Author Topic: How raptor compares to other rocket engines: Chart  (Read 11662 times)

Offline Sarigolepas

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Here are two charts comparing thrust density (thrust per unit of nozzle area) to specific impulse.





Offline Narfi

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Re: How raptor compares to other rocket engines: Chart
« Reply #1 on: 04/13/2024 11:34 pm »
Very informative graph and its impressive to see how the raptor is pushing the envelope. It would be cool to see the proprelant type as well maybe as circled group?

Offline TrevorMonty

Re: How raptor compares to other rocket engines: Chart
« Reply #2 on: 04/14/2024 12:11 pm »
ISPs give rough idea of propellant types. Not lot of methalox in there but give it a couple years and there will be another half dozen dots around middle.

Online InterestedEngineer

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Re: How raptor compares to other rocket engines: Chart
« Reply #3 on: 04/14/2024 02:27 pm »
what happened to the beautiful graphs?

Offline 1

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Re: How raptor compares to other rocket engines: Chart
« Reply #4 on: 04/14/2024 06:14 pm »
Remember folks, it's generally better to attach images and documents; especially if there's a chance of buffoonery from direct links.

I believe these are the two graphs from before; attached.

Online InterestedEngineer

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Re: How raptor compares to other rocket engines: Chart
« Reply #5 on: 04/14/2024 10:43 pm »
they were attachments... how mysterious.

Normally if there's a copyright issue a mod would post as such.

Online InterestedEngineer

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Re: How raptor compares to other rocket engines: Chart
« Reply #6 on: 04/14/2024 11:03 pm »
It is amazing that the thrust density of R1 SL is about the same as an SRB and R2SL exceeds that of an SRB.

So much technology in use by SLS is obsolete.

Offline 1

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Re: How raptor compares to other rocket engines: Chart
« Reply #7 on: 04/14/2024 11:55 pm »
they were attachments... how mysterious.

Normally if there's a copyright issue a mod would post as such.

No, they're embedded images; specifically of the "bbc_img" class trying to fetch from twimg. If I switch to a browser I don't normally use (read: one with no ad blocking, all scripting enabled, etc) then the images in the first post display correctly. I'm guess a similar situation's going on on your end. But we're going off topic now, so I'll leave it at that.

Offline deltaV

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Re: How raptor compares to other rocket engines: Chart
« Reply #8 on: 04/15/2024 01:46 am »
It is amazing that the thrust density of R1 SL is about the same as an SRB and R2SL exceeds that of an SRB.

So much technology in use by SLS is obsolete.

I agree that SLS is obsolete but I don't think the charts in this thread are a good illustration of how. If you took SLS's RS-25 engines and reduced the nozzle exit area by a factor of 6 you'd probably get an engine that's better than Raptor at both specific impulse and thrust density. I'm not saying that doing so would be a good idea, just that it shows the silliness of counting position on this chart as a valuable record.

Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.
« Last Edit: 04/15/2024 01:48 am by deltaV »

Online hplan

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Re: How raptor compares to other rocket engines: Chart
« Reply #9 on: 04/15/2024 02:22 am »
It is amazing that the thrust density of R1 SL is about the same as an SRB and R2SL exceeds that of an SRB.

So much technology in use by SLS is obsolete.

I agree that SLS is obsolete but I don't think the charts in this thread are a good illustration of how. If you took SLS's RS-25 engines and reduced the nozzle exit area by a factor of 6 you'd probably get an engine that's better than Raptor at both specific impulse and thrust density. I'm not saying that doing so would be a good idea, just that it shows the silliness of counting position on this chart as a valuable record.

Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

Since volume (and mass) grows as the cube of diameter but base surface area only grows as the square, high thrust density becomes increasingly important as rocket size grows.

I suspect what we see in these charts is that no one in the past has tried to build a rocket large and tall enough that maximizing thrust density became necessary.

(Either that or high thrust density is more or less co-extensive with high chamber pressure and raptors have very high chamber pressure.)
« Last Edit: 04/15/2024 02:25 am by hplan »

Online xvel

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Re: How raptor compares to other rocket engines: Chart
« Reply #10 on: 04/15/2024 02:26 am »

I agree that SLS is obsolete but I don't think the charts in this thread are a good illustration of how. If you took SLS's RS-25 engines and reduced the nozzle exit area by a factor of 6 you'd probably get an engine that's better than Raptor at both specific impulse and thrust density. I'm not saying that doing so would be a good idea, just that it shows the silliness of counting position on this chart as a valuable record.

Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

Nope, RS-25 is almost as wide as its big nozzle so cutting the nozzle wouldn't change thrust density much, raptor is so small that even with sea level nozzles it can be mounted so close that nozzles are touching and so its thrust density is a real thing and one of the most important things that enables superheavy to work at all
« Last Edit: 04/15/2024 02:28 am by xvel »
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Online InterestedEngineer

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Re: How raptor compares to other rocket engines: Chart
« Reply #11 on: 04/15/2024 04:11 am »
Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

The whole rocket system doesn't care about those two metrics, only the outcome:  Isp and thrust density.

Isp for far away from gravity wells, thrust density for deep in gravity wells.

Offline Twark_Main

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Re: How raptor compares to other rocket engines: Chart
« Reply #12 on: 04/15/2024 08:42 am »
Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

The whole rocket system doesn't care about those two metrics, only the outcome:  Isp and thrust density.

Isp for far away from gravity wells, thrust density for deep in gravity wells.

...and the students were enlightened.   8)
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Offline Twark_Main

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Re: How raptor compares to other rocket engines: Chart
« Reply #13 on: 04/15/2024 09:02 am »
This is why I argue the "optimal" booster engine cluster has the chambers densely packed together in a honeycomb, with nozzles extending downward to form sawtooth hexagons.

The actual curvature of the nozzle is rotationally symmetric (no weird structures or loads), it's just extended to fill the "corners" with more nozzle. This maximizes your achievable Isp vs thrust density curve (with a given chamber), since you don't waste any area on the bottom of the rocket.
"The search for a universal design which suits all sites, people, and situations is obviously impossible. What is possible is well designed examples of the application of universal principles." ~~ David Holmgren

Offline edzieba

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Re: How raptor compares to other rocket engines: Chart
« Reply #14 on: 04/15/2024 11:09 am »
Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

The whole rocket system doesn't care about those two metrics, only the outcome:  Isp and thrust density.

Isp for far away from gravity wells, thrust density for deep in gravity wells.
The whole rocket as a system doesn't even care about Specific Impulse: it cares about regular old Impulse. As rockets tend to not be easily and arbitrarily divisible once constructed (Otrag aside).

Offline Twark_Main

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Re: How raptor compares to other rocket engines: Chart
« Reply #15 on: 04/15/2024 11:34 am »
Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

The whole rocket system doesn't care about those two metrics, only the outcome:  Isp and thrust density.

Isp for far away from gravity wells, thrust density for deep in gravity wells.
The whole rocket as a system doesn't even care about Specific Impulse: it cares about regular old Impulse. As rockets tend to not be easily and arbitrarily divisible once constructed (Otrag aside).

Maybe so, but you can't plot just the rocket engines themselves (apart from the rest of the rocket) by that metric.

Thrust density and specific impulse are the critical metrics that connect the rocket engines to the whole rocket.
"The search for a universal design which suits all sites, people, and situations is obviously impossible. What is possible is well designed examples of the application of universal principles." ~~ David Holmgren

Online meekGee

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Re: How raptor compares to other rocket engines: Chart
« Reply #16 on: 04/15/2024 12:05 pm »
Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

The whole rocket system doesn't care about those two metrics, only the outcome:  Isp and thrust density.

Isp for far away from gravity wells, thrust density for deep in gravity wells.
The whole rocket as a system doesn't even care about Specific Impulse: it cares about regular old Impulse. As rockets tend to not be easily and arbitrarily divisible once constructed (Otrag aside).

Maybe so, but you can't plot just the rocket engines themselves (apart from the rest of the rocket) by that metric.

Thrust density and specific impulse are the critical metrics that connect the rocket engines to the whole rocket.
.. determining how large the fuel tanks need to be and whether it can then get off the ground..

And if you equate diameters and not use a conical design, it basically how tall the rocket can be, assuming hex packing as noted upthread.
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Offline Barley

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Re: How raptor compares to other rocket engines: Chart
« Reply #17 on: 04/15/2024 02:15 pm »
This is why I argue the "optimal" booster engine cluster has the chambers densely packed together in a honeycomb, with nozzles extending downward to form sawtooth hexagons.

The actual curvature of the nozzle is rotationally symmetric (no weird structures or loads), it's just extended to fill the "corners" with more nozzle. This maximizes your achievable Isp vs thrust density curve (with a given chamber), since you don't waste any area on the bottom of the rocket.
Question:  Is it necessary to have sawteeth in the interior of the pattern?

I'd guess gas would flow into in the voids and do the job of the missing sawteeth.


Online InterestedEngineer

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Re: How raptor compares to other rocket engines: Chart
« Reply #18 on: 04/15/2024 05:49 pm »
It's a 2D graph, you should choose the 2 variables that matter the most to rocket design.

Sure you could plot against 3 variables at a time.  I doubt we could read such a graph.  If I were to add another variable it would Thrust to Weight Ratio, which also matters to the rocket capability.

Throwing away the most important variables for rocket capability would be emphasizing some internal mechanism of the rocket engines, which may or may not matter.

You get two variables.  Choose wisely.  The OP did.


Offline Twark_Main

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Re: How raptor compares to other rocket engines: Chart
« Reply #19 on: 04/15/2024 08:39 pm »
Edit: chamber pressure vs. specific impulse would be a somewhat more meaningful chart.

The whole rocket system doesn't care about those two metrics, only the outcome:  Isp and thrust density.

Isp for far away from gravity wells, thrust density for deep in gravity wells.
The whole rocket as a system doesn't even care about Specific Impulse: it cares about regular old Impulse. As rockets tend to not be easily and arbitrarily divisible once constructed (Otrag aside).

Maybe so, but you can't plot just the rocket engines themselves (apart from the rest of the rocket) by that metric.

Thrust density and specific impulse are the critical metrics that connect the rocket engines to the whole rocket.
.. determining how large the fuel tanks need to be and whether it can then get off the ground..

And if you equate diameters and not use a conical design, it basically how tall the rocket can be, assuming hex packing as noted upthread.

...now you're comparing rockets (possibly non-existent, hypothetical rockets), not rocket engines.

You say you need fuel tanks. What will you assume for their mass ratio?

How do you choose between cylindrical and conical? What is the cone angle?


You "can" plot numbers like that, but it's meaningless. The numbers become mostly about a set of dubious assumptions unrelated to the rocket engine itself.
"The search for a universal design which suits all sites, people, and situations is obviously impossible. What is possible is well designed examples of the application of universal principles." ~~ David Holmgren

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