SpaceX Raptor engine - General Thread 2

[RotoSequence]

The Environmental Analysis released today (August 1st, 2019) contains nozzle specifications for Raptor:

Throat Radius (in)
4.362

Downstream radius of curvature (in)
1.309

Tangency angle (deg)
32.0

Nozzle lip exit angle (deg)
6.0
Nozzle exit diameter (in)
51.226

Nozzle throat to exit length (in)
60.06

[FutureSpaceTourist]

Also this info cross-posted from EA thread:

The PERCORP modelling of the Raptor thrust chamber included 1.2% of the total engine flow
(13.89 lb/s) as film coolant. Fuel-rich gas, used fuel film coolant, is injected through three slots
located in the converging section of the thrust chamber. The PERCORP code is not currently
capable of treating three discreet injection slots; however, since the slots are all within just a
0.71-inch axial length, the total film cooling effect on the exhaust plume can be reasonably
approximated using just a single. The PERCORP solution for the nominal 349. 6 lbf-s/lbm
engine specific impulse includes a 2.3% core mixing loss, yielding a characteristic velocity (C*)
efficiency of 98.6%.

[livingjw]

Raptor MR 3.6

[livingjw]

Raptor Performance seems about 6 seconds lower than expected.

[gongora]

Did you use the same chamber pressure (25.3MPa?) as the document?

[Ultrafamicom]

Raptor Performance seems about 6 seconds lower than expected.

Isn’t the flow rate of 525.5kg/s (13.89Ib/s/1.2%) at 1.7MN gives a SL specific impulse of 330.1s, which quite match public available data so far?

The specific impulse in the report seems like in mid atmosphere rather than vacuum, given the purpose of report.

[livingjw]

Did you use the same chamber pressure (25.3MPa?) as the document?

I will be doing that, along with a series of other small changes, but I doubt none of them will make up the 6 seconds. Chamber pressure variation has little effect on vacuum Isp.

John

[livingjw]

Raptor Performance seems about 6 seconds lower than expected.

Isn’t the flow rate of 525.5kg/s (13.89Ib/s/1.2%) at 1.7MN gives a SL specific impulse of 330.1s, which quite match public available data so far?

The specific impulse in the report seems like in mid atmosphere rather than vacuum, given the purpose of report.

Flow rate = 13.89 lb/s/.012 = 1158 lb/s = 525 kg/s

Thrust @ 349.6 sec Isp = 1158 lb/s * 349.6 lbf/(lbm/s) = 404,800 lbf = 1.801 MN

Edit: My 355.6 Isp vac estimate yields the quoted 332 sec sea level estimate long given by Elon.
Thrust @ 355.6 sec Isp = 1158 lb/s * 355.6 lbf/(lbm/s) = 411,600 lbf = 1.831 MN

- It could be that 349.6 sec is the  Isp at 10,000 m - 11,000 m altitude. That would match my Isp trend with altitude. It doesn't say what altitude, which usually means sea level or vacuum. 349.6 is too high for sea level .

[Kabloona]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

[su27k]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

I think it's compensation for the lack of full duration static fire like those they do for Falcon 9 at McGregor.

[livingjw]

I have included some results from my model and the 1 Aug Data from SpaceX document. My model gives sea level Isp at about 330 sec and with the published mass flow it yields 1.7 MN. This matches the stated thrust early in the report. The 349.6 Isp is a mystery. It is too high for sea level Isp and too low for vacuum Isp. It correlated with my model at 11.4 km altitude for what that is worth.

John

[cppetrie]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

I think it's compensation for the lack of full duration static fire like those they do for Falcon 9 at McGregor.

Given the staggered startup of FH, a longer period may be required to start so many engines and have them all reach steady state.

[CrazyHorse80]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

I think it's compensation for the lack of full duration static fire like those they do for Falcon 9 at McGregor.

Given the staggered startup of FH, a longer period may be required to start so many engines and have them all reach steady state.

I think they no longer have to do staggered startup for FH * so it could either be true for a few of the early launches or not required at all for SH.

* source: https://www.nasaspaceflight.com/2019/04/falcon-heavy-spacex-nasa-asteroid-redirect/

[ugordan]

I think they no longer have to do staggered startup for FH * so it could either be true for a few of the early launches or not required at all for SH.

* source: https://www.nasaspaceflight.com/2019/04/falcon-heavy-spacex-nasa-asteroid-redirect/

I don't think that's correct. Yes, they're no longer starting up side boosters earlier, but all cores simultaneously, however I strongly believe they're still starting engines in pairs of two, with maybe 100 ms delay between each. Only difference now is that this sequence starts simultaneously on all 3 cores.

On a somewhat related note, they certainly seem to be shutting down F9 engines in pairs at MECO, it's noticeable if you really pay attention to the engine plume decay.

[CrazyHorse80]

I can't find any more specific info about it, so you could be true. Anyhow, I've read that staggered startup on FH started at T-3, so how do you justify an increase of 5x the time to gather health status data for Raptor compared to Merlin? I think they'll do 15s static fires for some other reasons, but I'm no expert so I could easily be wrong.

[ugordan]

I think the longer static fires are precisely due to what su27k suggested in reply #609 above.

[DistantTemple]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

I think it's compensation for the lack of full duration static fire like those they do for Falcon 9 at McGregor.

In the EA, AIUI they need to give a realistic assessment of what they MIGHT need. It is better so state a little more even if that MAY not be needed. Conversely if they said 3 seconds, and ended up doing frequent and sometimes repeated 15 seconds, it would be significant extra noise nuisance. Maybe 15 seconds would usually be only for a new SH. Noise will be the biggest issue to those around the site and area.

[ThomasGadd]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

I think it's compensation for the lack of full duration static fire like those they do for Falcon 9 at McGregor.

Do we know they are not doing full duration static fire on the Raptor's at McGregor?

[Norm38]

If they can stagger start Merlin’s in pairs across the three cores of FH, why couldn’t they stagger start 6 Raptors at a time on SH?  Nice hexagonal symmetry.

[Confusador]

The environmental assessment says they're planning to do 15-second static fires of the Super Heavy on the pad. That seems like quite a long static fire. Any guesses as to why so long? Is there something special about the Raptor FFSC cycle that means you need to fire it longer to reach steady-state conditions for health checks?

I think it's compensation for the lack of full duration static fire like those they do for Falcon 9 at McGregor.

Do we know they are not doing full duration static fire on the Raptor's at McGregor?

su27k is referring to integrated stage firings of the Falcon 9, not the individual Merlin firings.  I have no reason to believe that they're going to stop test firing Raptors there, but I can't imagine them shipping Super Heavies or Starships up.