### Author Topic: ITS Propulsion – The evolution of the SpaceX Raptor engine  (Read 51346 times)

#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #40 on: 10/03/2016 09:53 PM »
Mr. Belluscio, a very nice article – thank you.

One note of correction: the 361s ISP you cite for the first stage's Raptors in vacuum is actually the sea level value for the three inner Raptors of the second stage. See pp. 36 of SpaceX's published PDF.

It says

Raptor Engines
3 Sea-Level - 361 Isp
6 Vacuum - 382 Isp

Meaning 3 Sea-Level engines and 6 Vacuum engines, with Isp 361 and 382 seconds in vacuum respectively.

It is easy to see that they mean the vacuum Isp for the Sea-Level engines as page 31 gives the sea-level Isp as 334 and the main use of the Sea-Level engines in the Ship will be for Earth ascent, Mars landing and Mars descent all of which are in near vacuum.

That seems a stretch of interpretation to me. If you state 'Sea-Level' and follow with an ISP value then... what might one suppose you are trying to communicate?

Is it possible that, for the three inner Raptors of the second stage, they have a third variant? After all, these engines need never fight Earth's gravity when velocity=0.

EDIT:
An exercise: go to the PDF and measure nozzle lengths. I'm working from the ITS cutaway view on page 26, and find the Raptors' nozzles on the first stage to be approximately 80% the length of those on second stage's inner three engines.
« Last Edit: 10/03/2016 10:01 PM by dglow »

#### baldusi

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #41 on: 10/03/2016 10:04 PM »
If you are not convinced, do 138MN/128MN*334 seconds=360.3seconds. Given the rounding on the MN, it is totally consistent with the 361s vacuum performance for Sea Level optimized Raptor.

#### SirKeplan

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #42 on: 10/03/2016 10:09 PM »
That seems a stretch of interpretation to me. If you state 'Sea-Level' and follow with an ISP value then... what might one suppose you are trying to communicate?

Is it possible that, for the three inner Raptors of the second stage, they have a third variant? After all, these engines need never fight Earth's gravity when velocity=0.
I can see where the confusion comes in, but if you compare with page 31 you see ISP is given as vacuum ISP, unless qualified with "(SL)"

on page 34 for the Spaceship it only makes sense to quote vacuum ISPs. for the sea level optimised engine we already know it's ISP at sea level, as it was stated earlier.

However, it is entirely possible the Sea-Level Raptors on the second stage are slightly different to on the first stage. the second stage does not have the same space constraints as the booster, and indeed if you measure the pixel sizes the second stage has wider nozzles in the images. this would allow the engine expansion to be slightly more optimal than if it used booster engines.

#### Nilof

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #43 on: 10/03/2016 10:19 PM »
The wikipedia edits are getting annoying. A few days ago I saw 382 indicated as the vaccum ISP of the ITS first stage and corrected it to ~360s . Apparently some confused soul changed it back to 382 seconds, looking back at the edit history I saw an edit war between a few other editors between the two values, and then at some point the vaccum isp was deleted outright.

The wikipedia article on the ITS seems to be Encyclopedia Astronautica-tier unreliable right now.

It would be so much nicer if anyone who edited rocket engine ISP's on any wiki was forced to sanity test said ISP's in RPA before making the edits...
« Last Edit: 10/03/2016 10:20 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #44 on: 10/03/2016 10:23 PM »
If you are not convinced, do 138MN/128MN*334 seconds=360.3seconds. Given the rounding on the MN, it is totally consistent with the 361s vacuum performance for Sea Level optimized Raptor.

(138*334)/128... yes, that is convincing.

When would we expect to see those three engines firing in a vacuum?

#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #45 on: 10/03/2016 10:26 PM »
The wikipedia edits are getting annoying. A few days ago I saw 382 indicated as the vaccum ISP of the ITS first stage and corrected it to ~360s . Apparently some confused soul changed it back to 382 seconds, looking back at the edit history I saw an edit war between a few other editors between the two values, and then at some point the vaccum isp was deleted outright.

The wikipedia article on the ITS seems to be Encyclopedia Astronautica-tier unreliable right now.

It would be so much nicer if anyone who edited rocket engine ISP's on any wiki was forced to sanity test said ISP's in RPA before making the edits...

SpaceX have not provided a formal ISP value for the first stage Raptors in vacuum, though Baldusi's math seems fair enough.

And yes, Wikipedia changes. Tragic, isn't it?
« Last Edit: 10/03/2016 10:26 PM by dglow »

#### Lars-J

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #46 on: 10/03/2016 10:38 PM »
If you are not convinced, do 138MN/128MN*334 seconds=360.3seconds. Given the rounding on the MN, it is totally consistent with the 361s vacuum performance for Sea Level optimized Raptor.

(138*334)/128... yes, that is convincing.

When would we expect to see those three engines firing in a vacuum?

After staging from the ITS booster, when climbing to LEO. (see the video) Also the martian atmosphere is practically a vacuum.

#### kch

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #47 on: 10/03/2016 10:44 PM »
The wikipedia edits are getting annoying. A few days ago I saw 382 indicated as the vaccum ISP of the ITS first stage and corrected it to ~360s . Apparently some confused soul changed it back to 382 seconds, looking back at the edit history I saw an edit war between a few other editors between the two values, and then at some point the vaccum isp was deleted outright.

The wikipedia article on the ITS seems to be Encyclopedia Astronautica-tier unreliable right now.

It would be so much nicer if anyone who edited rocket engine ISP's on any wiki was forced to sanity test said ISP's in RPA before making the edits...

SpaceX have not provided a formal ISP value for the first stage Raptors in vacuum, though Baldusi's math seems fair enough.

And yes, Wikipedia changes. Tragic, isn't it?

More amusing than tragic, though it does make it not-much-of-a-source as regards accurate information.  Useful mostly for the links to other sites.

#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #48 on: 10/03/2016 10:51 PM »
If you are not convinced, do 138MN/128MN*334 seconds=360.3seconds. Given the rounding on the MN, it is totally consistent with the 361s vacuum performance for Sea Level optimized Raptor.

(138*334)/128... yes, that is convincing.

When would we expect to see those three engines firing in a vacuum?

After staging from the ITS booster, when climbing to LEO. (see the video) Also the martian atmosphere is practically a vacuum.

Thank you! You're right, they're all firing at that point. It's on Mars departure when we see only the outside engines firing.
« Last Edit: 10/03/2016 10:59 PM by dglow »

#### Toast

##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #49 on: 10/03/2016 10:53 PM »
The 1MN dev. model of Raptor should be mass produced to replace Merlin to do away with the He system on F9 and FH.

That would be a massive change, a lot of the Falcon 9 design would have to go back to the drawing board. Plus, the Merlin is an extremely reliable engine, they've only had one failure out of almost three hundred engines that have launched. The helium system is problematic, but fixable. On the other hand, Raptor is a cutting-edge engine that's not fully developed yet, and that has unknown reliability. Switching to it now would result in an extremely protracted return to flight period, and might not improve reliability overall.

#### wardy89

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #50 on: 10/03/2016 11:05 PM »
This might be a stupid question but that does 1MN mean? some people have said that makes it about 1/3 size i would just like to understand the scaling ect.

Edit: please ignore this i have since answered my own question! MN=Meganewton which is 1000 Kilonewtons so roughly 1/3 thrust!
« Last Edit: 10/03/2016 11:23 PM by wardy89 »

#### AS-503

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #51 on: 10/03/2016 11:23 PM »
This might be a stupid question but that does 1MN mean? some people have said that makes it about 1/3 size i would just like to understand the scaling ect.

It means 1 Mega Newtons. Or 1,000,000 Newtons. Or 1,000,000 X 0.224 pounds (224,000 pounds of thrust).

#### Elmar Moelzer

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #52 on: 10/04/2016 01:01 AM »
SpaceX have not provided a formal ISP value for the first stage Raptors in vacuum, though Baldusi's math seems fair enough.
http://www.spacex.com/sites/spacex/files/mars_presentation.pdf
Page 36 gives the vacuum Isp for the SL Raptors.

#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #53 on: 10/04/2016 01:15 AM »
SpaceX have not provided a formal ISP value for the first stage Raptors in vacuum, though Baldusi's math seems fair enough.
http://www.spacex.com/sites/spacex/files/mars_presentation.pdf
Page 36 gives the vacuum Isp for the SL Raptors.

Actually, that page purports to give the Isp for three sea level Raptors, then the Isp for six vacuum Raptors, all of which belong to the second stage. What exactly this means is the discussion at hand.

Moreover, it appears none of these Raptors (on the second stage) are the same as those on the first – smaller nozzles all around on stage one.

#### Robotbeat

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #54 on: 10/04/2016 01:20 AM »
SpaceX have not provided a formal ISP value for the first stage Raptors in vacuum, though Baldusi's math seems fair enough.
http://www.spacex.com/sites/spacex/files/mars_presentation.pdf
Page 36 gives the vacuum Isp for the SL Raptors.

Actually, that page purports to give the Isp for three sea level Raptors, then the Isp for six vacuum Raptors, all of which belong to the second stage. What exactly this means is the discussion at hand.

Moreover, it appears none of these Raptors (on the second stage) are the same as those on the first – smaller nozzles all around on stage one.
Honestly, the discussion is silly. Try running RPA Lite, and the only way to make any sense of what was given is the simplest explanation:
382s is for vac-optimized Raptor at vacuum.
~360s is for sl-optimized Raptor at vacuum.
332s is for sl-optimized Raptor at sea level.

Let's not over-complicate it because the diagram may show slight /apparent differences in nozzle size. Occam's Razor.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #55 on: 10/04/2016 01:28 AM »
SpaceX have not provided a formal ISP value for the first stage Raptors in vacuum, though Baldusi's math seems fair enough.
http://www.spacex.com/sites/spacex/files/mars_presentation.pdf
Page 36 gives the vacuum Isp for the SL Raptors.

Actually, that page purports to give the Isp for three sea level Raptors, then the Isp for six vacuum Raptors, all of which belong to the second stage. What exactly this means is the discussion at hand.

Moreover, it appears none of these Raptors (on the second stage) are the same as those on the first – smaller nozzles all around on stage one.
Honestly, the discussion is silly. Try running RPA Lite, and the only way to make any sense of what was given is the simplest explanation:
382s is for vac-optimized Raptor at vacuum.
~360s is for sl-optimized Raptor at vacuum.
332s is for sl-optimized Raptor at sea level.

Let's not over-complicate it because the diagram may show slight /apparent differences in nozzle size. Occam's Razor.

CAD files, according to Musk... > 'a diagram'.
We're working with what we've been given.
Goodness knows many on this board have worked with less.

I don't care about the first stage vacuum Isp value; Baldusi convinced me on that.
But SpaceX have shown us three different nozzle sizes, a detail I hope you'll agree is relevant here.

#### Dante80

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #56 on: 10/04/2016 01:30 AM »
Here is how I view this.

1. There is one Raptor engine.
2. It has three different nozzles. 40:1, 50:1 and 200:1
3. The smallest 40:1 nozzle is for booster engines (so as to fit). The SL Isp is 334s and the Vac Isp is unknown (around 360s would be a good guess).
4. The 50:1 nozzle is for the spaceship/tanker landing engines. The Vac Isp is 361s, and the SL Isp is unknown (around 335s would be a good bet).
5. The 200:1 nozzle is for the spaceship/tanker vacuum engines. The Vac Isp is 382s and the SL Isp (if those engines are used for abort) is unknown.
6. The CAD Raptor image that SpaceX gave us was for the booster 40:1 sea level Raptor.
« Last Edit: 10/04/2016 01:32 AM by Dante80 »

#### Elmar Moelzer

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #57 on: 10/04/2016 01:47 AM »
382s is for vac-optimized Raptor at vacuum.
~360s is for sl-optimized Raptor at vacuum.
332s is for sl-optimized Raptor at sea level.
I agree! Giving anything but the vacuum Isp for a second stage engine makes no sense.

#### dglow

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #58 on: 10/04/2016 01:53 AM »
382s is for vac-optimized Raptor at vacuum.
~360s is for sl-optimized Raptor at vacuum.
332s is for sl-optimized Raptor at sea level.
I agree! Giving anything but the vacuum Isp for a second stage engine makes no sense.

...correct. Except this second stage returns to and lands on Earth.

#### FutureSpaceTourist

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##### Re: ITS Propulsion – The evolution of the SpaceX Raptor engine
« Reply #59 on: 10/04/2016 03:01 AM »
Let me add my congratulations and thanks for a great article. Very educational for an engine tech novice like me!

How was it determined that this was a 1MN 1/3 scale engine?
I didn't see it any forum posts.
Didn't see it in any an announcement.

I was wondering about this too and haven't seen any posts (including in L2), although the forum has been a bit busy of late!

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