Author Topic: Lots of little Raptors  (Read 62712 times)

Offline Burninate

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Lots of little Raptors
« on: 01/21/2015 10:24 pm »
The most recent comments on BFR indicate that they are optimized at 500klbf per engine, and simply using lots of engines to generate sufficient thrust.  I am trying to model this concept for "100 tons useful cargo to the surface of Mars", and reusable return of the lander, which is likely to represent a launch vehicle of roughly 200 tons to LEO, and a bunch of refueling missions.

Raptor will be a full-flow staged combustion engine producing 500klbf per engine with (based on older comments that may no longer be reliable) a vacuum Isp of 363 seconds and a sea-level Isp of 321 seconds.

What I would like to know, is what are the dimensions we should expect?  What I'm having trouble with is actually fitting all those engines into the rear of the rocket;  I have no context to understand how closely they should be spaced without triggering cascading failures if one explodes, or how big the engine bells should be.  10-15m seems to be the consensus on fairing diameter, but my intuitive guesses of scale (2m bell diameter with ~3-4m centerline spacing) are clearly wrong, because they indicate 15m is not big enough for this launch vehicle.

*Please distinguish between centerline-to-centerline distance, diameter of the bells, and airgap distance between the outer edges of the bells, or this will get confusing.

**Please distinguish between two different engines: A sea-level optimized Raptor, and a vacuum-optimized Raptor.  My packing problem is with the SL raptor, the VAC raptor I have not begun modelling

***Would the fact that this is a tightly packed array of engines allow for smaller nozzles?
« Last Edit: 01/21/2015 10:37 pm by Burninate »

Offline llanitedave

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Re: Lots of little Raptors
« Reply #1 on: 01/21/2015 10:29 pm »
I thought the vacuum ISP was supposed to be 380 seconds.
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Offline Burninate

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Re: Lots of little Raptors
« Reply #2 on: 01/21/2015 10:34 pm »
I thought the vacuum ISP was supposed to be 380 seconds.

Maybe?

as of http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ it was 363/321 at 1Mlbf, but later...

"A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (705 tonnes-force) of sea-level thrust, 8,200 kN (840 tonnes-force) of vacuum thrust, and a specific impulse of 380 s for a vacuum version.[1] Earlier information had estimated the design Isp under vacuum conditions as only 363 s.[2]"

And then a few days ago, it was 500klbf.

Should 380s(vac) be extrapolated from a 1550klbf engine back to a 500klbf engine?

380s is the most common estimate used for generic methane propulsion technology.
« Last Edit: 01/21/2015 11:27 pm by Burninate »

Offline spacenut

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Re: Lots of little Raptors
« Reply #3 on: 01/21/2015 11:20 pm »
I am not a rocket scientist, but figured at least 7-8m in width for nine 1 million lb thrust engines.  This would be say 150 tons to LEO.  A triple version or superheavy would be 27 million and deliver say 450-500 tons to LEO.  Kind of like copies of the Falcon 9 and Falcon 9H versions.  Wow, this would make either a really wide rocket or a say 18 engines on 3 cores for a heavy version. 

Wouldn't 10-12 meters be the limit?

Offline Mongo62

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Re: Lots of little Raptors
« Reply #4 on: 01/21/2015 11:26 pm »
I thought the vacuum ISP was supposed to be 380 seconds.

Maybe?

as of http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ it was 363/321 at 1Mlbf, but later...

"A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (705 tonnes-force) of sea-level thrust, 8,200 kN (840 tonnes-force) of vacuum thrust, and a specific impulse of 380 s for a vacuum version.[1] Earlier information had estimated the design Isp under vacuum conditions as only 363 s.[2]"

From the Redditt AMA Elon Musk:

Quote
MCT will have meaningfully higher specific impulse engines: 380 vs 345 vac Isp. For those unfamiliar, in the rocket world, that is a super gigantic difference for stages of roughly equivalent mass ratio (mass full to mass empty).

Offline Burninate

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Re: Lots of little Raptors
« Reply #5 on: 01/21/2015 11:41 pm »
I thought the vacuum ISP was supposed to be 380 seconds.

Maybe?

as of http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ it was 363/321 at 1Mlbf, but later...

"A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (705 tonnes-force) of sea-level thrust, 8,200 kN (840 tonnes-force) of vacuum thrust, and a specific impulse of 380 s for a vacuum version.[1] Earlier information had estimated the design Isp under vacuum conditions as only 363 s.[2]"

From the Redditt AMA Elon Musk:

Quote
MCT will have meaningfully higher specific impulse engines: 380 vs 345 vac Isp. For those unfamiliar, in the rocket world, that is a super gigantic difference for stages of roughly equivalent mass ratio (mass full to mass empty).
Okay, thanks, that mystery is solved - must have missed that comment.  What nozzle diameter is required for a practical reusable FFSC CH4-LOX 500klbf engine, in vac and at SL?
« Last Edit: 01/21/2015 11:41 pm by Burninate »

Offline Mongo62

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Re: Lots of little Raptors
« Reply #6 on: 01/22/2015 12:24 am »
Okay, thanks, that mystery is solved - must have missed that comment.  What nozzle diameter is required for a practical reusable FFSC CH4-LOX 500klbf engine, in vac and at SL?

No idea. I don't think that anybody outside of SpaceX knows.

As a crude estimate, the M1D had a SL thrust of 161klbf and a bell diameter of about 1m. The Raptor:M1D thrust ratio would be 3.1056:1, and given an equivalent thrust:area ratio the SL Raptor engine bell would be about 1.76m in diameter.

edit -- meant M1D, not M1C
« Last Edit: 01/22/2015 11:59 am by Mongo62 »

Offline llanitedave

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Re: Lots of little Raptors
« Reply #7 on: 01/22/2015 02:33 am »
At 1.76m engine bells, looks like you can fit 19 of them comfortably on a 10m diameter stage.  Think that would be enough for a start?


« Last Edit: 01/22/2015 02:36 am by llanitedave »
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Offline BobCarver

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Re: Lots of little Raptors
« Reply #8 on: 01/22/2015 02:59 am »
You need gimballing space:

Offline ArbitraryConstant

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Re: Lots of little Raptors
« Reply #9 on: 01/22/2015 03:25 am »
Okay, thanks, that mystery is solved - must have missed that comment.
Yeah, I don't think I've seen anything to support an engine that can operate at SL having 380s vac ISP, that's the vac optimized nozzle.

363s in vac is pretty amazing for a first stage engine tough.

Offline Timothy Mc

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Re: Lots of little Raptors
« Reply #10 on: 01/22/2015 03:39 am »
15m would be the limit

Offline DanielW

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Re: Lots of little Raptors
« Reply #11 on: 01/22/2015 03:49 am »
I fully expect the final design to be 880klbf by averaging the logarithms of what they want 1550klbf with what is optimum for t/w 500klbf. This being the most precise mathematical method of placing a tongue firmly in cheek. I would not be shocked if it ended up around there though.

Offline Burninate

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Re: Lots of little Raptors
« Reply #12 on: 01/22/2015 04:15 am »
Okay, thanks, that mystery is solved - must have missed that comment.  What nozzle diameter is required for a practical reusable FFSC CH4-LOX 500klbf engine, in vac and at SL?

No idea. I don't think that anybody outside of SpaceX knows.

As a crude estimate, the M1C had a SL thrust of 161klbf and a bell diameter of about 1m. The Raptor:M1D thrust ratio would be 3.1056:1, and given an equivalent thrust:area ratio the SL Raptor engine bell would be about 1.76m in diameter.
Um... did it?

Wow.

WP fail.  It lists M1C as 1.676m diameter, but following the ref that's based on an old Falcon 1 design, and that ref says M1C-Falcon 9 will be 1.37m, but it sounds like even there, it's obviously not that big.

A modeller's account and also delicious equations for me to munch on later:
While trying to understand Merlin 1D and in particular "Merlin 1D+"* in depth, I've iterated my calculations a few times and have reached internal coherence and good balance with reality using the following characteristics/specs. 

Merlin 1D..Merlin 1D Vac..Merlin 1D+..Merlin 1D+ Vac
Nozzle diameter, m1.073.031.073.03


I've tried to draw the Falcon 9 first stage with your diameter, but I can't get it to fit. A circle of 8 engines with a diameter of 1,07 m each is going to have an outer diameter of at least 3,8 m as drawn in my CAD program, and that's with the engine nozzles touching each other. But if you look at images of the launch, the engines do not protrude outside the first stage diameter. And there's a gap between the engines.

If I limit the outer diameter of the 8 engines to 3,66 m and allow some spacing between them, the nozzle diameter is around 96,5 cm (my drawing was in 1:144).

I've also tried measuring the diameter from the second photo. Ignoring the distortion, the space between the center engine and the outer engines is 0.452 times the diameter of the center engine. So the total diameter of the ring of 8 engines is (3 + (2*0.452)) times the diameter of one engine nozzle. If the total ring diameter is 366 cm, then one engine must be 93,5 cm in diameter.
I fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old). 
I fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old).

Chamber pressure need not enter into it In the 1D model*.  Thrust is

    F = q ve + Ae (pe - pa) ,

where pa is the ambient pressure.  Therefore the difference between sea-level (pa = pSL = 1 atm) thrust and vacuum (pa = 0) thrust is

    Fvac - FSL = Ae pSL ,

so

    Ae = (Fvac - FSL) / pSL .

If we take the total thrust of the Falcon 9's first stage at sea level and in vacuo from the Falcon 9 web page and divide by nine, we get single-engine thrusts of 653.9 and 741.3 kN, respectively.  For pSL = 101.325 kPa, we get Ae = 0.863 m2 and hence a diameter of 1.048 m, which agrees closely with the value you've calculated.  On the other hand, if we look at the web page for the Merlin engine itself, we're told that the engine's vacuum thrust is just 716 kN.  This lower thrust gives an exit area of 0.613 m2 and a diameter of just 0.883 m.  This fits within the geometric limit found by Hobbes-22.

Now, it could be that this simple analysis violates some constraints imposed by your more extensive model.  I would think, though (and please correct me if I'm wrong), that the the other constraints have are pretty loose, given SpaceX's reluctance to give engineering specifics.



* Actually, I suppose that's not strictly true.  If we're going to assume that flow separation occurs once the pressure drops more than a certain amount below ambient, then the effective nozzle area would depend on chamber pressure.  Thus far, though, we've been assuming there's no flow separation.  As far as I know (which isn't very far), flow separation is usually avoided these days.  (The sustainer of the classic Atlas was over-expanded at sea level to the point that flow separation did occur, but that was back when men were men :) .)  Anyway, allowing for flow separation would tend to increase our estimate of the nozzle's size.  Since we're pretty close to the size allowed by geometry already, this suggests that separation does not occur.

Also
At 1.76m engine bells, looks like you can fit 19 of them comfortably on a 10m diameter stage.  Think that would be enough for a start?
No :)

By the way, what do you use for modelling?
« Last Edit: 01/22/2015 04:30 am by Burninate »

Offline Burninate

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Re: Lots of little Raptors
« Reply #13 on: 01/22/2015 04:33 am »
As a crude estimate, the M1C had a SL thrust of 161klbf and a bell diameter of about 1m. The Raptor:M1D thrust ratio would be 3.1056:1, and given an equivalent thrust:area ratio the SL Raptor engine bell would be about 1.76m in diameter.
First: Can we expect this relationship to hold, all else being equal, or is there some kind of higher power of area figure to worry about?

Second: Can we expect that going, hypothetically, from M1D -> M1D-grade FFSC methane engine, with the same thrust, would change the nozzle diameter?

Offline go4mars

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Re: Lots of little Raptors
« Reply #14 on: 01/22/2015 04:38 am »
15m would be the limit
Based on what?
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Offline Burninate

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Re: Lots of little Raptors
« Reply #15 on: 01/22/2015 05:01 am »
Measuring the rough dimensions of one of the pics in that thread against known tank diameter, I get ~930mm diameter and ~150mm between adjacent engine bells, with about 20mm variance (perspective distorts).

I had not expected the engines to be that close together.  That changes some things.
« Last Edit: 01/22/2015 05:03 am by Burninate »

Offline TomH

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Re: Lots of little Raptors
« Reply #16 on: 01/22/2015 05:10 am »
You need gimballing space:


With that many engines, some could be fixed and others able to gimbal. With fast computers and advanced electronic accelerometers and gyros, all the thrust vectoring could be done by only a few of the engines.
You also have the option of skirt fairings.
« Last Edit: 01/22/2015 05:14 am by TomH »

Offline Burninate

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Re: Lots of little Raptors
« Reply #17 on: 01/22/2015 05:11 am »
You need gimballing space:

Do the outer 8 engines gimbal in 2 dimensions each, or only one?

Offline Lars-J

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Re: Lots of little Raptors
« Reply #18 on: 01/22/2015 05:27 am »
You need gimballing space:

Do the outer 8 engines gimbal in 2 dimensions each, or only one?

They all gimbal independently in 2 dimensions.

If they didn't, there wouldn't be as much engine out capability. People who argue that only a few engines should gimbal seem to forget the scenario where a gimballing engine shuts down - if all the engines did not gimbal, the rocket would be in trouble. As it is, they all adjust slightly to thrust together through the center of mass.

EDIT: I added some images:
 - Image #1 shows the outer engines installed in the octaweb structure, and you can see two gimbal actuators for each engine (outlined in green)
 - Image #2 shows gimbal actuators being installed (all M1D engines are identical as far as I can tell)
 - Image #3 the finished engine with the actuators
« Last Edit: 01/22/2015 05:39 am by Lars-J »

Offline Hotblack Desiato

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Re: Lots of little Raptors
« Reply #19 on: 01/22/2015 06:46 am »
You need gimballing space:


With that many engines, some could be fixed and others able to gimbal. With fast computers and advanced electronic accelerometers and gyros, all the thrust vectoring could be done by only a few of the engines.
You also have the option of skirt fairings.

They could also try it like the russians/sovjets did with the N1: No gimballing at all, instead adjusting the thrust of the engines to steer the rocket. throttle one side from 100% to 99% or 98%, and it will slowly turn. Pro: No need for gimballing hydraulics. Contra: needs to be done very careful. Of course, this is just an option for rockets with a lot of engines, but as far as we know, BFR will have those.

They will probably go for a 15m wide rocket, 10m was calculated for the BFR in falcon heavy style, but already confirmed that they won't use a 3-core configuration. I guess, they could keep the option of going for triple-cores.

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