totm nov 2023 SpaceX Discussion Thread

[tater]

8 minutes ago, cubinator said:

I think the extended mass inside that fairing would be the cause of that penalty.

With the exception of Starlink sats, I think most anything under a fairing is volume limited more than mass limited. The added mass of the extended fairing comes out of the payload mass, and it likely has a drag penalty as well (probably minor).

[RealKerbal3x]

The BN1 thrust dome looks to have 20 holes around its outer skirt. They don't seem to be plumbed for engines but with a full set of 8 on the inner ring that's spots for 28 Raptors!

 

 

[wumpus]

Buried lede?

From what I've heard, the Falcon 9 fairing is one of the biggest limitations  of the Falcon Heavy (the other being the mass stress limitations of the Falcon 9, making the maximum total mass unlikely to be much more than the maximum expendable mass to LEO of the Falcon 9).  But between this and the classified USSF-67 mission, they have found a customer willing to pay to have this limitation removed (at hundreds of millions of dollars).

Presumably Spacex intends to obsolete Falcon Heavy with Starship as fast as they can, but it is nice to see that a launch vehicle that is already in production and relatively inexpensive get more capability.

49 minutes ago, cubinator said:

Center core gets expended anyway even if it lands because it's cursed

They better get the hang of it, because I'm sure Super Heavy Booster will come down as least as hot (perhaps it will have to assume the "skydiver position" like Starship).  On the other hand, I've never seen any mention of the difference between 1/3rd expendable Falcon Heavy vs. full recovered or fully expended, but I strongly suspect that it has close to the capability of fully expended.

[RealKerbal3x]

1 minute ago, wumpus said:

They better get the hang of it, because I'm sure Super Heavy Booster will come down as least as hot (perhaps it will have to assume the "skydiver position" like Starship).

Super Heavy will do a boostback burn and return to the launch site. It'll also separate from Starship even earlier into flight than Falcon 9's first stage. So it'll enter the atmosphere slower than the slowest Falcon 9 reentry. 

[kerbiloid]

1 hour ago, RealKerbal3x said:

The BN1 thrust dome looks to have 20 holes around its outer skirt.

24, like in KSP local standard.

Isn't it a proof that they use KSP-proven designs?

[zolotiyeruki]

4 hours ago, wumpus said:

Presumably Spacex intends to obsolete Falcon Heavy with Starship as fast as they can, but it is nice to see that a launch vehicle that is already in production and relatively inexpensive get more capability.

For stuff going beyond LEO, wouldn't SS's payload have to include whatever stage is necessary to take the real payload to its final destination?  Or are SpaceX planning to build SS so that it can re-enter from GTO?

[StrandedonEarth]

1 minute ago, zolotiyeruki said:

For stuff going beyond LEO, wouldn't SS's payload have to include whatever stage is necessary to take the real payload to its final destination?  Or are SpaceX planning to build SS so that it can re-enter from GTO?

Well, eventually it’s supposed to be able to re-enter from Mars...

they’d probably have to run the numbers to see what is cheaper: include the kick stage as payload or send up the tankers. Or send up a bare F9, dock the payload to the second stage and use that for the kick.... Okay, that defeats the purpose of SS. 

Really, it would depend on the mass of the payload. But SS is designed for BLEO work, although I suppose LEO tankers could have a lighter/thinner heat shield

[sevenperforce]

47 minutes ago, zolotiyeruki said:

For stuff going beyond LEO, wouldn't SS's payload have to include whatever stage is necessary to take the real payload to its final destination?  Or are SpaceX planning to build SS so that it can re-enter from GTO?

Starship is supposed to be able to re-enter from Mars, so re-entry from GTO should be no problem. Starship could even do direct-to-GEO missions for most comsats, although I'm not sure if it can manage that without refueling. GTO payload without refueling is just 21 tonnes. 

For science payloads going out of Earth's SOI entirely, a Starship could refuel in LEO and then do the burn to wherever -- Mercury, Jupiter, Saturn, Pluto, anything. Starship would eject the payload immediately after the burn, flip over, and do a retrograde burn to bring apoapsis back inside Earth's SOI. The Starship could then aerobrake back.

For massively huge payloads, SpaceX could do an expendable Starship with just three RVacs and no heat shield or flaps, and then refuel it in elliptical orbit. Huge burn at periapsis. Could easily send a ten-tonne payload out of the solar system without gravity assists.

[Clamp-o-Tron]

1 minute ago, sevenperforce said:

For massively huge payloads, SpaceX could do an expendable Starship with just three RVacs and no heat shield or flaps, and then refuel it in elliptical orbit. Huge burn at periapsis. Could easily send a ten-tonne payload out of the solar system without gravity assists.

IIRC, SS needs the sea-level Raptors firing for the first 1/3 or so of its flight to orbit, because the TWR on just Rvacs is too low for the trajectory SH would put it on. Your concept stands, though.
Maybe some nuclear-ion or just straight NTR propulsion for more dV, so SS could just go to near-TLI, deploy the kickstage, divert to a Lunar encounter, then take a free return and aerobrake back to LEO, giving it more capacity then having to turn around.

[Ultimate Steve]

52 minutes ago, sevenperforce said:

For massively huge payloads, SpaceX could do an expendable Starship with just three RVacs and no heat shield or flaps, and then refuel it in elliptical orbit. Huge burn at periapsis. Could easily send a ten-tonne payload out of the solar system without gravity assists.

You are underselling this. It's a tweet, so take it with a grain of salt, but a while back elon mentioned that this theoretical expendable upper stage could have a dry mass as low as 40 tons. This (1340t wet 140t dry (100t payload 40t dry 1200 prop load) 382s) gives about 8.4km/s of delta v, just shy of the 9km/s solar escape velocity from LEO.

Not even going to elliptical orbit, with just LEO refueling, expendable starship can deliver 100 tons of payload anywhere in the solar system except pluto, although for uranus and neptune it won't exactly be quick (you'd probably want to do elliptical refueling there).

[Spaceman.Spiff]

6 minutes ago, Ultimate Steve said:

Not even going to elliptical orbit, with just LEO refueling, expendable starship can deliver 100 tons of payload anywhere in the solar system except pluto, although for uranus and neptune it won't exactly be quick (you'd probably want to do elliptical refueling there).

Hopefully that means they could bring along a ton of storable prpellants and better science and cameras.

Lets see a capture at Neptune! We have the technology!

Take me some frikin pictures! Yeeeah!

[tater]

Raptor thrust should be fine, the solution if they needed TWR would be more Rvacs (staging is already effectively at 0 ambient pressure). SS only needs SL raptors for landing.

[sevenperforce]

3 hours ago, Clamp-o-Tron said:

IIRC, SS needs the sea-level Raptors firing for the first 1/3 or so of its flight to orbit, because the TWR on just Rvacs is too low for the trajectory SH would put it on. Your concept stands, though.
Maybe some nuclear-ion or just straight NTR propulsion for more dV, so SS could just go to near-TLI, deploy the kickstage, divert to a Lunar encounter, then take a free return and aerobrake back to LEO, giving it more capacity then having to turn around.

Elon said it could be done with three RVacs and nothing else, so I'm assuming SH would do a lofted trajectory. It would be less efficient for the initial launch, sure, but more efficient for the final burn.

[xebx]

Starship only need 3 Raptor Vac and 3 Raptor SL (SL can be shut down at around 4500m/s) to deliver approximately :

120t (high DeltaV loss = SH come back to launch pad)

or 180t (low-med DeltaV loss = SH land on a boat)

 or 240t (no recovery).

(SH : 69MN, 215t dry weight, 3400t fuel, SLT=1.35; Starship : 105t dry weight, 1170t fuel, Twr=0.98 at staging, once in orbit 3 Raptor Vac can do all the job even fully refueled; DeltaV Starship 8700 m/s+11t fuel for landing)

Edited February 12, 2021 by xebx

[RyanRising]

6 hours ago, tater said:

Raptor thrust should be fine, the solution if they needed TWR would be more Rvacs (staging is already effectively at 0 ambient pressure). SS only needs SL raptors for landing.

If that’s the case, why does the Moonship concept have SL raptors? It also operates only in vacuum, so what would it need that capability for that a deep-space Starship wouldn’t?

[magnemoe]

21 minutes ago, RyanRising said:

If that’s the case, why does the Moonship concept have SL raptors? It also operates only in vacuum, so what would it need that capability for that a deep-space Starship wouldn’t?

They need more than the 3 vacuum engines to reach orbit of earth efficient, the center SL engines is also important for safety as if one of the vacuum engines fails you need engines with gimbal to compensate. Guess they also design the controll system for 3 vacuum and 3 SL engines and changing this will require making an new autopilot. 

[RyanRising]

Just now, magnemoe said:

They need more than the 3 vacuum engines to reach orbit of earth efficient, the center SL engines is also important for safety as if one of the vacuum engines fails you need engines with gimbal to compensate. Guess they also design the controll system for 3 vacuum and 3 SL engines and changing this will require making an new autopilot. 

Possibly, but ISTM that you could remove the sea-level engines in the middle and add 3 more RVacs around the perimeter, for 6-fold symmetry. If one of those fails, you can shut down the opposing one (though that will result in lower total thrust), and if it doesn't fail you have a higher specific impulse throughout the ascent. That's what I think was being suggested for this upper stage, anyway. And if I dare say, I don't think a variation on the autopilot that can handle 6 RVacs instead of 3 RVacs + 3 Raptor SL would be that much of a challenge for SpaceX. They seem... reasonably decent at writing rocket control software.

[RCgothic]

Elon has previously agreed a pure tanker with stretched tanks and 9 raptors (6Vac, 3SL) might make sense.

[derega16]

6 vac 3 sl...wait isn't that the original ITS engine configuration?

[tater]

They have tested the Rvac that can run at SL (so it's not a pure vac engine), the important differences are that they don't throttle, and don't gimbal. I assume the reason the lunar SS renders have SL Raptors is that they exist (they have made/tested far more of them), and perhaps because they throttle.

[RCgothic]

6 hours ago, xebx said:

Starship only need 3 Raptor Vac and 3 Raptor SL (SL can be shut down at around 4500m/s) to deliver approximately :

120t (high DeltaV loss = SH come back to launch pad)

or 180t (low-med DeltaV loss = SH land on a boat)

 or 240t (no recovery).

(SH : 69MN, 215t dry weight, 3400t fuel, SLT=1.35; Starship : 105t dry weight, 1170t fuel, Twr=0.98 at staging, once in orbit 3 Raptor Vac can do all the job even fully refueled; DeltaV Starship 8700 m/s+11t fuel for landing)

Btw, I'd expect a fully expendable starship to have a substantially lighter dry weight. No fins, no heatshield, no header tanks and disposable fairing.

At 5% dry mass, disposable starship would be ~62.5t. F9US is 3.9%. Expendable Starship could even be better because of the way square cube works.

Expendable Starship may be a fairly niche application for distant destinations once they get orbital refuelling worked out. They absolutely do not want to be expending these things.

But it kind of makes sense as an easily acheivable early milestone. Personally if I were Elon I'd like to do it just to smash the "double Saturn V to LEO" marker.

[sevenperforce]

2 hours ago, tater said:

They have tested the Rvac that can run at SL (so it's not a pure vac engine), the important differences are that they don't throttle, and don't gimbal. I assume the reason the lunar SS renders have SL Raptors is that they exist (they have made/tested far more of them), and perhaps because they throttle.

I think the RVac can throttle just fine. But you're right, it can't gimbal.

5 hours ago, RCgothic said:

Elon has previously agreed a pure tanker with stretched tanks and 9 raptors (6Vac, 3SL) might make sense.

I don't remember him saying six vacuum Raptors.

[RCgothic]

17 minutes ago, sevenperforce said:

I think the RVac can throttle just fine. But you're right, it can't gimbal.

I don't remember him saying six vacuum Raptors.

Found it:

 

 

[RyanRising]

14 minutes ago, RCgothic said:

Found it:

Darn it, I just went on a forum adventure looking for that too. Well, glad you found it. Definitely more him agreeing to six vacuum engines than coming out with it himself, but it's still some validation on the idea.

[tater]

1 hour ago, sevenperforce said:

I think the RVac can throttle just fine. But you're right, it can't gimbal.

Might be wrong, I thought he said they'd not throttle—I bet I'm mixing it up with the fixed SL raptors on SH.

(I should have said likely wrong, almost certainly wrong, or certainly wrong, lol)

Edited February 12, 2021 by tater