totm nov 2023 SpaceX Discussion Thread

[MaverickSawyer]

1 hour ago, sevenperforce said:

Does Raptor use film cooling, regenerative cooling, or both?

If not film cooling, that could be outlet flow separation due to the pressure differential. I suspect this nozzle is lip-optimized a la SSME.

I'm thinking, based off the pic Elon posted of the plume, that it's both. See how there's the core plume with the nest edges and clearly defined shock diamonds, and then the more ragged sheath that forms around it a few feet from the bell? If you look closely, there's a faint plume of unburned gas ahead of it... and there's a visible gap between the core plume and the edge of the engine bell.

I wonder if they're injecting methane into the bell where the nozzle transitions from sea level optimization to the more vacuum optimized area. Could be to dampen any flow instability at the transition zone...

[DDE]

3 hours ago, sevenperforce said:

This particular Raptor has achieved the second-highest chamber pressure of ANY rocket engine EVER. It is 2.2% away from being the highest-chamber-pressure engine of all time.

RD-171MV may up the ante a bit. Should make it to the test stand soonish.

There’s also Energomash’s experimentation with an annular pulse-detonation engine. How do you even measure the chamber pressure in a spinning explosion?

Edited February 7, 2019 by DDE

[MaverickSawyer]

7 minutes ago, DDE said:

RD-171MV may up the ante a bit. Should make it to the test stand soonish.

There’s also Energomash’s experimentation with an annular pulse-detonation engine. How do you even measure the chamber pressure in a spinning explosion?

Measure static pressure 180* out of phase with the detonation?

[KSK]

On a more serious note, do we know if there was any green flame this time? Although presumably the results from the first test weren't alarming enough to preclude a test at full mission required power.

Although in view of the numbers being thrown around here, I think that needs to be re-written as Full Mission Powaahhhh!!

[sevenperforce]

36 minutes ago, KSK said:

On a more serious note, do we know if there was any green flame this time? Although presumably the results from the first test weren't alarming enough to preclude a test at full mission required power.

Elon said that it was burning off a little copper:

Dunno where from.

The combustion chamber is copper for the sake of thermal conductivity, so there was probably a little brazing inside that got blown out the nozzle. Nothing to worry about.

Edited February 7, 2019 by sevenperforce

[tater]

The fact that they are doing this until late at night is fascinating. Some might honestly be for better images, but clearly they are burning the midnight... methane.

[Silavite]

1 hour ago, tater said:

The fact that they are doing this until late at night is fascinating. Some might honestly be for better images, but clearly they are burning the midnight... methane.

Spoiler

 

 

More on topic; how quickly can SpaceX start producing Raptors en-mass? Although the hopper only needs 3, each Superheavy and Starship combo will need 38(?) Raptors in total.

Edited February 7, 2019 by Silavite

[sevenperforce]

Elon explains the naming scheme for Starship and Super Heavy:

So the vehicle is the Starship and it uses a booster sometimes. The booster is called a Super Heavy. Got it.

 

Edited February 7, 2019 by sevenperforce

[sevenperforce]

If this holds (which, let's face it, is not terribly likely given SpaceX's naming convention track record), I guess we'll see usage like this:

• "Tune in tomorrow on our YouTube channel to see the inaugural launch of Starship Heart of Gold on its journey to Mars."
• "Ten minutes to launch, and Starship is completing LOX loading, along with its Super Heavy booster."
• "T minus sixty seconds. Starship is on internal power."
• "Starship with Super Heavy, go for launch."
• "Starship Heart of Gold has cleared the tower."
• "We have just received confirmation that Starship has passed through Max-Q, the point of maximum dynamic pressure on the Starship and its Super Heavy booster."
• "In just under twenty seconds from now, Starship will separate from its Super Heavy booster as the latter's engines shut down and continue into orbit."
• "Starship's main engines are throttling down on their boost into orbit. Meanwhile, the Super Heavy booster that lifted Starship to the edge of space is about to perform its entry burn on the way back to Cape Canaveral."
• "We have confirmation of a good engine cutoff and a good nominal parking orbit for Starship Heart of Gold! Stand by for the initiation of Super Heavy's landing burn."
• "Super Heavy has touched down after successfully launching Starship Heart of Gold! Ground crew, prepare for Starship tanker launch at 0600 hours."

I can get behind that.

Just not under it.

Edited February 7, 2019 by sevenperforce

[tater]

 

[Mad Rocket Scientist]

 

Confirmation of what was already expected.

[StrandedonEarth]

2 hours ago, sevenperforce said:

• "Tune in tomorrow on our YouTube channel to see the inaugural launch of Starship Heart of Gold on its journey to Mars."
• "Ten minutes to launch, and Starship is completing LOX loading, along with its Super Heavy booster."
• "T minus sixty seconds. Starship is on internal power."
• "Starship with Super Heavy, go for launch."
• "Starship Heart of Gold has cleared the tower."
• "We have just received confirmation that Starship has passed through Max-Q, the point of maximum dynamic pressure on the Starship and its Super Heavy booster."
• "In just under twenty seconds from now, Starship will separate from its Super Heavy booster as the latter's engines shut down and continue into orbit."
• "Starship's main engines are throttling down on their boost into orbit. Meanwhile, the Super Heavy booster that lifted Starship to the edge of space is about to perform its entry burn on the way back to Cape Canaveral."
• "We have confirmation of a good engine cutoff and a good nominal parking orbit for Starship Heart of Gold! Stand by for the initiation of Super Heavy's landing burn."
• "Super Heavy has touched down after successfully launching Starship Heart of Gold! Ground crew, prepare for Starship tanker launch at 0600 hours."

90 minutes later...

• In an infinitely improbable occurrence, Starship Heart of Gold has landed beside its SuperHeavy booster after an Abort-Once-Around. Rumors are swirling that all the space toilets have been mysteriously replaced by bowls of petunias....

Spoiler

Quote

I think the keys to life, the universe, and everything might actually be hidden in the reasons why a single, very large bowl of petunias seems to be endlessly falling through space in The Hitchhiker's Guide to the Galaxy.

You know the scene. The crew of the Heart of Gold uses the Infinite Improbability Drive, but instead of finding themselves made of pasta or blipped into the heart of a dying star, they instead spontaneously create a sperm whale and a large bowl of petunias which are now falling to their deaths. The whale, just now brought into existence, quickly has an entire journey of self-discovery before crashing to the ground. The petunias, meanwhile, only have time to think "Oh no, not again" before reaching their own splattered demise.

 

 

Edited February 8, 2019 by StrandedonEarth

[Xd the great]

7 hours ago, MaverickSawyer said:

I'm thinking, based off the pic Elon posted of the plume, that it's both. See how there's the core plume with the nest edges and clearly defined shock diamonds, and then the more ragged sheath that forms around it a few feet from the bell? If you look closely, there's a faint plume of unburned gas ahead of it... and there's a visible gap between the core plume and the edge of the engine bell.

I wonder if they're injecting methane into the bell where the nozzle transitions from sea level optimization to the more vacuum optimized area. Could be to dampen any flow instability at the transition zone...

It does not make sense for a full flow staged combustion engine to use film cooling technique, due to the wastage of fuel...

[RedKraken]

apparently

Quote

Engines like SSME, F-1, J-2, RS-27, Vulcain 2, RD-171 and RD-180 use film cooling technique for combustion chamber cooling

from a paper here : https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=41363.0;attach=1543146;sess=0

[MaverickSawyer]

36 minutes ago, Xd the great said:

It does not make sense for a full flow staged combustion engine to use film cooling technique, due to the wastage of fuel...

I'm not saying film cooling for the whole engine. I'm thinking it's more of a case of trying to manage the flow separation at the transition point. I'm not a fluid dynamics expert my any means, so my opinion is worth about as much as the electricity used for me to post this, but... eh. Just spitballing ideas here.

[Xd the great]

41 minutes ago, MaverickSawyer said:

I'm not saying film cooling for the whole engine. I'm thinking it's more of a case of trying to manage the flow separation at the transition point. I'm not a fluid dynamics expert my any means, so my opinion is worth about as much as the electricity used for me to post this, but... eh. Just spitballing ideas here.

I thought they used a simple atmo level nozzle here. Dunno, not a rocket engine expert here either.

Atmo level engines are underexpanded in vacuum, but that is no big deal.

Edited February 8, 2019 by Xd the great

[sevenperforce]

1 hour ago, MaverickSawyer said:

I'm not saying film cooling for the whole engine. I'm thinking it's more of a case of trying to manage the flow separation at the transition point. I'm not a fluid dynamics expert my any means, so my opinion is worth about as much as the electricity used for me to post this, but... eh. Just spitballing ideas here.

A full-flow staged-combustion engine cannot use film-cooling for the combustion chamber because it preburns both propellants prior to injection.

From a perspective of diameter alone, the Raptor's nozzle is not nearly large enough to support a vacuum-optimized expansion ratio. Thus, no flow separation.

59 minutes ago, Xd the great said:

I thought they used a simple atmo level nozzle here. Dunno, not a rocket engine expert here either.

Atmo level engines are underexpanded in vacuum, but that is no big deal.

Heuristic views of the nozzle suggest that it may be a convergence-based altitude-compensating curve. So you'd have internal flow separation stabilized by higher pressures around the rim.

Also, recall that this is not yet full thrust.

 

4 hours ago, Mad Rocket Scientist said:

Confirmation of what was already expected.

"Frag"....what a guy. 

[AVaughan]

16 hours ago, sevenperforce said:

If the Starhopper design requires 170 metric tonnes (not tons, Elon!) and they have three engines, then the expected GLOW of Starhopper is 510 tonnes. On Jan 23, I predicted a propellant mass upper bound (based on pixel tracing) of 531 tonnes, so the assumptions were generous.

Suppose Starhopper dries at 75 tonnes (which is ridiculously conservative). That gives 435 tonnes of props. Let's put Isp at 310 seconds to be more conservative. A realistic estimate of dV is 5.8 km/s. Hot damn.

Note that Elon was talking about the required thrust for Starship + SuperHeavy.  So whilst you can probably use those numbers to put an upper bound on the hoppers max takeoff mass (and hence max takeoff fuel load), I'm not sure that you can use that to confirm that the tanks can actually hold all that fuel.  (Personally I think it is likely that they will prefer hopper to have a takeoff TWR of over 1.5, so that an engine failure just after liftoff is not an automatic loss of vehicle.  Even if the tanks can hold 400+ tons of props, it is quite possible that they never will load the hopper to capacity). 

Edited February 8, 2019 by AVaughan

[sevenperforce]

6 hours ago, AVaughan said:

Note that Elon was talking about the required thrust for Starship + SuperHeavy.

Yep I just realized that. Durrr, there go my calculations.

Though now it can potentially tell us something about Starship. 170 metric tonnes times 7 engines comes to 1.19 kilotonnes, significantly less than the 1.34 kilotonnes of the 2017 IAC version. And that's with a 1:1 TWR, which is obviously not workable (but that's where Elon is going to put the numbers, because eternal optimism, and because the engines can run harder). The 2017 model had an estimated dry mass of 85 tonnes and "typical ascent payload" of 150 tonnes, meaning 6.5 km/s of dV.

Payload to LEO already dropped once from 150 tonnes to 100 tonnes between the 2017 IAC and the 2018 #DearMoon presentation; that was when they were still looking at composite. However, that is most likely the result of swapping vacuum Raptors out in favor of SL Raptors. I haven't seen a solid estimate on vacuum Isp for SL Raptors, but it's probably in the 355 s range, which gives almost the same amount of dV.

For Super Heavy's 31 engines, we're looking at a stack GLOW of 4.4 kilotonnes at a liftoff TWR of 1.2 or thereabouts. Subtracting Starship gives 3,200 tonnes for Super Heavy, which is actually greater than the 2017 IAC numbers. It may be more nimble off the pad than I imagined, or that may be part of a more radical redesign, or potentially he wasn't thinking about the implications of what he was saying.

[tater]

@sevenperforce, there's a tweet from Elon I saw recently that said that the change to SS has actually increased payload mass. Not sure what it increased relative to, however (up from the 100 tonne #dearmoon, presumably).

[sevenperforce]

20 minutes ago, tater said:

@sevenperforce, there's a tweet from Elon I saw recently that said that the change to SS has actually increased payload mass. Not sure what it increased relative to, however (up from the 100 tonne #dearmoon, presumably).

Yeah, dry mass goes down with SS even though it is denser, because it is stronger. Similar to replacing aluminum with titanium, etc.

Dry mass estimation is complete guesswork at this point, though. It was also never clear in the 2017 IAC version whether the "85 tonnes dry plus 150 payload" was for a cargo launcher or for the crewed/interplanetary version.

Dry mass also goes down because you don't need a bunch of PICA-X.

[tater]

8 minutes ago, sevenperforce said:

Yeah, dry mass goes down with SS even though it is denser, because it is stronger. Similar to replacing aluminum with titanium, etc.

Dry mass estimation is complete guesswork at this point, though. It was also never clear in the 2017 IAC version whether the "85 tonnes dry plus 150 payload" was for a cargo launcher or for the crewed/interplanetary version.

Dry mass also goes down because you don't need a bunch of PICA-X.

Yeah, they always show a crew vehicle, when of course a payload variant is what I'd expect to see actually fly long before a crew version.

 

[tater]

 

[sevenperforce]

1 hour ago, tater said:

Yeah, they always show a crew vehicle, when of course a payload variant is what I'd expect to see actually fly long before a crew version.

In the 2016 ITS variant, they specified that the crewed interplanetary ITS would have a dry mass of 150 tonnes and the tanker (which presumably could also be adapted for cargo) would have a dry mass of 90 tonnes. Quoted payload to LEO (presumably with cargo variant) was 300 tonnes.

Thus, when we have the IAC 2017 version at a "dry mass" of 85 tonnes and a payload of 150 tonnes, we don't know if the 150 tonnes is the payload for the cargo variant (with a dry mass lower than 85 tonnes) or if 85 is the cargo variant, and the 150 tonnes represents the 85-tonne vehicle plus a crew cabin of unspecified mass.

[sevenperforce]

1 hour ago, sevenperforce said:

In the 2016 ITS variant, they specified that the crewed interplanetary ITS would have a dry mass of 150 tonnes and the tanker (which presumably could also be adapted for cargo) would have a dry mass of 90 tonnes. Quoted payload to LEO (presumably with cargo variant) was 300 tonnes.

Thus, when we have the IAC 2017 version at a "dry mass" of 85 tonnes and a payload of 150 tonnes, we don't know if the 150 tonnes is the payload for the cargo variant (with a dry mass lower than 85 tonnes) or if 85 is the cargo variant, and the 150 tonnes represents the 85-tonne vehicle plus a crew cabin of unspecified mass.

I suppose I can do some math...

The above numbers are nice because they give us two data points for two variants of the same system. For the crew variant, 2400 tonnes mass at staging and 450 tonnes dry, using only Vacuum Isp, gives 6267 m/s; for the tanker variant, 2590 tonnes wet and 470 tonnes "dry" gives 6389 m/s. The difference of 122 m/s is attributable to the lower staging velocity for the heavier payload, relative to the first stage.

If we use the guiding assumption that staging velocity is going to be about the same (realistically, SpaceX wants staging at as high a velocity as the first stage can efficiently RTLS from, so it will be the same across designs), then 6.2-6.3 km/s is a good estimate of expected dV on Starship. 

I suppose this could be used to nail down the parameters of the 2017 IAC version, which in turn would speak to the #DearMoon version, which in turn would give us a new Starship estimate.