SpaceX Discussion Thread

[RyanRising 469]

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 2,308]

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

[derega16 212]

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

[tater 27,232]

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 2,308]

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 7,474]

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 2,308]

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 469]

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 27,232]

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 by tater

[sevenperforce 7,474]

2 minutes ago, tater said:

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.

Right, makes sense.

Back during the 2016 IAC presentation, I believe Elon said the vacuum Raptors would use differential throttle for pointing on orbit. From reporting on that presentation: "Per the notional design of the Tanker/Spaceship, only the three SL Raptors in the center of the vehicle are capable of gimbaling for precise attitude control during ascent and landing maneuvers. Differential throttling on the outer Vac engines is employed for control during in-space maneuvering."

I don't think it was in any of the slides, though; just something he said.

[tater 27,232]

Yeah, and in some talk where he said the outer SH engines would be fixed to the skirt, he said they were easier since they didn't have to throttle or gimbal. I mixed the 2 up.

Regardless, the intermediate version that we have pics of up the thread (the Rvac they test fired at SL) is probably perfectly capable of getting SS to orbit just those. The must underperform SL Raptors in thrust within some altitude (ambient pressure) range, but since they should be better in all ways up high, and the vehicle is also lighter as they go, it seems like a non-issue (besides which Elon talked about a stripped version with just 3 engines, right?).

Edited February 13 by tater

[RCgothic 2,308]

40 minutes ago, RyanRising said:

Definitely more him agreeing to six vacuum engines than coming out with it himself, but it's still some validation on the idea.

 

6 hours ago, RCgothic said:

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

Yup. 

[RyanRising 469]

5 hours ago, RCgothic said:

Yup. 

I may need to get more sleep, thanks

[tater 27,232]

 

[YNM 2,524]

Good to hear they're satisfied enough with the falling-aerodynamic data to start actually try stick the landing. When they'd land it from that position it'll be a new step of the game.

Edited February 13 by YNM

[tater 27,232]

[Spaceception 3,635]

A couple weeks to go in February, I wonder if they'll push to have Superheavy stacked by the end of the month.

[tater 27,232]

 

[RealKerbal3x 5,817]

Reason to be cautiously optimistic: 

 

[zolotiyeruki 476]

"Master the flip"?  It seems to me that they've already got a pretty good handle on the maneuvering algorithms.  They're 2 for 2 on that part.  It's the engines that are currently giving them trouble.

Speaking of which, did SpaceX ever say why the second engine didn't light on SN9?

[kerbiloid 11,279]

Spoiler

 

[RealKerbal3x 5,817]

47 minutes ago, zolotiyeruki said:

"Master the flip"?  It seems to me that they've already got a pretty good handle on the maneuvering algorithms.  They're 2 for 2 on that part.  It's the engines that are currently giving them trouble.

Speaking of which, did SpaceX ever say why the second engine didn't light on SN9?

Yeah, the flip seemed to work just fine on SN8. The only reason it didn't really work on SN9 was that only one engine was ever lit and there wasn't enough torque from that one engine gimballing to correct for the angular velocity from the flip.

And no, they only ever said that the engine failed to relight.

[tater 27,232]

 

[Silavite 223]

There's a pretty good chance of freezing rain overnight in Boca Chica.

Quote

Excerpt from NWS Brownsville Forecast Discussion

Model soundings still lean toward a classic warm nose freezing rain
signature late Sunday evening through Monday morning, however, snow
flurries or brief periods of light snow, mainly across Zapata and
Jim Hogg counties, may be possible Monday morning as cooler air
aloft arrives. Flyovers, interchanges, bridges, and elevated
surfaces will likely become icy in any precipitation Sunday night
into Monday morning. This may occur even when surface temperatures
are a degree or two above freezing. As a result, travel should be
avoided Sunday night through Monday morning.

The facility's proximity to the coast may help to prevent the ground from freezing, but exposed surfaces (like the entirety of SN10...) will likely get covered in a thin icy glaze.

[coyotesfrontier 298]

3 hours ago, Silavite said:

The facility's proximity to the coast may help to prevent the ground from freezing, but exposed surfaces (like the entirety of SN10...) will likely get covered in a thin icy glaze.

For most aircraft that might be a problem, but rockets are a different story