforce said:

Well he basically said there would need to be a reason. This reduces mass, allows fitting more engines (not fewer) into the same space, and also allows for some potential air augmentation flow, which would raise I_sp during early boost as well. Good reasons, even if not SSTO reasons.

True, but the net result (though air augmentation could be far more substantial here) is a higher average Isp. It's not going to beat 380, so we've got 50s to play with, and the average over S1 boost might be closer to 350 than 330. If the average Isp is 330, S1 has ~3600m/s dv. If the average is 350, it's 3800, 360 it's 3900, and at 370 it's 4000m/s. 10s buys you 100 m/s on the stack. Alternately, you cut the S1 prop mass to get back to nominal dv for S1. I'm just not seeing the huge cost savings with such marginal improvements (on a TSTO).

One other thing I don't know about is how few engines you can run in that plug config for landing.

[sevenperforce]

17 minutes ago, tater said:

True, but the net result (though air augmentation could be far more substantial here) is a higher average Isp. It's not going to beat 380, so we've got 50s to play with, and the average over S1 boost might be closer to 350 than 330. If the average Isp is 330, S1 has ~3600m/s dv. If the average is 350, it's 3800, 360 it's 3900, and at 370 it's 4000m/s. 10s buys you 100 m/s on the stack. Alternately, you cut the S1 prop mass to get back to nominal dv for S1. I'm just not seeing the huge cost savings with such marginal improvements (on a TSTO).

One other thing I don't know about is how few engines you can run in that plug config for landing.

The outer engines are not throttleable anyway, so Superheavy will only ever use its six core engines for landing.

Static entrainment in the air-augmentation configuration produces a 15% thrust boost, which gets you 379.5 s on the pad. It only goes up from there until about Mach 4, where it starts to drop off (but by that time you're out of the bulk of the atmosphere anyway). What dry mass numbers are you using for Superheavy?

I posted the same image to NSF, and someone pointed out that the nozzle would be under compression and would require reinforcement against buckling, which is something I did not think about.

Arguably the same approach could be used with an "inside-out" aerospike, where the nozzles point outward rather than inward and the nozzle is more like a hoopskirt around the base.

Edited October 3, 2019 by sevenperforce

[tater]

30 minutes ago, sevenperforce said:

What dry mass numbers are you using for Superheavy?

Musk said the stack was 5000t, so I subtracted SS, and called SH dry mass 9%, then looked at the dv of the stack.

Yeah, air augmentation is a much bigger deal than just the plug (hence the col turbo rocket). Even so, you run out of air fairly quickly, though I suppose what you buy with margin is the ability to either do a boostback with a higher staging velocity, or add an entry burn (same thing, put more smash on S2).

 

[XB-70A]

On 10/2/2019 at 5:40 AM, sh1pman said:

Look how the Starhopper broke through the concrete pad floor on landing.

 Either the pad was expendable, or the landing was harder than expected. So if Starhopper can break concrete, won’t the Starship’s tiny new legs just stick into the ground on Moon and Mars like knives into butter?

 

It didn't went really well for the man who took that picture...

More of his pics of Hopper are available here: https://imgur.com/a/TjJd6XR

[Guest]

Thanks for the link @XB-70A! 

Interesting story behind that pic!

I think it’s safe to say that trespassing on private property, photographing their prototypes and then publicising the pics isn’t the best way to develop a good relationship with SpaceX as he claims to want >_<...

whats more interesting is that landing seems a lot harder than previous pics suggested, making the change in exhaust colour before landing seem definitely not norminal. We may have come very close to seeing some fireworks, which makes me wonder if there is more to hopper’s abrupt retirement than meets the eye 

[StrandedonEarth]

Sounds like Jim is trying to patch things up...

 

[tater]

23 minutes ago, StrandedonEarth said:

Sounds like Jim is trying to patch things up...

 

Spoiler

I just hope this doesn't inspire fan art like the whole Kirk/Spock stuff (don't google it, seriously).

 

Edited October 4, 2019 by tater

[Wjolcz]

So, because Elon said that "if 20km works then orbit" and they want to achieve orbit with Mk5 then I'm assuming they are expecting at most 4 spectacular RUDs.

Hopefully the Mk1 will face towards the ocean when doing the flip-and-land bottle trick.

[ThatGuyWithALongUsername]

1 hour ago, Wjolcz said:

So, because Elon said that "if 20km works then orbit" and they want to achieve orbit with Mk5 then I'm assuming they are expecting at most 4 spectacular RUDs.

Hopefully the Mk1 will face towards the ocean when doing the flip-and-land bottle trick.

Well... Mk3, Mk4, or Mk5. He's mentioned all of them (?). So yeah, I'm guessing that's what's up with that.

 

Also ramen that the even numbered Starships in Florida are kind of in competition with the Boca Chica odd numbered Starships. So Mk1 and Mk2 would both do 20km, if they succeed then Mk3 and Mk4 would make orbit, one of them would be first depending on the team. Mk5 if one of them blows up.

But I'm still confused... they need the booster for orbit, right? Didn't Elon say they wouldn't build those until after Mk3 and Mk4 are complete? Wouldn't that provide a few solid months of testing Mk3 and Mk4?

[Wjolcz]

1 hour ago, ThatGuyWithALongUsername said:

But I'm still confused... they need the booster for orbit, right? Didn't Elon say they wouldn't build those until after Mk3 and Mk4 are complete? Wouldn't that provide a few solid months of testing Mk3 and Mk4?

Wouldn't surprise me if they skipped one or two of them and built the booster from their parts instead.

[sevenperforce]

43 minutes ago, ThatGuyWithALongUsername said:

Well... Mk3, Mk4, or Mk5. He's mentioned all of them (?). So yeah, I'm guessing that's what's up with that.

Also ramen that the even numbered Starships in Florida are kind of in competition with the Boca Chica odd numbered Starships. So Mk1 and Mk2 would both do 20km, if they succeed then Mk3 and Mk4 would make orbit, one of them would be first depending on the team. Mk5 if one of them blows up.

But I'm still confused... they need the booster for orbit, right? Didn't Elon say they wouldn't build those until after Mk3 and Mk4 are complete? Wouldn't that provide a few solid months of testing Mk3 and Mk4?

It may be that the process of bent-metal construction is much quicker than the process of actually outfitting and installing everything they need, and so Elon is expecting construction on Superheavy to be well underway by the time Mk1 is actually ready for its first flight.

If they start bending metal for Mk3 in the next few weeks and they shave construction time in half (single-weld rings, lessons learned), Boca Chica could complete it and begin construction of their first Superheavy in as little as four months. Depending on when Mk1 flies, it may take that long to analyze the data and make any necessary adjustments to Mk3's software or hardware; meanwhile Superheavy will complete.

15 hours ago, tater said:

Musk said the stack was 5000t, so I subtracted SS, and called SH dry mass 9%, then looked at the dv of the stack.

Yeah, air augmentation is a much bigger deal than just the plug (hence the col turbo rocket). Even so, you run out of air fairly quickly, though I suppose what you buy with margin is the ability to either do a boostback with a higher staging velocity, or add an entry burn (same thing, put more smash on S2).

The 10-meter "skirt" at the base is just begging to be modded into an airflow duct. I wonder if they would be able to open a flow path around the lower bulkhead such that air was uniformly directed into the gap between the core engines and the outer engines.

Static entrainment would not be as significant because the inlet is too far from the exhaust. Part of me thinks that this configuration would effectively create a "virtual nozzle" with air trapped between the inner and outer exhaust plumes...but perhaps without a continued physical duct that added thrust would not be transferred to the base of Superheavy at all.