Author Topic: Lots of little Raptors  (Read 62711 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.

Offline Pete

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Re: Lots of little Raptors
« Reply #20 on: 01/22/2015 07:30 am »
They could also try it like the russians/sovjets did with the N1

Ah yes.
Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?

Besides, one of the functions of gimballing is to provide roll control. That is a bit difficult to do, with fixed engines with only throttle tweaks.
« Last Edit: 01/22/2015 07:31 am by Pete »

Offline Eerie

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Re: Lots of little Raptors
« Reply #21 on: 01/22/2015 07:41 am »
Ah yes.
Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?

Weren't N1 failures related to plumbing, not gimballing?

Offline Lars-J

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Re: Lots of little Raptors
« Reply #22 on: 01/22/2015 07:44 am »
Ah yes.
Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?

Weren't N1 failures related to plumbing, not gimballing?

Yes, but it was still a terrible - truly terrible - idea. For every engine you lose, you need to turn off the opposite number. And differential throttling is not very responsive. Plus as noted by Pete, you need separate roll thrusters. No, it's just a terrible idea that was chosen for the N1 because they were desperate.
« Last Edit: 01/22/2015 07:46 am by Lars-J »

Offline Eerie

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Re: Lots of little Raptors
« Reply #23 on: 01/22/2015 08:49 am »
Yes, but it was still a terrible - truly terrible - idea. For every engine you lose, you need to turn off the opposite number.

Why is it terrible? Russian engines are pretty reliable, aren't they? Anyway, if Wikipedia is to be believed, all N1 rockets were in control until the plumbing malfunctioned, meaning that the idea was sound.

I'm sure that if USSR decided to continue with the development of N1, they would fix the plumbing problem eventually.
« Last Edit: 01/22/2015 08:50 am by Eerie »

Offline malenfant

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Re: Lots of little Raptors
« Reply #24 on: 01/22/2015 10:57 am »
N1 did not have separate vernier engines for roll.  Certain opposing pairs of engines were designated for roll control and were canted over in fixed positions.  Roll is therefore provided by differential thrust.

It's still not a great system.  It limits the engine-out options because once again you have only a limited number of steering engines.  Also as has already been said, throttle response is slower than actuators.

Offline Eerie

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Re: Lots of little Raptors
« Reply #25 on: 01/22/2015 11:57 am »
It's still not a great system.  It limits the engine-out options because once again you have only a limited number of steering engines.  Also as has already been said, throttle response is slower than actuators.

I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.

Offline spacenut

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Re: Lots of little Raptors
« Reply #26 on: 01/22/2015 12:42 pm »
So, if a 15m core, how many engines could fit 15m?  Seems like a lot, and it seems a 3 core version would be more practical.  Also seems that a 1 million lb engine would also be more practical.  One for a single engine smaller diameter launcher, two, 9 could fit a 10 meter core like a Falcon 9.  Three a 3 core version would be the Mars direct launcher.  Just my opinion.  They must not trust in space assembly and docking much at SpaceX.

Offline hkultala

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Re: Lots of little Raptors
« Reply #27 on: 01/22/2015 01:25 pm »
It's still not a great system.  It limits the engine-out options because once again you have only a limited number of steering engines.  Also as has already been said, throttle response is slower than actuators.

I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.

Yep, we can dismiss it becauses of OTHER reasons:

1) Landing.

BFR is going to be reusable. The amount of steering power needed with the amount of thrust used during landing just cannot work.
The engine used for landing has to be either:
1a) Gimbaled
2b) Pressure-fed steering thruster engine like draco/superdraco, with MANY of engines used for landing. would mean lots of extra weight and another type of propellant etc; complex, expensive and weights a lot.

2a) Engine-out-capasity

Losing one engine would mean shutting down 2 engines. Losing 2 engines means shutting down 4 engines. And then there is the roll control. If done the N-1-way, what if these lost engines are these roll control engines.


Offline guckyfan

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Re: Lots of little Raptors
« Reply #28 on: 01/22/2015 01:31 pm »
They must not trust in space assembly and docking much at SpaceX.

I don't follow that conclusion. They plan for refuelling in space which is not easier than in space docking.

Docking however requires more structural mass to result in a stable configuration and they are after the best possible T/W ratio possible. That is not what you get when docking modules in space.

Offline malenfant

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Re: Lots of little Raptors
« Reply #29 on: 01/22/2015 02:47 pm »
It's still not a great system.  It limits the engine-out options because once again you have only a limited number of steering engines.  Also as has already been said, throttle response is slower than actuators.

I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.

FWIW the third flight failed when the stage broke up due to excessive roll.  Of course everything is fixable and I understand that roll control was increased after that flight.

Point is it's not a solution you would choose if you had options.  SpaceX have options, the most obvious of which is to increase the stage diameter.

My personal opinion is that Raptor will be bigger anyway.  Elon Said T/W optimised around this number.   He never said they were settled on building it to this number.  Trades are clearly ongoing and there are other factors beside T/W.

Offline JasonAW3

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Re: Lots of little Raptors
« Reply #30 on: 01/22/2015 03:47 pm »
They could also try it like the russians/sovjets did with the N1

Ah yes.
Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?

Besides, one of the functions of gimballing is to provide roll control. That is a bit difficult to do, with fixed engines with only throttle tweaks.

Considering that the Antares rocket and the N-1 used the same engines, I'm gonna agree with you on this.
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Offline malenfant

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Re: Lots of little Raptors
« Reply #31 on: 01/22/2015 03:51 pm »
They could also try it like the russians/sovjets did with the N1

Ah yes.
Given the stellar fail/success rate of the N1, we can assume that this technique to be robust and reliable, right?

Besides, one of the functions of gimballing is to provide roll control. That is a bit difficult to do, with fixed engines with only throttle tweaks.

Considering that the Antares rocket and the N-1 used the same engines, I'm gonna agree with you on this.

Elon calls that "Reasoning by analogy."

It does not follow.

Offline S.Paulissen

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Re: Lots of little Raptors
« Reply #32 on: 01/22/2015 04:44 pm »
It is unclear if Raptor will use the same Ae/Ao ratio as merlin 1D, so it is also unclear that raptor will simply have a scaled up merlin chamber/nozzle.  Having a 320 sea level ISP, even with higher energy methane, suggests that a chamber pressure somewhat higher than Merlin 1D and thus necessitating a somewhat larger Ae/Ao.
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Offline Lars-J

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Re: Lots of little Raptors
« Reply #33 on: 01/22/2015 05:06 pm »
It's still not a great system.  It limits the engine-out options because once again you have only a limited number of steering engines.  Also as has already been said, throttle response is slower than actuators.

I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.

We've given you lots of reasons, are you ignoring them?

Offline Eerie

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Re: Lots of little Raptors
« Reply #34 on: 01/22/2015 05:37 pm »
We've given you lots of reasons, are you ignoring them?

Not at all. I'm not saying SpaceX should recreate N1, god forbid. I was just arguing against the custom of using the N1 as a negative example, just because it failed 4 times and then was cancelled. It failed for a specific reason, and the lesson to learn from it should be related to that reason.
« Last Edit: 01/22/2015 05:39 pm by Eerie »

Offline AncientU

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Re: Lots of little Raptors
« Reply #35 on: 01/22/2015 06:11 pm »
It's still not a great system.  It limits the engine-out options because once again you have only a limited number of steering engines.  Also as has already been said, throttle response is slower than actuators.

I'm not saying it's great, but that there are no indications that it failed, AFAIK. So there is no reason to dismiss it based on the experience of N1.

FWIW the third flight failed when the stage broke up due to excessive roll.  Of course everything is fixable and I understand that roll control was increased after that flight.

Point is it's not a solution you would choose if you had options.  SpaceX have options, the most obvious of which is to increase the stage diameter.

My personal opinion is that Raptor will be bigger anyway.  Elon Said T/W optimised around this number.   He never said they were settled on building it to this number. Trades are clearly ongoing and there are other factors beside T/W.

This.  The somewhat crazy numbers of mini-Raptors engines required to make a BFR leads one to this conclusion.  Lower thrust-to-weight might not be optimum (not such a huge problem on first stage anyway), but neither is dozens of engines.  The bigger raptors previously mentioned (1-1.5Mlb/f) would still be prime for the first stage while the optimized 0.5Mlb/f mini-raptor for the second and/or MCT.

Note/question: Can anyone discuss the shape for a T/W curve around maximum?  Is it sharply peaked or relatively flat-topped? Does it bias toward heavier/higher thrust or away?  Basically, how much loss from optimum can we expect if T/W is doubled/tripled?
« Last Edit: 01/22/2015 06:14 pm by AncientU »
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Offline Lobo

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Re: Lots of little Raptors
« Reply #36 on: 01/22/2015 06:39 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?

I think you're thinking about it the wrong way.  Not that long ago Elon (or someone else and SpaceX) made a comment about in orbit refueling.  That coupled with the recent comments about Raptor only being 500klbs makes me thing SpaceX could be changing paradigm.   They may be going away from a ginormous 12-15m LV with 15Mlbs of thrust or more, to something smaller and easier to handle.  (By "smaller" I mean more Saturn V size).  Engines that are smaller and easier to handle, cores that are smaller and easier to handle, etc.   And then they can utilize the volume that helps to make reusability beneficial.  More smaller launches with full reusability rather than fewer larger ones.  At the end of the day, BFR only needs to be large enough to get a dry MCT to LEO.  It can be fueled and crewed in orbit with subsequent BFR launches while prepping for a MArs mission. 

Myself, I'm thinking the core may be around 10m in diameter and maybe 15-20 engines per core max.  Something that'll put maybe 150mt into LEO (or whatever a dry MCT weighs). 
They can put more engines on it, but if we're talking 30+ engines, with an engine that's not been developed yet....seems like they'd make Raptor bigger rather than going with such a crazy number of engines.  The fact they are going to only be 500klbs makes me think while there will be "a lot" of them, there won't be a zillion of them.  So BFR itself will be of a smaller scale than most of us were thinking when we were looking at Meuller's comments of around 1Mlbs of thrust, and Elon's comments of around 15Mlbs of total thrust (and assuming 9 engines like Falcon, so that's like 1.6Mlbs of thrust per Raptor).

They'd have the ability to process and ready several BFR's at the same time at whatever launch complex they use for it, so they can launch an MCT up and then subsequent fueling missions in fairly short order.  Just a matter of pad turn around then, and making it farily rapid.  They could put up a couple of MCT's at the same time, and then fuel them up for multiple missions in the same Mars launch window. 
Cores and upper stages come back to the launch site and are rolled right into a HIF to be stacked with another tanker 2nd stage and then rolled out for another fueling mission.

So how does 15-20 engines on a 10m core look?

Offline Hyperion5

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Re: Lots of little Raptors
« Reply #37 on: 01/22/2015 06:42 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.

You're overestimating the bell size of these engines.  There's a very similar Russian SC methalox called the RD-0162 under design by KBKhA (see here: http://www.kbkha.ru/?p=8&cat=11&prod=59).  It is remarkably similar to the Raptor as currently sketched out.  It would produce 203.6 tf of thrust at sea level, has a 321/356 Isp split (booster version), possesses a 97:1 t/w ratio, and gets all of that out of an engine measuring 3.55 m long with a 1.65 m diameter engine bell.  Given the Raptor has a 321/363 Isp split and will produce 230 tf at sea level in its booster form, you can see why the Russian engine is a good model for the Raptor.  We've had a number of estimates on chamber pressure for the Raptor already done that wind up showing a very similar chamber pressure to the Russian engine.  My educated guess thus is that we'll see the Raptor engine with a 1.75-1.8 m diameter engine bell, is 3.6-3.7 meters long, and will have a t/w ratio somewhere between 90:1 and 110:1.   

*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?

Well I talked to one of our experts, and he sketched a 20-engine BFR out while assuming the Raptor had a 1.6 m nozzle.  He ringed the entire edge of the rocket's diameter with those engines and found he could stuff that many engines into a 12 m diameter.  Assuming he didn't put any of those engines into the center, you could presumably stuff another 4-7 of them into the remaining central space.  I believe the general rule of thumb on engine spacing is that they should be spaced apart from each other at a distance minimally equalling 10% of their diameter. 

Offline Lars-J

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Re: Lots of little Raptors
« Reply #38 on: 01/22/2015 07:02 pm »
I think you're thinking about it the wrong way.  Not that long ago Elon (or someone else and SpaceX) made a comment about in orbit refueling.  That coupled with the recent comments about Raptor only being 500klbs makes me thing SpaceX could be changing paradigm.   They may be going away from a ginormous 12-15m LV with 15Mlbs of thrust or more, to something smaller and easier to handle.  (By "smaller" I mean more Saturn V size).  Engines that are smaller and easier to handle, cores that are smaller and easier to handle, etc.   And then they can utilize the volume that helps to make reusability beneficial.  More smaller launches with full reusability rather than fewer larger ones.  At the end of the day, BFR only needs to be large enough to get a dry MCT to LEO.  It can be fueled and crewed in orbit with subsequent BFR launches while prepping for a MArs mission.

THIS. I have long argued that the 15m diameter super duper BFR is just over the top.

A smaller BFR, max 10m in diameter, would per plenty. As long a it can put a dry/empty MCT in orbit (or lift an MCT that will take itself to orbit), no more performance is needed. SpaceX gains nothing by building anything bigger than they need to.

Offline TomH

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Re: Lots of little Raptors
« Reply #39 on: 01/22/2015 07:14 pm »
...People who argue that only a few engines should gimbal seem to forget the scenario where a gimballing engine shuts down...

Actually, I did not forget that. This discussion regards a vehicle with 19-30 engines. Do you need some spares in case one or more of the gimbaling engines goes out? Yes. Does every single engine out of so many need to thrust vector? I am doubtful. If not all engines need to gimbal, can they be placed closer together? My guess is a little bit, but I do not know for sure.

Offline guckyfan

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Re: Lots of little Raptors
« Reply #40 on: 01/22/2015 07:15 pm »
I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle.  With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".

Offline TomH

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Re: Lots of little Raptors
« Reply #41 on: 01/22/2015 07:19 pm »
Why is it terrible? Russian engines are pretty reliable, aren't they?

Depends which engine you are talking about. The Antares that blew up recently was using an old Soviet engine that had been sitting in storage for decades-a bit spiffed up-but still old.

Offline Eerie

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Re: Lots of little Raptors
« Reply #42 on: 01/22/2015 08:05 pm »
I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle.  With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".

Won't a single rocket capable of sending 100t to the surface of Mars have to lift like 500+t to LEO? Even with 15m diameter BFR, is it plausible? Sea Dragon was supposed to lift 550t and be 23m in diameter.
« Last Edit: 01/22/2015 08:05 pm by Eerie »

Offline TomH

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Re: Lots of little Raptors
« Reply #43 on: 01/22/2015 08:10 pm »
I keep wondering how deeply Raptor will throttle. While CRS-5/SpX-5 did have the hydraulic fluid problem, there is also the issue that an almost empty F9 first stage is lighter than the minimum thrust of one Merlin, thus the T/W > 1.0 at landing. Timing must be precise so that V = 0 and Altitude = 0 occur at the precise moment in time. Grasshopper was ballasted, making it possible to hover and then land gently. F9 cannot do that, but F9 flights are paid for by other companies or agencies. BFR will be an out of pocket expense for Musk, and it will be a very large out of pocket expense. Building the thing to be reusable from the very beginning seems important.

If we were to assume that everything scaled exactly between F9 stage 1 and BFR stage 1 (I know it will not, but the principle applies), BFR stage 1 would not be able to hover. You can still land with precise timing, but you take less risk if T/W = 1.0 is right in the middle of the throttle range-giving you the ability to hover and throttle up/down very gently. So how do you give it the ability to hover?

1. Make the engine throttle more deeply.
2. Make the engine smaller, the stage bigger. or both, so that the stage 1 mass moves into the middle of the throttle range.
3. Build a smaller center engine that is just for landing. Such an engine might have a very sensitive and deep throttle. While expensive, it is less expensive than LOV on landing.

Then there is the problem that if you keep making the stage larger, the engines smaller, or both, that full throttle of the center engine gives T/W < 1.0. Now you need to use more than one engine for landing. If two, neither is the center engine. If you use three, do you use a line segment through the center or an equilateral triangle around the center? Actually, by the time you get to the point of needing that many, the total engine count seems ridiculously high.

I do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.

In summary, since the first stage of this LV needs to have high confidence of landing and reusability, the engine count needs to take into consideration how deeply the engine can throttle and a calculation that results in T/W = 1.0 (at landing) being in the middle of the throttle range. Optimum engine count needs to match optimum ability to hover and execute a very controlled landing.
« Last Edit: 01/22/2015 08:13 pm by TomH »

Offline nadreck

Re: Lots of little Raptors
« Reply #44 on: 01/22/2015 08:22 pm »

I do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.



Why? This seems to get repeated over several threads. A computer monitors all the inputs and can 1000 (or 10,000, or even 100,000) times a second have a model of exactly how far from centre its current course with current thrust takes it. It can that many times a second adjust its commands that control grid fins, gymballing, thrust, and RCS.  If it stops and hovers it risks having to make larger and larger corrections (oscillating) to actually stay in place then slowly land. Hover and land slowly will be far more difficult to program, and will have lower tolerances for wind (in aviation it would be called cross wind, but for a rocket landing vertically all wind is cross wind).

It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline ScepticMatt

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Re: Lots of little Raptors
« Reply #45 on: 01/22/2015 08:31 pm »
I keep wondering how deeply Raptor will throttle.
With the greater number of engines, can't they just use proportionally fewer engines for the landing burn?

Offline RonM

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Re: Lots of little Raptors
« Reply #46 on: 01/22/2015 08:39 pm »

I do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.



Why? This seems to get repeated over several threads. A computer monitors all the inputs and can 1000 (or 10,000, or even 100,000) times a second have a model of exactly how far from centre its current course with current thrust takes it. It can that many times a second adjust its commands that control grid fins, gymballing, thrust, and RCS.  If it stops and hovers it risks having to make larger and larger corrections (oscillating) to actually stay in place then slowly land. Hover and land slowly will be far more difficult to program, and will have lower tolerances for wind (in aviation it would be called cross wind, but for a rocket landing vertically all wind is cross wind).

What TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.

It's another reason not to hover, just come in and land.


Offline nadreck

Re: Lots of little Raptors
« Reply #47 on: 01/22/2015 08:57 pm »

I do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.



Why? This seems to get repeated over several threads. A computer monitors all the inputs and can 1000 (or 10,000, or even 100,000) times a second have a model of exactly how far from centre its current course with current thrust takes it. It can that many times a second adjust its commands that control grid fins, gymballing, thrust, and RCS.  If it stops and hovers it risks having to make larger and larger corrections (oscillating) to actually stay in place then slowly land. Hover and land slowly will be far more difficult to program, and will have lower tolerances for wind (in aviation it would be called cross wind, but for a rocket landing vertically all wind is cross wind).

What TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.

It's another reason not to hover, just come in and land.

Sorry I should only have quoted half that paragraph: "Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful."

That is what I am saying Why? to.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline guckyfan

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Re: Lots of little Raptors
« Reply #48 on: 01/22/2015 09:05 pm »
What TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.

It's another reason not to hover, just come in and land.

Not at all. During the landing burn the engine will give all control authority needed. The big reason to come in and land is saving fuel.

Offline nadreck

Re: Lots of little Raptors
« Reply #49 on: 01/22/2015 09:23 pm »
What TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.

It's another reason not to hover, just come in and land.

Not at all. During the landing burn the engine will give all control authority needed. The big reason to come in and land is saving fuel.

I completely disagree, I think the tolerances for wind, engine thrust response, and engine gymballing response will be much lower at hover than even the second to last second before touch down. This isn't about the grid fins at that point in time, it is about the differences in the amount of time disruptive factors have to act and how much control authority must be used that introduces other movement to deal with the disruptive factors in hover.  I am not saying the vehicle won't be able to hover, but I am saying that it is simply harder to control hovering than even descending at 10m/s.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Burninate

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Re: Lots of little Raptors
« Reply #50 on: 01/22/2015 09:25 pm »
I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle.  With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".

Won't a single rocket capable of sending 100t to the surface of Mars have to lift like 500+t to LEO? Even with 15m diameter BFR, is it plausible? Sea Dragon was supposed to lift 550t and be 23m in diameter.

It's not merely "100t useful cargo to the surface of Mars", what I am inferring Musk actually means is that it's "100t useful cargo to the surface of Mars [and back to LEO]" (as we wouldn't want to leave passengers behind, would we?).  My numbers are still coming along, but it's looking like - no, 550t to LEO in one launch is not required, but mission IMLEO will be well above 550t.  Putting a refueling stop on the Martian surface and in low Earth orbit, and staging reusably two times in the initial launch, would put your delta V bottleneck at the return from Mars, at around 6km/s as a single stage, which isn't unmanageable;  I am undecided on Earth cargo landing options, however, which I'm leaving out for now.

I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle.  With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".

Right, the rocket equation doesn't care if your dry mass is payload or engine mass or structural tank mass;  One thing I am seriously stuck on at the moment, though, is transferring *crew* or not.  For MCT to be man-rated to existing standards during launch it should have an intact abort, and (separately from the reason for this thread, the SL engines), the Raptor Vacuum engines are looking really enormous to get enough into the upper stage for both failsafe, and enough thrust to do a pad abort.  I'm still iterating here.
« Last Edit: 01/22/2015 10:48 pm by Burninate »

Offline The Amazing Catstronaut

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Re: Lots of little Raptors
« Reply #51 on: 01/22/2015 09:41 pm »
I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle.  With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".

Won't a single rocket capable of sending 100t to the surface of Mars have to lift like 500+t to LEO? Even with 15m diameter BFR, is it plausible? Sea Dragon was supposed to lift 550t and be 23m in diameter.

Sea Dragon was a design that was big, dumb, and littered with fairly deliberate mass inefficiencies to make it cheaper to expend.

BFR is big, intelligent, smart and re-usable. Also it doesn't suffer the inefficiencies of one enormous engine.

Edit: They are (probably) going to end up being comparable in lift capacity. Remember, the MTC (alone) is meant to land 100 tonnes onto the surface of mars, SSTO into space, and then fly back to Earth. She's meant to be able to haul a significant crew (we're not yet sure if that's 50 crew or 100 crew, but she could easily do it unmanned).

This is going to require one enormous monocore rocket. A one core rocket is what they said they'd build, so I'm hedging a monstrous one core rocket is what we're going to get. It's the only thing that fills the criteria.

Just because nobody's ever done it before shouldn't appear overly problematic - that's part and parcel of doing just about anything in Space. It's space: bar science, economics, tech and romance, that's almost the entire point.

Edit Edit: If oldspace can build a Saturn 5 contemporary in the present day, with the present political restrictions, then the Newest of Newspace can certainly attempt a rivalling project. Where's the issue?
« Last Edit: 01/22/2015 09:47 pm by The Amazing Catstronaut »
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Offline Lobo

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Re: Lots of little Raptors
« Reply #52 on: 01/22/2015 09:50 pm »
I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle.  With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".

I agree.  That's why I said "dry" not "empty".  You'd want to launch it with large cargo already stowed.  You then fuel it.  And finally send up the crew in another MCT that's basically just a LEO taxi version for a large crew.  That crew may have some provisions/personal effects with them at that, but all of the big stuff would already be on board. 

If MCT is really to land 100mt of cargo on the surface, how muct will it weigh dry?  We don't have much in the way of spacecraft to compare too.
The Apollo LEM was about 10mt dry (descent stage), but could land around 6mt of "cargo" in a fully fueled ascent stage, astros, provisions, cargo and equipment. 
Could a dryer MCT weight about 50mt, and carry 100mt of cargo, and then another X tonnes of propellant when fully fueled?  That would put the minimum required single BFR to loft about 150mt.  I think it could be around Saturn V size and do that.

Keep in mind it doesn't need to land on Earth with 100mt of cargo, just on Mars in 1/3 gravity.  That 100mt of cargo will only weigh around 33mt sitting in MCT.  So MCT doesn't necessarily have to be build strong/heavy enough to take the loads 100mt would put on it during an Earth EDL.  It just needs to withstand it under Earth gravity during launch.


Offline guckyfan

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Re: Lots of little Raptors
« Reply #53 on: 01/22/2015 10:18 pm »
The Apollo LEM was about 10mt dry (descent stage), but could land around 6mt of "cargo" in a fully fueled ascent stage, astros, provisions, cargo and equipment. 
Could a dryer MCT weight about 50mt, and carry 100mt of cargo, and then another X tonnes of propellant when fully fueled?  That would put the minimum required single BFR to loft about 150mt.  I think it could be around Saturn V size and do that.

As a wild uneducated guess I thought of 60t for the empty MCT so we are in the same ballbpark. Yes 150 to 160t is  needed with fuel to spare for first stage RTLS. This leaves no margin for a escape pod or escape engines. Likely at least one, maybe two engines extra for engine loss capability during the long flight and to add ability to separate from a failing first stage if not enough to speed away from a fireball. Early small crews may board separately. Later, when 100 colonists go to Mars there will be a long and hopefully positive history behind the vehicle that allows the confidence to launch them in MCT.

Offline Lobo

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Re: Lots of little Raptors
« Reply #54 on: 01/22/2015 11:01 pm »
The Apollo LEM was about 10mt dry (descent stage), but could land around 6mt of "cargo" in a fully fueled ascent stage, astros, provisions, cargo and equipment. 
Could a dryer MCT weight about 50mt, and carry 100mt of cargo, and then another X tonnes of propellant when fully fueled?  That would put the minimum required single BFR to loft about 150mt.  I think it could be around Saturn V size and do that.

As a wild uneducated guess I thought of 60t for the empty MCT so we are in the same ballbpark. Yes 150 to 160t is  needed with fuel to spare for first stage RTLS. This leaves no margin for a escape pod or escape engines. Likely at least one, maybe two engines extra for engine loss capability during the long flight and to add ability to separate from a failing first stage if not enough to speed away from a fireball. Early small crews may board separately. Later, when 100 colonists go to Mars there will be a long and hopefully positive history behind the vehicle that allows the confidence to launch them in MCT.

In my thought, all missions would require staging in LEO with several of these "smaller" BFR's to fuel them up, and then some sort of LEO taxi, probably a modified MCT for that purpose, takes the crew up.  This LEO taxi could have some sort of LAS system.  The one going to Mars doesn't need it.  The taxi would take up, up to 100 colonists (eventually) with their personal gear. 
Also, I'd picture probably some EDS stage boosting the MCT to escape or near escape, so it would only have to do it's final escape burn, and then EDL at mars.  It obviously needs enough on board fuel to get itself off of Mars and back through TEI.  Not sure if it'd brake into LEO or return directly to the Earth's surface for refurbishment.  I think SpaceX mentioned something a bit back about them maybe not coming back down to Earth??  Can't remember exactly. 

In this way, your reusable smaller BFR gets more launches per MArs mission, and more use which makes the case for reusability better.  If there's a Mars launch window every 2 years, and You have one ginormous BFR that will launch MCT with all of it's crew and fuel directly to Mars, that means you have one HUGE rocket, and it still will only launch a couple of times every 2 years.
With a smaller HLV and LEO staging, there'd be several BFR launches for each MCT going to Mars that launch window, which would maximize reusability and use.  Could even start staging a year in advance of TMI.  Put the MCT up, then start fueling it up (assuming little boiloff of methalox in orbit), doing in orbit checkouts, etc.  Then send the crew up just prior to TMI.  Another MCT could be being staged in the same way, but staggered so it does TMI like a week or two later.  I think each Mars launch window is around 1 month or so?
If it takes say 5 BFR launches to stage one MCT mission to Mars, and eventually they send 3 MCT's per launch window (for example) That's 15 BFR launches over the course of like a year, with each core and 2nd stage coming back and going back into a HIF for reintegration.  If they have multiple HIF's and a pad that can launch every 2-3 weeks, that's plausible.  And could be done at 39A or 39B as it BFR wouldn't be too powerful for those to handle.  No new ginormous 15+Mlb thrust ginormous BFR launch pad needed.


Offline llanitedave

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Re: Lots of little Raptors
« Reply #55 on: 01/23/2015 01:36 am »

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?


Modeling?  That weren't no model, that was just a picture on LibreOffice Draw, scaled by the standard grid.
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Offline guckyfan

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Re: Lots of little Raptors
« Reply #56 on: 01/23/2015 07:33 am »
In my thought, all missions would require staging in LEO with several of these "smaller" BFR's to fuel them up, and then some sort of LEO taxi, probably a modified MCT for that purpose, takes the crew up.  This LEO taxi could have some sort of LAS system.  The one going to Mars doesn't need it.  The taxi would take up, up to 100 colonists (eventually) with their personal gear. 

Moved my reply to the MCT thread because we are OT for the little Raptor thread.

Offline hrissan

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Re: Lots of little Raptors
« Reply #57 on: 01/23/2015 10:12 am »
If you need just single engine to gimbal for landing (and second stage steering), you should design the gimballing system into engine, but as soon as you have an engine which can gimbal, why not make all engines gimbal? Merlin actuators weight just 4kg each (http://jasc-controls.com/jasc-industry-listing/space/space-actuators/thrust-vector-control-actuator-part-101424-5/), i understand that there is more weight in the (ball) joint and rods, if anyone knows how much they weight, would be welcome to discussion, but judjung by their size, they do not weight much.

I do not know if the liquid methane actuators exist, never heard of one and could not find it in the internet, using methane as a hydraulic liquid sounds unique and novel task. So if SpaceX wants to gimbal Raptor, it will have to design a liquid methane actuator (my bet) or include separate pump/contour for conventional hydraulic liquid.

And regarding stage diameter, if the tooling is very expensive and can make just single diameter tanks, SpaceX might choose bigger than needed diameter initially with lots of space between engines and overall shorter rocket, expecting to elongate the rocket and add more engines in future revisions.

Offline alang

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Re: Lots of little Raptors
« Reply #58 on: 01/23/2015 11:52 am »
Is the fixation on Mars mission profiles making people lose sight that the most important thing necessary to transform space travel is cheap access to low earth orbit? Once cheap access to LEO becomes possible then many other possibilities open up, including slow shipment of non-human freight to Mars and other places by ion propulsion (how the payload gets there doesn't have to decided by SpaceX), not to mention the fact that cheap launches could inspire lower value payloads. Perhaps this kind of thinking is starting to affect the design of SpaceX's next vehicle.
I'm sure this has been discussed many times, so could someone please point me at a thread on this forum about optimising cost to LEO, without any other consideration being involved?

Offline guckyfan

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Re: Lots of little Raptors
« Reply #59 on: 01/23/2015 12:07 pm »
Is the fixation on Mars mission profiles making people lose sight that the most important thing necessary to transform space travel is cheap access to low earth orbit? Once cheap access to LEO becomes possible then many other possibilities open up, including slow shipment of non-human freight to Mars and other places by ion propulsion (how the payload gets there doesn't have to decided by SpaceX), not to mention the fact that cheap launches could inspire lower value payloads. Perhaps this kind of thinking is starting to affect the design of SpaceX's next vehicle.
I'm sure this has been discussed many times, so could someone please point me at a thread on this forum about optimising cost to LEO, without any other consideration being involved?

I don't think anyone has lost sight on that. After all the fully reusable BFR is all about low cost to LEO. It's what makes MCT feasible. Lots of cheap fuel to LEO.

Other architectures need to be measured on MCT. SpaceX seems to have decided against slow missions. They want their ship back for reuse, so fast not only for crew but for cargo as well. Which of the two approaches is more feasible in the long run we will see.


Offline GORDAP

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Re: Lots of little Raptors
« Reply #60 on: 01/23/2015 01:37 pm »
Well, I've been away from this forum for a few days and it's been fun reading this thread in one sitting.

I think Lars-J and Lobo make a great point about there really being no need to posit a 1st stage diameter of greater than 10 meters.  As llanitedave's diagram shows, 19 engines is a nice number of equi-separated nozzles that fit within a circle, and at even 1.8 meters in diameter, this leaves .25 meters between each bell, which is 14% of the diameter, thus fitting comfortably within Hyperion5's criteria of 10% separation.  BobCarver is concerned about room for gimballing - does this leave enough room for that?  It would seem so, as I'd expect gimballing to be performed in a fairly coordinated manner - hard to see why one engine would gimbal hard left, while the one adjacent to it would gimbal hard right.

AncientU perceptively points out that in Musk's AMA he said that T/W appears to be optimized around 500 klbs thrust, but that doesn't imply that they will fix upon that value for a thrust that is optimal for the entire design, right?  At, say, 790 klbs of thrust, 19 engines would give slightly greater total thrust than the 15 mlbs that Musk alluded to.

So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

Offline Eerie

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Re: Lots of little Raptors
« Reply #61 on: 01/23/2015 02:22 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters

10m diameter rocket will be too high, perhaps?

Offline symbios

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Re: Lots of little Raptors
« Reply #62 on: 01/23/2015 02:49 pm »
You can even get 20 engines (calculated total 2m for each engine incl margin for gimbaling) in there if you let the bell stick out about 30 cm outside of the circle. That would also give the center engine more room to move around.

Advantage of 19 is that it has it's own symmetry that 20 does not have.

I do not know what the disadvantage would be if the engine bell would be outside of the tanks flow protection.
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Offline TomH

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Re: Lots of little Raptors
« Reply #63 on: 01/23/2015 03:38 pm »
Is the fixation on Mars mission profiles making people lose sight that the most important thing necessary to transform space travel is cheap access to low earth orbit? Once cheap access to LEO becomes possible then many other possibilities open up, including slow shipment of non-human freight to Mars and other places by ion propulsion (how the payload gets there doesn't have to decided by SpaceX), not to mention the fact that cheap launches could inspire lower value payloads. Perhaps this kind of thinking is starting to affect the design of SpaceX's next vehicle.
I'm sure this has been discussed many times, so could someone please point me at a thread on this forum about optimising cost to LEO, without any other consideration being involved?

This thread is about Raptor engines. Raptor is being developed by SpaceX, which is owned by Elon Musk. He also started Paypal (which he sold) and currently owns Tesla Motors, as well as huge battery and solar enterprises. Musk is developing the Raptor engine, a monster rocket, and the Mars Crew Transport with his own money, solely for the purpose of colonising Mars. He owns the project and he is developing it for his own personal reasons, which do not include making a profit. This thread regards the size of Raptor engines and how many would be optimal for a Musk's Mars ambition. It seems rather odd, and also carries a mildly negative connotation, for you to say that people in this thread have a Mars fixation and should focus on cheap access to LEO. Musk is paying for it himself and getting to Mars is his goal, not getting to LEO. You state that transport of heavy payload to Mars doesn't need to be decided by SpaceX. Whom do you propose is going to develop it? Only NASA has a stated goal of getting to Mars, but the only thing actually under design are SLS and Orion.

Offline oldAtlas_Eguy

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Re: Lots of little Raptors
« Reply #64 on: 01/23/2015 03:51 pm »
Its not 19 engines on a 10m stage but 25 and here is why:

On an F9 to fit exactly 8 engines in the outer ring requires the bell to be .93m and the engine to engine seperation to be 1.07m.

The thrust is related to the square of the bell diameter. So taking the sqrt of the 500/147 Raptor/M1D and multiply by the center to center spacing gives 1.83m Using this spacing value you get exactly 14 engins in the outer ring, 10 in the middle ring and the 1 center engine = 25 engines.

Now for estimated payload capability of such a LV take the total thrust of 25 Raptor engins / total thrust of 9 M1Ds both at sL thrust levels and you get the payload increase factor for a scalled paylod capability for a reusable 1st stage. Now multiply by the LEO capability of a reusable F9 of 13.5mt and you get 128mt. Now calculate the ISP advantage by using the Raptor ISP and dividing by yhe M1D ISP 325/280 and multiplying the payload times this advantage number which gives an estimate of 149mt for the 10m BFR using the Raptor 500klbf engines.

Offline symbios

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Re: Lots of little Raptors
« Reply #65 on: 01/23/2015 04:07 pm »
Its not 19 engines on a 10m stage but 25 and here is why:

On an F9 to fit exactly 8 engines in the outer ring requires the bell to be .93m and the engine to engine seperation to be 1.07m.

The thrust is related to the square of the bell diameter. So taking the sqrt of the 500/147 Raptor/M1D and multiply by the center to center spacing gives 1.83m Using this spacing value you get exactly 14 engins in the outer ring, 10 in the middle ring and the 1 center engine = 25 engines.

Now for estimated payload capability of such a LV take the total thrust of 25 Raptor engins / total thrust of 9 M1Ds both at sL thrust levels and you get the payload increase factor for a scalled paylod capability for a reusable 1st stage. Now multiply by the LEO capability of a reusable F9 of 13.5mt and you get 128mt. Now calculate the ISP advantage by using the Raptor ISP and dividing by yhe M1D ISP 325/280 and multiplying the payload times this advantage number which gives an estimate of 149mt for the 10m BFR using the Raptor 500klbf engines.

That cut through a lot of discussions, were have you been when we needed you ;D

Anyone who can confirm this? Sound so simple put this way that I wonder that no one said it before...  :o
(I do not doubt you Atlas, but I do not know enough so more input is good.)

Edit: Also when SpaceX change the trust to 165 they probably will not change bell diameter so with that you will get more than 149mt.
« Last Edit: 01/23/2015 04:10 pm by symbios »
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Offline llanitedave

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Re: Lots of little Raptors
« Reply #66 on: 01/23/2015 04:20 pm »

Sounds like good reasoning to me as well.  One question, though.


The thrust is related to the square of the bell diameter.


Doesn't that assume equal chamber pressures?  Could a higher pressure allow a still smaller bell diameter?
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Offline Jdeshetler

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Re: Lots of little Raptors
« Reply #67 on: 01/23/2015 04:30 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

This photo was posted before, the overall height of 10m stage is 120' higher than 12m stage which 10m stage need  much larger, longer and more expensive infrastructure (applied to FSS, lighting towers, HIF, storages & Transporter Erectors) but maybe SpaceX have already figured it out...

Offline oldAtlas_Eguy

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Re: Lots of little Raptors
« Reply #68 on: 01/23/2015 04:32 pm »

Sounds like good reasoning to me as well.  One question, though.


The thrust is related to the square of the bell diameter.
Doesn't that assume equal chamber pressures?  Could a higher pressure allow a still smaller bell diameter?



The initial ratio is based on all other items being the same including the ISP. When you increase the payload capability by the ISP advantage without changing the bell size the higher preassure is accounted for by a higher expansion ratio and higher exit velocity while keeping the bell diameter the same.

All of these calculations are still just back-of-the-envelope estimates.

Now lets look at the number of engines from the exact oposite direction. The BFR min payload goal is 180mt. So starting with that and reversing through all the calculation in order to get nuymber of engines of 31 and then using the engine number you get a stage diameter of ~12.33m.

Offline GORDAP

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Re: Lots of little Raptors
« Reply #69 on: 01/23/2015 04:41 pm »
On an F9 to fit exactly 8 engines in the outer ring requires the bell to be .93m and the engine to engine seperation to be 1.07m.

The thrust is related to the square of the bell diameter. So taking the sqrt of the 500/147 Raptor/M1D and multiply by the center to center spacing gives 1.83m Using this spacing value you get exactly 14 engins in the outer ring, 10 in the middle ring and the 1 center engine = 25 engines.


oldAtlas, I'm having trouble following your math.  is the 1.83m the bell diameter of the Raptor under your scenario, or the center to center engine spacing along the outer ring?  In either case, how'd you get there:  If I multiply either .93m or 1.07m (your F9 figures) by the sqrt of 500/147, I don't get 1.83. ???

Offline GORDAP

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Re: Lots of little Raptors
« Reply #70 on: 01/23/2015 04:46 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

This photo was posted before, the overall height of 10m stage is 120' higher than 12m stage which 10m stage need  much larger, longer and more expensive infrastructure (applied to FSS, lighting towers, HIF, storages & Transporter Erectors) but maybe SpaceX have already figured it out...

Jdeshetler, I think those photos/designs were done before there was any any SpaceX announced target of how many tons of payload were to be landed on Mars.  And the person doing them was speculating on some maximum possible value of payload, perhaps in excess of 200 tons to Mars surface.  Now that we have heard Musk say the target is 100 tons, I think the target designs could all be scaled back correspondingly, and a 10 meter core would not be nearly as tall as previously believed.  But I could certainly be wrong about my assumptions.  Does anyone know more firmly what assumptions were used in the design that produced these pics?

Offline Lars-J

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Re: Lots of little Raptors
« Reply #71 on: 01/23/2015 05:07 pm »
Now lets look at the number of engines from the exact oposite direction. The BFR min payload goal is 180mt. So starting with that and reversing through all the calculation in order to get nuymber of engines of 31 and then using the engine number you get a stage diameter of ~12.33m.

Where in the world do you get a BFR min payload of 180mt? Please explain your assumptions.

Offline oldAtlas_Eguy

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Re: Lots of little Raptors
« Reply #72 on: 01/23/2015 05:07 pm »
On an F9 to fit exactly 8 engines in the outer ring requires the bell to be .93m and the engine to engine seperation to be 1.07m.

The thrust is related to the square of the bell diameter. So taking the sqrt of the 500/147 Raptor/M1D and multiply by the center to center spacing gives 1.83m Using this spacing value you get exactly 14 engins in the outer ring, 10 in the middle ring and the 1 center engine = 25 engines.



oldAtlas, I'm having trouble following your math.  is the 1.83m the bell diameter of the Raptor under your scenario, or the center to center engine spacing along the outer ring?  In either case, how'd you get there:  If I multiply either .93m or 1.07m (your F9 figures) by the sqrt of 500/147, I don't get 1.83. ???

OOPS! I missed giving one item in the initial calulation for center to center value and that was the sqrt equation is actually sqrt(500/((325/280)147)).  You multiply the M1D thrust by the increased exit velocity advantage in order to get a smaller bell for the same thrust.
« Last Edit: 01/23/2015 05:09 pm by oldAtlas_Eguy »

Offline Lars-J

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Re: Lots of little Raptors
« Reply #73 on: 01/23/2015 05:10 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

This photo was posted before, the overall height of 10m stage is 120' higher than 12m stage which 10m stage need  much larger, longer and more expensive infrastructure (applied to FSS, lighting towers, HIF, storages & Transporter Erectors) but maybe SpaceX have already figured it out...

Jdeshetler, I think those photos/designs were done before there was any any SpaceX announced target of how many tons of payload were to be landed on Mars.  And the person doing them was speculating on some maximum possible value of payload, perhaps in excess of 200 tons to Mars surface.  Now that we have heard Musk say the target is 100 tons, I think the target designs could all be scaled back correspondingly, and a 10 meter core would not be nearly as tall as previously believed.  But I could certainly be wrong about my assumptions.  Does anyone know more firmly what assumptions were used in the design that produced these pics?

Indeed - and with LEO refueling being in the picture now, it changes things further. The stack does not need to have 3 stages (or MCT on *top* of a 2-stage BFR). It makes the most sense to let MCT be its own 2nd stage - since it needs to be capable of that Delta-V anyway. This would shorten the stack.
« Last Edit: 01/23/2015 05:12 pm by Lars-J »

Offline fast

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Re: Lots of little Raptors
« Reply #74 on: 01/23/2015 05:18 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

This photo was posted before, the overall height of 10m stage is 120' higher than 12m stage which 10m stage need  much larger, longer and more expensive infrastructure (applied to FSS, lighting towers, HIF, storages & Transporter Erectors) but maybe SpaceX have already figured it out...

Jdeshetler, I think those photos/designs were done before there was any any SpaceX announced target of how many tons of payload were to be landed on Mars.  And the person doing them was speculating on some maximum possible value of payload, perhaps in excess of 200 tons to Mars surface.  Now that we have heard Musk say the target is 100 tons, I think the target designs could all be scaled back correspondingly, and a 10 meter core would not be nearly as tall as previously believed.  But I could certainly be wrong about my assumptions.  Does anyone know more firmly what assumptions were used in the design that produced these pics?

Indeed - and with LEO refueling being in the picture now, it changes things further. The stack does not need to have 3 stages (or MCT on *top* of a 2-stage BFR). It makes the most sense to let MCT be its own 2nd stage - since it needs to be capable of that Delta-V anyway. This would shorten the stack.

Third stage on that pic. must be MCT, otherwise why it have such strange biconical shape?

Offline oldAtlas_Eguy

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Re: Lots of little Raptors
« Reply #75 on: 01/23/2015 05:18 pm »
Now lets look at the number of engines from the exact oposite direction. The BFR min payload goal is 180mt. So starting with that and reversing through all the calculation in order to get nuymber of engines of 31 and then using the engine number you get a stage diameter of ~12.33m.

Where in the world do you get a BFR min payload of 180mt? Please explain your assumptions.

Back when the BFR/MCT was first talked about by SpaceX, Ms Shotwell gave the payload values of between 180-210mt. Those goals for payload could have changed or the goal for BFR is to eventially get to those values with a engine thrust upgrade of 24%.  Or it could be some other LV improvement giiving a higher payload such as the US design  propelant factors being significantly better than the F9 US. If that is the case then the assumptions about the number of engines or stage diameter could be significantly different.

Offline Lars-J

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Re: Lots of little Raptors
« Reply #76 on: 01/23/2015 05:23 pm »
Now lets look at the number of engines from the exact oposite direction. The BFR min payload goal is 180mt. So starting with that and reversing through all the calculation in order to get nuymber of engines of 31 and then using the engine number you get a stage diameter of ~12.33m.

Where in the world do you get a BFR min payload of 180mt? Please explain your assumptions.

Back when the BFR/MCT was first talked about by SpaceX, Ms Shotwell gave the payload values of between 180-210mt. Those goals for payload could have changed or the goal for BFR is to eventially get to those values with a engine thrust upgrade of 24%.  Or it could be some other LV improvement giiving a higher payload such as the US design  propelant factors being significantly better than the F9 US. If that is the case then the assumptions about the number of engines or stage diameter could be significantly different.

Ok, that makes sense. But I do think some of those assumptions are no longer valid, now that LEO depots/refueling is in the picture. The question - of course - is how to make everything work with Musk's "100 tons of cargo to Mars" statement. It could be interpreted a number of ways.

Offline oldAtlas_Eguy

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Re: Lots of little Raptors
« Reply #77 on: 01/23/2015 05:33 pm »
Another item in the core size diameter assumptions is the shroud diameter calulations.

The shroud diameter is related to the payload mass capability by the cube root of the increase in payload capability. Note this only gives very rough estimates.

Using 180mt as the target BFR payload results in a shroud diameter of 11.9m.  So a 10m core would need a hamerhead shroud but a 12+m core would not.

Edit: BTW a 12m core would have the same aerodynamic losses as a hamerhead 12m shroud on a 10m core.  If I was designing a new LV I would plan on using a core diameter that equals or exceeds the requred shroud diameter for the possible payloads including increases of payload capability after upgrades.
« Last Edit: 01/23/2015 05:37 pm by oldAtlas_Eguy »

Offline DLR

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Re: Lots of little Raptors
« Reply #78 on: 01/23/2015 05:43 pm »
I believe the BFR will be 12.5m wide or wider, since I think SpaceX will go with a capsule-shaped MCT based on Dragon. To land "a hundred tonnes" on Mars, such a capsule would have to have a diameter of 15m or more.

What is the downside of making the rocket wider?

Offline The Amazing Catstronaut

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Re: Lots of little Raptors
« Reply #79 on: 01/23/2015 06:14 pm »

So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

When put like that; no, I'm not sure I can. ;3 There's been some excellent arguments voiced here and I can't readily disagree with them.


However, are we certain the math works out for a 19 engine BFR of roughly Saturn 5 scale for landing a 60 tonne MTC + 100 tonnes of cargo + all the fuel + consumables that it's going to use in flight down on the surface of mars? Does a 10m diameter give enough internal volume for 100 people? Nobody wants to stuck in an area the size of a people carrier for months and months in space.

Edit: to be succinct; are we sure 19 engines is truly enough engines? It's going to need to lift more than just dry mass + fuel + cargo.
« Last Edit: 01/23/2015 06:15 pm by The Amazing Catstronaut »
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Offline spacenut

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Re: Lots of little Raptors
« Reply #80 on: 01/23/2015 06:32 pm »
I think three Mar's cyclers as proposed by Buz Aldrin coupled with the large reusable launch rocket and a large reusable lander would keep continuous supplies, people, and equipment going to Mars.  If a large nuclear ion powered reusable spacecraft could be built in orbit and lowered the time to Mars to 6 weeks that would be ideal.  I read somewhere with continuous 1g acceleration and deceleration to and from Mars, that would solve the time and gravity problems using ion propulsion. 

Getting this BFR built is the first step.  Now I see why a 15m diameter would be better because of the height of the existing infrastructure. 

Offline GORDAP

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Re: Lots of little Raptors
« Reply #81 on: 01/23/2015 07:41 pm »

However, are we certain the math works out for a 19 engine BFR of roughly Saturn 5 scale for landing a 60 tonne MTC + 100 tonnes of cargo + all the fuel + consumables that it's going to use in flight down on the surface of mars? Does a 10m diameter give enough internal volume for 100 people? Nobody wants to stuck in an area the size of a people carrier for months and months in space.

Edit: to be succinct; are we sure 19 engines is truly enough engines? It's going to need to lift more than just dry mass + fuel + cargo.

Amazing, I'm far from a rocket surgeon, but my BOE calculations lead me to think 19 engines of, say, 750 klbs sea level thrust would get the job done.

If the takeoff GLOW is 8.9 mlbs, this gives an initial 1.6 'g' of thrust (net of .6 after gravity), which I think is an acceptable gravity loss.  I'm assuming the MCT is the 2nd stage, and that each stage must provide 4700 m/s delta v.  Assuming an average Isp of 342 for the 1st stage give about 6.7 mlbs of fuel.  If the stage has a mass ratio of 25 (average of the 2 different FH cores), then it will weigh 269 klbs.  And if we reserve 25k for S1 landing fuel, this leaves roughly 1.9 mlbs for the fully fueled MCT on top.

If the MCT has a mass ratio of 15 (it's got a lot of 'stuff' besides tanks and engines), it'll weigh about 90 klbs - which is conservative compared to the 60 klbs estimate others have posted.  Anyway, assuming a 380 Isp for the MCT, this gives 1.36 mlbs for to-orbit fuel, leaving 363 klbs for payload.  This is 160 mt (sorry bout the mixed units) which is well above Musk's recently stated value of 100 mt.  But then again I haven't accounted for many things like margins or LAS (a biggie), but I'd think the 60% surplus should cover that easily.

I do agree that a 10 meter diameter for the MCT is probably too small for the volume one would want on a roughly capsule shaped body.  Probably more in the 14-15 meter range.  So if it's true that you aerodynamically pay fully for the widest part of the stack, then it's perhaps foolish to stick with a 10 meter 1st stage.  I guess I stubbornly cling to it because I think Tom Mueller stated that was what it would be a good while back, and as far as I know no one at SpaceX has said differently.

Offline TomH

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Re: Lots of little Raptors
« Reply #82 on: 01/23/2015 10:00 pm »
I read somewhere with continuous 1g acceleration and deceleration to and from Mars, that would solve the time and gravity problems using ion propulsion

Assuming you want to place 100 mT on the surface, mass at beginning of TMI may be perhaps 250 mT. At 1g, that's going to require a thrust of 550k lb. That's more than the thrust of one raptor at full throttle. I don't think you'll get that kind of thrust out of an ion engine regardless of its power source. You will build up, then have to burn off, an ungodly amount of kinetic energy. Now if you had an antimatter engine, and could employ so much total impulse, a 1g engine would be nice to have, but a 1g ion engine for such a huge mass is just far too massive in itself.

Offline Lobo

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Re: Lots of little Raptors
« Reply #83 on: 01/23/2015 10:18 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters

10m diameter rocket will be too high, perhaps?

It shouldn't be if we're talking something more along the lines of 140-150mt to LEO capacity rather than the ginormous 300mt monsters we'd previously been speculating on.  Saturn V wasn't too tall, and would be considerably shorter if it had just two methalox stages, rather than a kerolox stage (actually smaller than the methalox stage), two hydrolox stages that are much larger than methalox would be and one of those stages was 6.6m wide rather than 10m.  Just eyeballing things, If you had roughly the same thrust as Saturn V and two methalox stages at 10m wide like the S-1C and S-II stages, it's probably be about the same height as just the S-1C and S-II stages only (or INT-21), with a PLF of whatever height is desired on top of that. 

Offline Lobo

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Re: Lots of little Raptors
« Reply #84 on: 01/23/2015 10:29 pm »
Its not 19 engines on a 10m stage but 25 and here is why:

On an F9 to fit exactly 8 engines in the outer ring requires the bell to be .93m and the engine to engine seperation to be 1.07m.

The thrust is related to the square of the bell diameter. So taking the sqrt of the 500/147 Raptor/M1D and multiply by the center to center spacing gives 1.83m Using this spacing value you get exactly 14 engins in the outer ring, 10 in the middle ring and the 1 center engine = 25 engines.

Now for estimated payload capability of such a LV take the total thrust of 25 Raptor engins / total thrust of 9 M1Ds both at sL thrust levels and you get the payload increase factor for a scalled paylod capability for a reusable 1st stage. Now multiply by the LEO capability of a reusable F9 of 13.5mt and you get 128mt. Now calculate the ISP advantage by using the Raptor ISP and dividing by yhe M1D ISP 325/280 and multiplying the payload times this advantage number which gives an estimate of 149mt for the 10m BFR using the Raptor 500klbf engines.

That cut through a lot of discussions, were have you been when we needed you ;D

Anyone who can confirm this? Sound so simple put this way that I wonder that no one said it before...  :o
(I do not doubt you Atlas, but I do not know enough so more input is good.)

Edit: Also when SpaceX change the trust to 165 they probably will not change bell diameter so with that you will get more than 149mt.

I'd be surprised if there were that many engines on the MPS.  Not like it can't be done, but I think if they were looking at needing that many, they'd make Raptor larger to get the number to a more reasonable number. 

I don't know if there's a "sweet spot" exactly between making engines smaller and easier to make and transport, and the plumbing complexity of using a zillion little engines, but I'd think there would be.  I know FH will have 27 engines but that's only due to it being a tri core version of a 9-engine rocket, not because 27 engines is desirable unto itself. 

25 just seems like it'd be outside of that sweet spot.  Maybe if Raptor already existed for another smaller LV, and they decided to make a new LV using an exsiting enigne.  Then fitting them all in may be easier/cheaper than developing a new larger engine.  But Raptor is being developed specifically for BFR, so I'd think they'd design it of a size that won't require a crazy number.  I know Elon said there "would be a lot of them" but that's pretty subjective.  The 9 engines on F9 is a lot of engine for a typical LV core.  15 would seem like "a lot" too to most.

Offline Lobo

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Re: Lots of little Raptors
« Reply #85 on: 01/23/2015 10:30 pm »
So, can anyone give a good reason for us to expect that the BFR will be greater than 10 meters in diameter and have greater than 19 Raptors on the 1st stage?

This photo was posted before, the overall height of 10m stage is 120' higher than 12m stage which 10m stage need  much larger, longer and more expensive infrastructure (applied to FSS, lighting towers, HIF, storages & Transporter Erectors) but maybe SpaceX have already figured it out...

Wasn't that assuming like a 300mt-ish LV?  I think we're talking maybe half that now.

Offline Lobo

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Re: Lots of little Raptors
« Reply #86 on: 01/23/2015 10:34 pm »
Now lets look at the number of engines from the exact oposite direction. The BFR min payload goal is 180mt. So starting with that and reversing through all the calculation in order to get nuymber of engines of 31 and then using the engine number you get a stage diameter of ~12.33m.

Where in the world do you get a BFR min payload of 180mt? Please explain your assumptions.

Back when the BFR/MCT was first talked about by SpaceX, Ms Shotwell gave the payload values of between 180-210mt. Those goals for payload could have changed or the goal for BFR is to eventially get to those values with a engine thrust upgrade of 24%.  Or it could be some other LV improvement giiving a higher payload such as the US design  propelant factors being significantly better than the F9 US. If that is the case then the assumptions about the number of engines or stage diameter could be significantly different.

Ok, that makes sense. But I do think some of those assumptions are no longer valid, now that LEO depots/refueling is in the picture. The question - of course - is how to make everything work with Musk's "100 tons of cargo to Mars" statement. It could be interpreted a number of ways.

@this.  Obviously we are all specualting, but Elon's recent comments seem to indicate a change of concept.  Raptor was originally a 650klb engine too. Then 1Mlb.  Then hinted that it would be 1.67Mlb (if a 9 engien F9 like MPS were used...another assumption on our part).  Now we're down to 500klbs with talk of LEO fueling and staging.  So I don't know that we can say 180mt minimum any more.
BFR's minimum will be whatever a loaded, but dry MCT weighs, as a single BFR will have to get that at a minimum into LEO.  We have some comments about 100mt of cargo, but little else to really estimate MCT's dry weight to be. 

Offline ImUtrecht

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Re: Lots of little Raptors
« Reply #87 on: 01/23/2015 11:55 pm »
Elon said that BFR would dwarf Saturn 5
I speculate since FH has already 27 engines that BFR will have a lot more.
Whole lotta little raptors speculation thread ? I say 27 x 2 = 54   :P
And like the M1D very high thrust to weight ratio... 150 or more ??


« Last Edit: 01/24/2015 12:04 am by ImUtrecht »

Offline Lars-J

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Re: Lots of little Raptors
« Reply #88 on: 01/24/2015 12:37 am »
Elon said that BFR would dwarf Saturn 5
I speculate since FH has already 27 engines that BFR will have a lot more.
Whole lotta little raptors speculation thread ? I say 27 x 2 = 54   :P
And like the M1D very high thrust to weight ratio... 150 or more ??

There is more to a rocket than engine count.  ::) Unless you think the F9 dwarfs the Saturn V.
« Last Edit: 01/24/2015 12:38 am by Lars-J »

Offline The Amazing Catstronaut

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Re: Lots of little Raptors
« Reply #89 on: 01/24/2015 09:02 am »

There is more to a rocket than engine count.  ::) Unless you think the F9 dwarfs the Saturn V.

That's a little bit facetious/straw man (or, as we call it here in the UK, an Aunt Sally). You know we're talking about raptors here, not any of the merlin family.

Edit: To clarify further, Utrecht's point was as sound as anyone's since we don't actually know the amount of engines the BFR will have. Even the dimensions of the engines talked about are still heavily ambiguous. Since FH has proven to feature a 3x engine count to F9, you can see the line of thought.

I agree with you however; there's unlikely to be a linear relationship with any SpaceX rocket we've seen before. However, you have to admit, from F1 to FH, there's been an undeniable tendency to strap together more engines.
« Last Edit: 01/24/2015 09:20 am by The Amazing Catstronaut »
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Offline fast

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Re: Lots of little Raptors
« Reply #90 on: 01/24/2015 10:01 am »
Maybe someone will start a poll about number of raptors on BFR first stage...

Offline hkultala

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Re: Lots of little Raptors
« Reply #91 on: 01/24/2015 12:07 pm »
You need gimballing space:


Not much, as all engines will be gimballing to same direction (pitch/yaw control) or around same center point(roll control).

Though maybe some space will be needed because if possibility of faulty actuator on some engine which makes it not gimball.

Also the nozzles of the outer engines do not need to fit inside the core diameter, they can "come out".


Offline hkultala

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Re: Lots of little Raptors
« Reply #92 on: 01/24/2015 12:23 pm »
About the diameter of the engines..

Raptor will be staged combustion with much higher pressure than Merlin 1d which is gas generator.

Doesn't this higher pressure allow smallr nozzle for same thrust?

So the nozzle area will not grow by factor of 3 but much less?

RD-191 has about 87% of the rumoured thrust of thrust as raptor (2 MJ) and diameter of 1.45m. Pressure is expected to be in same range. Extrapolating the area from rd-191 would give diameter 1.55m for raptor.




Offline Burninate

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Re: Lots of little Raptors
« Reply #93 on: 01/24/2015 10:06 pm »
About the diameter of the engines..

Raptor will be staged combustion with much higher pressure than Merlin 1d which is gas generator.

Doesn't this higher pressure allow smallr nozzle for same thrust?

So the nozzle area will not grow by factor of 3 but much less?

RD-191 has about 87% of the rumoured thrust of thrust as raptor (2 MJ) and diameter of 1.45m. Pressure is expected to be in same range. Extrapolating the area from rd-191 would give diameter 1.55m for raptor.
Can I get any opinions on this proposition from anyone else?  What is the scaling law here?

Offline Lars-J

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Re: Lots of little Raptors
« Reply #94 on: 01/24/2015 10:28 pm »

Edit: To clarify further, Utrecht's point was as sound as anyone's since we don't actually know the amount of engines the BFR will have. Even the dimensions of the engines talked about are still heavily ambiguous. Since FH has proven to feature a 3x engine count to F9, you can see the line of thought.

I agree with you however; there's unlikely to be a linear relationship with any SpaceX rocket we've seen before. However, you have to admit, from F1 to FH, there's been an undeniable tendency to strap together more engines.

And you miss to the point that this line of thought only applies if Raptor is the same size as Merlin 1D. And we KNOW that is not the case.

Offline ImUtrecht

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Re: Lots of little Raptors
« Reply #95 on: 01/25/2015 02:53 pm »

Edit: To clarify further, Utrecht's point was as sound as anyone's since we don't actually know the amount of engines the BFR will have. Even the dimensions of the engines talked about are still heavily ambiguous. Since FH has proven to feature a 3x engine count to F9, you can see the line of thought.

I agree with you however; there's unlikely to be a linear relationship with any SpaceX rocket we've seen before. However, you have to admit, from F1 to FH, there's been an undeniable tendency to strap together more engines.

And you miss to the point that this line of thought only applies if Raptor is the same size as Merlin 1D. And we KNOW that is not the case.


Elon Musk said that he first thought of upscaling FH

"Goal is 100 metric tons of useful payload to the surface of Mars. This obviously requires a very big spaceship and booster system. ... At first, I was thinking we would just scale up Falcon Heavy, but it looks like ... [the] default plan is to have a sea level and vacuum version of Raptor, much like Merlin. ... [Raptor will boast] a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them :)".

I do not understand why this line of thought only applies when Raptor has the same side as the M1D.
On what is that assumption based ?
We do not know the diameter of BFR and here are speculations that Raptor can have a diameter from 1.50 meters to 1.85 m1

Offline Lars-J

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Re: Lots of little Raptors
« Reply #96 on: 01/25/2015 06:35 pm »
I do not understand why this line of thought only applies when Raptor has the same side as the M1D.
On what is that assumption based ?

Because YOU are the one making the silly statement that a Raptor powered LV must have more engines that FH - when we don't even know what the final thrust of Raptor will be. Your line of thinking (bigger rocket has more engines!) only applies if the BFR is powered by an engine of the same power as M1D.

I'm not sure how many ways I can explain this. Are we lost in translation here?

Offline The Amazing Catstronaut

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Re: Lots of little Raptors
« Reply #97 on: 01/25/2015 07:30 pm »

Edit: To clarify further, Utrecht's point was as sound as anyone's since we don't actually know the amount of engines the BFR will have. Even the dimensions of the engines talked about are still heavily ambiguous. Since FH has proven to feature a 3x engine count to F9, you can see the line of thought.

I agree with you however; there's unlikely to be a linear relationship with any SpaceX rocket we've seen before. However, you have to admit, from F1 to FH, there's been an undeniable tendency to strap together more engines.

And you miss to the point that this line of thought only applies if Raptor is the same size as Merlin 1D. And we KNOW that is not the case.

Noted, but it still doesn't disqualify the meaning of his message. Just because the argument has (valid) points against it doesn't mean the argument isn't worth tabling.

Edit: Just to be clear, I am aware that Raptor is significantly larger than the Merlin 1D.

Edit Edit: I'd assume the misunderstanding between Lars-J and Utrecht that's currently going on is to do with phrasing issues more than anything else.
« Last Edit: 01/25/2015 07:35 pm by The Amazing Catstronaut »
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Offline Lobo

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Re: Lots of little Raptors
« Reply #98 on: 01/26/2015 07:26 pm »

Edit: To clarify further, Utrecht's point was as sound as anyone's since we don't actually know the amount of engines the BFR will have. Even the dimensions of the engines talked about are still heavily ambiguous. Since FH has proven to feature a 3x engine count to F9, you can see the line of thought.

I agree with you however; there's unlikely to be a linear relationship with any SpaceX rocket we've seen before. However, you have to admit, from F1 to FH, there's been an undeniable tendency to strap together more engines.

And you miss to the point that this line of thought only applies if Raptor is the same size as Merlin 1D. And we KNOW that is not the case.


Elon Musk said that he first thought of upscaling FH

"Goal is 100 metric tons of useful payload to the surface of Mars. This obviously requires a very big spaceship and booster system. ... At first, I was thinking we would just scale up Falcon Heavy, but it looks like ... [the] default plan is to have a sea level and vacuum version of Raptor, much like Merlin. ... [Raptor will boast] a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them :)".

I do not understand why this line of thought only applies when Raptor has the same side as the M1D.
On what is that assumption based ?
We do not know the diameter of BFR and here are speculations that Raptor can have a diameter from 1.50 meters to 1.85 m1

Again...keep in mind that despite Elon's statement, if they are doing staging and fueling in LEO, then BFR won't need to get MCT +100mt of "useful payload" all in one launch.  BFR can be smaller, and be used and reused several time while preparing an MCT for departure to Mars.  BFR only needs to be big enough to get a dry (but loaded with cargo) MCT into LEO in one launch.  (As I assume they won't be doing in-orbit cargo transfer).  Using 500klb Raptors and two stage to LEO, 15-20 engines might be enough to get that done.  The booster might actually stage pretty low and slow so it can get back to the landing site easier.  The 2nd stage may be what does the "heavy lifting", and it could have a a few vacuum Raptors on it.  The whole 2nd stage [presumably] will come back after them mission and land at the launch site, so no boost back issues there. 
They may go with a core that's -just- powerful and fueled enough to get a big 2nd stage and the cargo off the ground and to some "low and slow" staging point.  Fewer engines on the core than one might think necessary, but also the core would not carry a particularly large amount of fuel like we're used to seeing in 2-stage LV's. 

Like Saturn INT-21/Skylab LV.  Granted, the propellants were different than they'd be in BFR's two stages, but the S-II wasn't much shorter of a stage than the S-1C itself.  It was a really -big- 2nd stage and I've read numbers that put it's actual maximum potential LEO capacity at close to 120mt.  Far more than Skylab actually needed.  And the S-1C staged relatively "low and slow".  The S-II had the thrust to really supplied a lot of dV for INT-21/Skylab LV.

And the S-1C was "just" 7500klbs of thrust.  15 Raptors at 500klbs each would do that in terms of raw thrust, and their ISP advantage vs. F-1's, less mass of fuel would be used for the same burn (although with greater volume of methalox vs. kerolox). 
After staging, if vacuum Raptor really does 380s impulse, that's only be 41s less than the S-II's J2's.   So the hydrolox 2nd stage of the INT-21 isn't much advantage over BFR's 2nd stage.
I'd think a 15 Raptor core BFR could be roughly the size of the S-1C (less dense fuel, but more efficient engines) with a 2nd stage maybe 1/2 to 1/3 the size of the S-II.  LH2 is about 1/6 as dense as LCH4 (if I did my math right), and if Raptor is fairly close to J2 in efficiency, I think 1/2 to 1/3 as tall as the S-II is a decent WAG?

Titan would be another example of that.  Booster staging pretty low and slow, but with a very large 2nd stage in the core which air-lit after SRB separation. 

So, for INT-21 performance, BFR could be the same diameter but shorter than INT-21.   For more performance than INT-21...to get a dry MCT plus 100mt into LEO, then add engines and stretch the stages accordingly.  But I still don't think we're talking a really monstrous LV any more.  Maybe a little taller than INT-21 with 20-ish engines, 10m diameter, and I think we might be able to get that dry MCT plus cargo to LEO.

I think that'll be more the scale and ratio BFR actually ends up being (although anything can happen, of course).  And 20 engines I think by most people's definition will be "a lot of them".

:-)





Offline Lars-J

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Re: Lots of little Raptors
« Reply #99 on: 01/26/2015 07:38 pm »
One thing that is probably safe to assume, though, is that the outer ring of Raptors will be a number that is divisible by 4 (due to 4 legs). This will simplify the thrust structure and leg attachment.

Offline spacenut

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Re: Lots of little Raptors
« Reply #100 on: 01/26/2015 09:29 pm »
If he is going to use launch pad 39a, the maximum it was designed for is 11-12 million lbs thrust.  At 500k lb thrust, they would need around 20 raptors.  It could have 18-20 circling two in the middle of a 10-15m wide rocket.  This could get 200-225 tons to LEO.  If the vehicle refueled in LEO, might get 100 tons to Mars without cargo transfer.  Two raptors in the middle should be able to land the first stage.   

If the Raptor was 1 million lb thrust, they could do the same thing as Falcon 9 on a 12 meter core for 9 million plus lbs thrust.  This could work also at pad 39a.  A three core heavy version could probably go to Mars and back with one launch but would require a new launch site. 

Offline TomH

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Re: Lots of little Raptors
« Reply #101 on: 01/26/2015 09:48 pm »
If he is going to use launch pad 39a, the maximum it was designed for is 11-12 million lbs thrust.

Do those parameters still hold in light of the way they're rebuilding the pad? It's been awhile since I've seen it discussed, but aren't there acoustic max limits due to all the urbanization that's happened in the area?

Offline spacenut

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Re: Lots of little Raptors
« Reply #102 on: 01/26/2015 09:53 pm »
I do not know what SpaceX is doing with 39a.  I do know from what someone said, a short wide BFR would not have to be so tall.  If they do improve the pressure the pad could take, then they might be shooting for something bigger. 

Offline Lobo

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Re: Lots of little Raptors
« Reply #103 on: 01/26/2015 11:31 pm »
If he is going to use launch pad 39a, the maximum it was designed for is 11-12 million lbs thrust.  At 500k lb thrust, they would need around 20 raptors.  It could have 18-20 circling two in the middle of a 10-15m wide rocket.  This could get 200-225 tons to LEO.  If the vehicle refueled in LEO, might get 100 tons to Mars without cargo transfer.  Two raptors in the middle should be able to land the first stage.   

If the Raptor was 1 million lb thrust, they could do the same thing as Falcon 9 on a 12 meter core for 9 million plus lbs thrust.  This could work also at pad 39a.  A three core heavy version could probably go to Mars and back with one launch but would require a new launch site.

SLS will have approximately 8.5Mlbs of thrust.  I don't know if accustically solids are any "worse" than liquid, but BFR will be all liquid.
IF SLS were to live long enough to get the Dark Knights upgrade when the 5-seg casings run out, it will have more like 10.2Mlbs of thrust, as the Dark Knights are supposedly around 4.5Mlbs of thrust each.  As I don't think liquid boosters are really in the picture any more, if 39B couldn't handle that 10.2Mlbs of thrust physically or accoustically, I don't think they'd be looking at them.

So BFR could have 20 500klb engines and shouldn't exceed SLS launching from KSC with Dark Knights.  How much more they can go above that accoustically I don't know. 

Be interesting to know if accoustically the area can handle 11-12Mlbs of thrust, that the pads physically are supposed to be able to handle.
11Mlbs would only be 2 more Raptors...22 total.  12Mlbs would be 4 more Raptors, 24 total.  So then you are hitting the physical limits of the pads, regardless of any accoustic limits.
« Last Edit: 01/26/2015 11:37 pm by Lobo »

Offline ArbitraryConstant

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Re: Lots of little Raptors
« Reply #104 on: 01/26/2015 11:39 pm »
If he is going to use launch pad 39a, the maximum it was designed for is 11-12 million lbs thrust.
Where does this constraint come from though?

The pad? Or, like, having to not shatter windows in surrounding communities?

Offline spacenut

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Re: Lots of little Raptors
« Reply #105 on: 01/27/2015 02:17 am »
From what I understand.  The Saturn V started at 7.5 million lbs thrust.  The engines were to be upgraded to 1.8 million lbs thrust each or 9 million lbs.  I read on this forum, with all the Direct talk that the pads were designed for 11 million lbs thrust.  The Saturn V J2 engines on the upper stages were also to be upgraded to 250k thrust each for 1.25 million second stage and 250k 3rd stage.  This would have increased the throw weight for an increased LEM size.  Also 6-8 Saturn V's were to build a potential Mars exploration vehicle and lander by 1984, before Nasa's budget was cut. 

So, unless the pads are upgraded or a new pad built at the Texas location, 9-10 million lbs thrust is probably the maximum Raptor will do as well as SLS black knights.  Either could get 150 tons to LEO.  Which one will be cheaper?


Offline DJPledger

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Re: Lots of little Raptors
« Reply #106 on: 01/27/2015 08:25 am »
SpaceX have said that the BFR will be too big for LC-39A and they will need a new launch site for it. This indicates that it will have more than 12Mlbf at liftoff. This means that if Raptor for the 1st stage is 500klbf the BFR will need more than 24 of them. BFR could have as many as 37 Raptors under a 15m core with a center engine surrounded by 3 concentric hexagons of 6, 12, and 18 engines. This configuration easily allows all the engines to fit under the core without protruding out in the way of the landing legs while providing the needed thrust.

Honestly I don't believe that SpaceX will put 30 plus engines on their BFR because they won't want to go through the nightmare of the N-1. Too many engines on the 1st stage is potentially a disaster for reliability which was testified by all 4 failures of the N-1. Not to mention all the launch aborts associated with so many engines so even if the launch doesn't fail the launch aborts may cause Mars synods to be missed.

I think SpaceX will change their mind on the BFR 1st stage engine and bite the bullet and go for a scaled up Raptor to keep the no. of engines on it's 1st stage to 9 in the proven octaweb configuration of the F9 v1.1. The 500klbf Raptor then be used exclusively on the upper stage/MCT. The 500klbf Raptor is the right size for the MCT spacecraft but way too small for the BFR 1st stage.

Offline spacenut

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Re: Lots of little Raptors
« Reply #107 on: 01/27/2015 03:07 pm »
That's why I think the original 1.8 million lb thrust engine would probably be what washes our, or at least the 1 million lb. thrust.  1.8 = 16.2million in a Raptor 9 configeration.  A rocket this size gets what 250 tons to LEO?


Offline Lars-J

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Re: Lots of little Raptors
« Reply #108 on: 01/27/2015 04:21 pm »
SpaceX have said that the BFR will be too big for LC-39A and they will need a new launch site for it. This indicates that it will have more than 12Mlbf at liftoff. This means that if Raptor for the 1st stage is 500klbf the BFR will need more than 24 of them.

What is the source for your first statement? Because I think that such a comment - if made - was made when Raptor was assumed to be much bigger. I do think the BFR (along with Raptor) has been scaled down since then, but I have no proof of that.

Offline Lobo

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Re: Lots of little Raptors
« Reply #109 on: 01/27/2015 05:29 pm »
SpaceX have said that the BFR will be too big for LC-39A and they will need a new launch site for it. This indicates that it will have more than 12Mlbf at liftoff. This means that if Raptor for the 1st stage is 500klbf the BFR will need more than 24 of them. BFR could have as many as 37 Raptors under a 15m core with a center engine surrounded by 3 concentric hexagons of 6, 12, and 18 engines. This configuration easily allows all the engines to fit under the core without protruding out in the way of the landing legs while providing the needed thrust.

Yes, they did.  But they also said that Raptor would be 650Klbs of thrust, and then later 1000klbs of thrust.  Now we are to 500klbs of thrust.  I think there's been some major changes of paradigm with Elon's recent comments, as well as the comments about in space staging not too long ago.  So I don't think you can take Shotwell's comments last summer about BFR being too big for 39A as iron clad.  Things are changing, and will continue to do so.  39A really seems like an -ideal- place to launch a BFR from, or at least a Block 1 BFR.  If BFR were to kill off SLS, then that opens up yet another HLV pad near SpaceX's existing HLV pad.  Maybe originally SpaceX was looking at a scale of BFR that could launch MCT directly to Mars with 100mt of payload.  That was when they were looking at 1Mlb Raptors.  Now they are looking at staging in LEO and doing multiple launches per Mars mission.  BFR doesn't need to be as big, which probably makes things considerably easier, as going with a ginormous 300mt class LV with 1Mlbs+ thrust engines would have a certain amount of unique issues that going with something maybe half that class wouldn't have so much.


Honestly I don't believe that SpaceX will put 30 plus engines on their BFR because they won't want to go through the nightmare of the N-1. Too many engines on the 1st stage is potentially a disaster for reliability which was testified by all 4 failures of the N-1. Not to mention all the launch aborts associated with so many engines so even if the launch doesn't fail the launch aborts may cause Mars synods to be missed.

I think SpaceX will change their mind on the BFR 1st stage engine and bite the bullet and go for a scaled up Raptor to keep the no. of engines on it's 1st stage to 9 in the proven octaweb configuration of the F9 v1.1. The 500klbf Raptor then be used exclusively on the upper stage/MCT. The 500klbf Raptor is the right size for the MCT spacecraft but way too small for the BFR 1st stage.

I agreed, but I don't know that N-1 is a good example.  A lot of the issues with that were vibration shaking loose the plumbing, and the Soviets not being able to test the stages prior to launch as the US could test the Saturn stabes.  We also have far more advanced computer programs for doing vibrational analysis prior to trying to launch.  Also likely BFR's engines will be able to gimbal so one engine shutting down doesnt' mean you need to shut down an opposite enigne like N-1 did. 

N-1 had issues with so many engines, but it's not like that couldn't have been resolved with better testing a tools.  I don't know that's there's inherrently any reason you can't have large numbers of engines on a single core like N-1.
However, that said, I agree I don't think there will be -that- many engines.  I think a very large number of engines does introduce some issues of plumbing and just being combersome.  Especially since they are still developing the engine for it, and could just make the engine larger rather than using 30+ of them.
I think the new info about a 500klb Raptor also indicate that BFR is going to be smaller than origainlly envisioned, and flown and reused more per Mars mission.  (which makes a lot of sense if you are using reusability as part of your paradigm, you want as high flight rate as possible.  Otherwise you have a ginormous BFR that you are only launching once or twice per Mars Launch window for quite some time.  SpaceX then would have SLS).

I'm thinking 15-20 500klbs Raptors.  That would be "a lot of them", but not an unreasonably large number.

Offline guckyfan

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Re: Lots of little Raptors
« Reply #110 on: 01/27/2015 05:43 pm »
Just a reminder. They are going to fly 27 Merlin engines on a Falcon Heavy. They plan to fly many of those. So is there a limit that prohibits a few more than 27 engines on one launch vehicle if they need the thrust?

It gives them a nice production line, even with few cores to be built. It gives them just one engine type for first and upper stage as the larger Raptor could not.

My prediction they make the core just big enough that they can launch MCT including payload but without fuel to LEO. And big enough to do RTLS after delivering.

Offline Lobo

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Re: Lots of little Raptors
« Reply #111 on: 01/27/2015 05:45 pm »
From what I understand.  The Saturn V started at 7.5 million lbs thrust.  The engines were to be upgraded to 1.8 million lbs thrust each or 9 million lbs.  I read on this forum, with all the Direct talk that the pads were designed for 11 million lbs thrust.  The Saturn V J2 engines on the upper stages were also to be upgraded to 250k thrust each for 1.25 million second stage and 250k 3rd stage.  This would have increased the throw weight for an increased LEM size.  Also 6-8 Saturn V's were to build a potential Mars exploration vehicle and lander by 1984, before Nasa's budget was cut. 

So, unless the pads are upgraded or a new pad built at the Texas location, 9-10 million lbs thrust is probably the maximum Raptor will do as well as SLS black knights.  Either could get 150 tons to LEO.  Which one will be cheaper?

@this.

I think 39A and 39B were built to handle an eventual conceptualized Nova Class LV with eight 1.5Mlbs F-1's.  I think Von Braun called it the Saturn C-8, where the Saturn V was the Saturn C-5 and it dated back to the 1959-1962 era, prior to 39A and B being consructed.
I don't know that the 1.8Mlbs F-1A was in the plans yet when they were constructed.  I believe that came later.  The original F-1's were still in development at that time.  So that Saturn C-8 I think was the maximum thrust of reference designed into the construction of that pads.
8X1.5Mlb F-1 engine on a Nova booster is 12Mlbs. the maximum the pads can structurally handle.  Or so I've heard.

The Saturn C-8 here:

http://space50.com/apollo/apollo-the-saturn-rocket/7964-in-the-beginning.html




Offline Lobo

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Re: Lots of little Raptors
« Reply #112 on: 01/27/2015 05:51 pm »
Just a reminder. They are going to fly 27 Merlin engines on a Falcon Heavy. They plan to fly many of those. So is there a limit that prohibits a few more than 27 engines on one launch vehicle if they need the thrust?

It gives them a nice production line, even with few cores to be built. It gives them just one engine type for first and upper stage as the larger Raptor could not.

My prediction they make the core just big enough that they can launch MCT including payload but without fuel to LEO. And big enough to do RTLS after delivering.

Again, I don't think there's any inherrent reason you can't have a large number of engines.  But keep in mind that FH is a tri-core LV, with each MPS only having -9- engines, not 27, and each core essently being it's own independant rocket.   It sounds like BFRwon't be tri-core, so I don't think you can draw any conclusions as to how many engines would be on BFR by how many are on FH.

FH is 3 9-engine rockets, launching together, in effect.  So the question to ask, is, how many more engines than 9 will SpaceX put on a core, as they'll only have ever operated a core with 9 engines on it.


Offline Owlon

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Re: Lots of little Raptors
« Reply #113 on: 01/27/2015 05:53 pm »
Just a reminder. They are going to fly 27 Merlin engines on a Falcon Heavy. They plan to fly many of those. So is there a limit that prohibits a few more than 27 engines on one launch vehicle if they need the thrust?

It gives them a nice production line, even with few cores to be built. It gives them just one engine type for first and upper stage as the larger Raptor could not.

My prediction they make the core just big enough that they can launch MCT including payload but without fuel to LEO. And big enough to do RTLS after delivering.

This is my thinking exactly. In fact, I would think 27 engines on one vehicle would be easier than 27 engines on three cores with cross-feed.

Offline Lars-J

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Re: Lots of little Raptors
« Reply #114 on: 01/27/2015 06:08 pm »
Just a reminder. They are going to fly 27 Merlin engines on a Falcon Heavy. They plan to fly many of those. So is there a limit that prohibits a few more than 27 engines on one launch vehicle if they need the thrust?

No theoretical limit, but certainly a practical one. Remember that they are not flying 27 engines because it is ideal - they are using what they have. But the BFR will be more of a clean sheet design. New size, new fuel, new engines.

Offline guckyfan

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Re: Lots of little Raptors
« Reply #115 on: 01/27/2015 06:28 pm »

No theoretical limit, but certainly a practical one. Remember that they are not flying 27 engines because it is ideal - they are using what they have. But the BFR will be more of a clean sheet design. New size, new fuel, new engines.

Yes, and knowing that they say they have decided on a large number of engines instead of few very big ones. I suppose they have conciously made that decision. I like others believe they may upgrade thrust per engine later when they need more launches. And I expect they will design their rocket in a way that they can upgrade without major redesign. Making the core and engine spacing wide enough that upgrade will be not much more than stretching the tank.

You know, I am one who does not suppose they are fools.


Offline mme

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Re: Lots of little Raptors
« Reply #116 on: 01/27/2015 07:00 pm »

No theoretical limit, but certainly a practical one. Remember that they are not flying 27 engines because it is ideal - they are using what they have. But the BFR will be more of a clean sheet design. New size, new fuel, new engines.

Yes, and knowing that they say they have decided on a large number of engines instead of few very big ones. I suppose they have conciously made that decision. I like others believe they may upgrade thrust per engine later when they need more launches. And I expect they will design their rocket in a way that they can upgrade without major redesign. Making the core and engine spacing wide enough that upgrade will be not much more than stretching the tank.

You know, I am one who does not suppose they are fools.
Agreed.  According to Musk's quote, they choose the size because T/W was better using smaller engines:

From http://www.parabolicarc.com/2015/01/06/highlights-elon-musks-reddit-session/
Quote
Elon Musk: Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them - See more at: http://www.parabolicarc.com/2015/01/06/highlights-elon-musks-reddit-session/#sthash.Eyy4Y5Vl.dpuf

There's no evidence that the rocket got smaller, at least no reference I am aware of.  My "guess" is they'll have 27 of them just because it keeps the rocket the same size.  And yes, the plumbing required for that does blow my mind.

And of course, it's SpaceX, so they aren't committed until they start bending metal.  And even then they aren't totally committed to a specific solution, so I could be wrong.  (And to be clear, I like that about them.)
Space is not Highlander.  There can, and will, be more than one.

Offline Lobo

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Re: Lots of little Raptors
« Reply #117 on: 01/27/2015 07:31 pm »
There's no evidence that the rocket got smaller, at least no reference I am aware of.  My "guess" is they'll have 27 of them just because it keeps the rocket the same size.  And yes, the plumbing required for that does blow my mind.

And of course, it's SpaceX, so they aren't committed until they start bending metal.  And even then they aren't totally committed to a specific solution, so I could be wrong.  (And to be clear, I like that about them.)

I think the comments about the BFR being a) Single core, b) having an engine with about half the thrust as previously cited (by Meuller), and c) SpaceX's previous comments about in-space staging, taken all together is evidence that the rocket got smaller.  At least smaller than what was speculated to be necessary to send MCT and 100t of cargo to Mars in a single launch.

Again...the engine is still in development.  And was cited to be 1Mlbs of thrust just a year ago by the engine's -designer-.   So I'm finding it hard to believe a clean-sheet engine and a clean-sheet booster will result in upwards of 30 engines on a single boat tail.

I think if they were still looking at 15Mlbs of thrust (as referenced by Elon some time ago, although unsure if it was sea level or vacuum), they would be designing Raptor at a thrust level that it wouldn't take 30 on a single boat tail. 

That's purely my opinion, but upwards of 30 engines just seems....excessive...when you aren't trying to use an existing engine on a much larger LV, and have the ability to match the engine to the LV.


Offline Lobo

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Re: Lots of little Raptors
« Reply #118 on: 01/27/2015 07:50 pm »
Just a reminder. They are going to fly 27 Merlin engines on a Falcon Heavy. They plan to fly many of those. So is there a limit that prohibits a few more than 27 engines on one launch vehicle if they need the thrust?

It gives them a nice production line, even with few cores to be built. It gives them just one engine type for first and upper stage as the larger Raptor could not.

My prediction they make the core just big enough that they can launch MCT including payload but without fuel to LEO. And big enough to do RTLS after delivering.

This is my thinking exactly. In fact, I would think 27 engines on one vehicle would be easier than 27 engines on three cores with cross-feed.

How so?

Each core is only feeding 9 engines via the octoweb.  Each core is a separate rocket, with it's own individual plumbing and tanks.
To put 27 engines on a single core, you have to take your propellant main feed line and divide it 27 -equal- ways to feed your engines, vs. just 9 engines.  Certainly not impossible (other than the vibration issues they were working on, the N-1 did it), but I would think a much more difficult and complex task.

I'm no rocket engineer, but I am a mechanical engineer that knows a little of fluid dynamics.  The fewer bends, turns, and valves fluid goes through the better, generally speaking.  Although the S-1C only had 5 engines, they opted to feed each engine LOX  and RP-1 directly rather than manifold it them.  I'm guessing they did that because it's more efficient that way and they wanted optimal performance?  (Although I believe the S-II's propellants were manifolded).
I'm not real sure of the N-1's 1st stage plumbing, but it must have been quite something.


Offline TomH

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Re: Lots of little Raptors
« Reply #119 on: 01/27/2015 09:47 pm »
In addition to other N-1 issues mentioned, I believe there were some metallurgy issues with them not being able to perfect some alloys that we had perfected. Also, the engine design was not completely perfected and refined at that point in time. N-1 cannot be equivocated with a high Raptor count. All the other variables are different. Thirty plus engines does give a higher probability of having one engine out. OTOH, the probability of a single engine out causing LOM falls to nill. Modern controllers shut an engine (liquid) down long before it explodes. Lose one engine and lose only about 3% of thrust. You just throttle up the other engines a hair, run them longer, or both.

Lobo, I think all your numbers on the pad, Nova, F-1A are correct. I remember them all exactly the same way.

Offline Lobo

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Re: Lots of little Raptors
« Reply #120 on: 01/27/2015 10:46 pm »
N-1 cannot be equivocated with a high Raptor count. All the other variables are different. Thirty plus engines does give a higher probability of having one engine out. OTOH, the probability of a single engine out causing LOM falls to nill. Modern controllers shut an engine (liquid) down long before it explodes. Lose one engine and lose only about 3% of thrust. You just throttle up the other engines a hair, run them longer, or both.

Yea, I'm sure SpaceX could make that many engines work.  N-1 probably would have gotten it to work too had it not been cancelled.  I think they had originally scheduled like 7 test flights to work out the bugs.  That's how the Soviets kinda did it back then.  PUt it together, launch it.  See why it blows up.  Fix it.  Launch again.  Fix again.  rinse, repeat until it works.  Particularly with N-1 as they had no way to actually test the stage other than launching it.    If BFR was actually over 10m wide and upwarsd of 15Mlbs of thrust I'm not sure how they'd test it.  I don't know if they could transport it to some inland facility like F9's test pad in Texas, unless it was by chance built and launching from that location.  Can they put the thing on the actual launch pad and hold it down and test it?
If it's 10m wide and not too powerful, they might be able to test the new booster at Stennis.  It should be able to accomodate such a stage I think.
Not sure what the plan is but I'm sure they'll have means to ground test the booster before it actually flies.  So however complex the plumbing turns out to be, they'll be able to make sure it's working right.


Lobo, I think all your numbers on the pad, Nova, F-1A are correct. I remember them all exactly the same way.

Ok, I was just pulling off the top of my head from memory.  Thanks.

Offline MP99

Re: Lots of little Raptors
« Reply #121 on: 01/30/2015 08:56 am »

I think the new info about a 500klb Raptor also indicate that BFR is going to be smaller than origainlly envisioned, and flown and reused more per Mars mission.  (which makes a lot of sense if you are using reusability as part of your paradigm, you want as high flight rate as possible.  Otherwise you have a ginormous BFR that you are only launching once or twice per Mars Launch window for quite some time.  SpaceX then would have SLS).

There is obviously a penalty for full reuse, IE including the upper stage. Even more so for GTO payloads.

It seems possible that the goal is to up the flight rate of BFR by eliminating Falcon Heavy, and fully-reusing BFR instead.

Without having run any numbers, ISTM that a less-ginourmous BFR could be more economical.

Cheers, Martin

Offline Burninate

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Re: Lots of little Raptors
« Reply #122 on: 01/30/2015 01:02 pm »
Another query:
How compatible might a Raptor (for the purposes of this query a generic FFSC ) be with CO/O2 fueled operation?

Offline docmordrid

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Re: Lots of little Raptors
« Reply #123 on: 01/30/2015 01:23 pm »
IIRC the ISP for CO2 is only 290 seconds while CH4 is in the high 360+ second range, so the answer is CO2's low efficiency makes it less practical.
« Last Edit: 01/30/2015 01:27 pm by docmordrid »
DM

Offline Burninate

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Re: Lots of little Raptors
« Reply #124 on: 01/30/2015 01:32 pm »
IIRC the ISP for CO2 is only 290 seconds while CH4 is in the high 360+ second range, so the answer is CO2's low efficiency makes it less practical.
290 seconds is not prohibitive for the possibility I am pursuing;  Destroying the interior of the chamber with deposits that cannot be removed except in an Earthside factory, is.

Offline Lobo

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Re: Lots of little Raptors
« Reply #125 on: 01/30/2015 05:58 pm »

I think the new info about a 500klb Raptor also indicate that BFR is going to be smaller than origainlly envisioned, and flown and reused more per Mars mission.  (which makes a lot of sense if you are using reusability as part of your paradigm, you want as high flight rate as possible.  Otherwise you have a ginormous BFR that you are only launching once or twice per Mars Launch window for quite some time.  SpaceX then would have SLS).

There is obviously a penalty for full reuse, IE including the upper stage. Even more so for GTO payloads.

It seems possible that the goal is to up the flight rate of BFR by eliminating Falcon Heavy, and fully-reusing BFR instead.

Without having run any numbers, ISTM that a less-ginourmous BFR could be more economical.

Cheers, Martin

Agreed on the less-ginormous BFR being more economical for heavier commercial/government paylaods.  Ditto that for multi-launch Mars Missions.  Coupled with something less ginormous with relatively "small" engines being inherently cheaper and easier to process and handle.
In my mind, I keep thinking around 150mt to LEO.  A 50mt dry MCT and 100mt of stowed cargo.  A dry MCT could be heavier than that, that's just a nice round number for speculation at this level.  :-)

I don't know that I agree on such a less ginormous BFR eliminating FH though.  I could see it eliminating a fully expendable FH, or maybe FH that needs the central core expended.   Although those payloads would probably be rare.  But I just don't see how even a less ginormous BFR could launch from LC-40, SLC-4E, or Boca Chica.  And I think FH-R with all 3 cores RTLS will be of a performance level (23mt to LEO, 6-7mt to GTO) that it will probably have a relatively high flight rate. 
Part of the advantage of F9/FH is the ability for it's stages to be road transportated around, where any sort of BFR will be limited to barge ocean transport.
« Last Edit: 01/30/2015 06:05 pm by Lobo »

Offline BobHk

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Re: Lots of little Raptors
« Reply #126 on: 01/30/2015 06:22 pm »

I think the new info about a 500klb Raptor also indicate that BFR is going to be smaller than origainlly envisioned, and flown and reused more per Mars mission.  (which makes a lot of sense if you are using reusability as part of your paradigm, you want as high flight rate as possible.  Otherwise you have a ginormous BFR that you are only launching once or twice per Mars Launch window for quite some time.  SpaceX then would have SLS).

There is obviously a penalty for full reuse, IE including the upper stage. Even more so for GTO payloads.

It seems possible that the goal is to up the flight rate of BFR by eliminating Falcon Heavy, and fully-reusing BFR instead.

Without having run any numbers, ISTM that a less-ginourmous BFR could be more economical.

Cheers, Martin

Agreed on the less-ginormous BFR being more economical for heavier commercial/government paylaods.  Ditto that for multi-launch Mars Missions.  Coupled with something less ginormous with relatively "small" engines being inherently cheaper and easier to process and handle.
In my mind, I keep thinking around 150mt to LEO.  A 50mt dry MCT and 100mt of stowed cargo.  A dry MCT could be heavier than that, that's just a nice round number for speculation at this level.  :-)

I don't know that I agree on such a less ginormous BFR eliminating FH though.  I could see it eliminating a fully expendable FH, or maybe FH that needs the central core expended.   Although those payloads would probably be rare.  But I just don't see how even a less ginormous BFR could launch from LC-40, SLC-4E, or Boca Chica.  And I think FH-R with all 3 cores RTLS will be of a performance level (23mt to LEO, 6-7mt to GTO) that it will probably have a relatively high flight rate. 
Part of the advantage of F9/FH is the ability for it's stages to be road transportated around, where any sort of BFR will be limited to barge ocean transport.

Quote
where any sort of BFR will be limited to barge ocean transport.

But if they get landing and re-use down pat couldn't the stage hop from manufacturing center to launch pad?

Offline DanielW

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Re: Lots of little Raptors
« Reply #127 on: 01/30/2015 06:44 pm »
Does lots of raptors open the possibility of a sudo plug nozzle on a squat rocket? Just wondering if they could an isp bump at higher altitudes but putting raptors in a ring around a picax plug. Landing would require at least 3 working engines. I am assuming no gimbaling. I am not sure if there would be either enough control authority of a high enough response rate with just differential throttling.

Offline Burninate

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Re: Lots of little Raptors
« Reply #128 on: 01/30/2015 09:09 pm »
  Destroying the interior of the chamber with deposits that cannot be removed except in an Earthside factory, is.

 ???
Possibility: Raptor on CO+O2 suffers from coking buildup which limits use to five burns
Possibility: Raptor on CO+O2 explodes
Possibility: Raptor on CO+O2 corrodes something important

I don't know if any of these things is reasonable or not.  I am asking whether Raptor on CO+O2 is even likely to be usable at all, and whether it is likely to be as reusable as Raptor on CH4+O2.

Offline sanman

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Re: Lots of little Raptors
« Reply #129 on: 01/31/2015 04:21 am »

I think the new info about a 500klb Raptor also indicate that BFR is going to be smaller than origainlly envisioned, and flown and reused more per Mars mission.  (which makes a lot of sense if you are using reusability as part of your paradigm, you want as high flight rate as possible.  Otherwise you have a ginormous BFR that you are only launching once or twice per Mars Launch window for quite some time.  SpaceX then would have SLS).

There is obviously a penalty for full reuse, IE including the upper stage. Even more so for GTO payloads.

It seems possible that the goal is to up the flight rate of BFR by eliminating Falcon Heavy, and fully-reusing BFR instead.

Without having run any numbers, ISTM that a less-ginourmous BFR could be more economical.

Cheers, Martin

If your Mars launch window is limited, then wouldn't it mean that bigger rocket is better, to carry as much as possible thru limited window?

Ideally, it would be nice to set your rocket's size and flight rate based on the volume of customers flying - but unfortunately, it seems there's no way to know that in advance. (Unless Musk could get people to sign up in advance, and then base his design on the number of people booked. Is there such a thing as the Traveling Colonist Problem?)

« Last Edit: 01/31/2015 04:22 am by sanman »

Offline TomH

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Re: Lots of little Raptors
« Reply #130 on: 01/31/2015 07:32 am »
If your Mars launch window is limited, then wouldn't it mean that bigger rocket is better, to carry as much as possible thru limited window?

That window is for Trans-Martian Injection. You have two yeas to assemble and transfer fuel to a modular MCT somewhere between LEO and L1 before sending the thing on its way to Mars. Is it better to have a smaller rocket that flies a number of times each cycle to the rendezvous point, or a gigantic rocket that sits in a hangar for two years at a time between short flights? The argument could be made that you could send a much larger assembly through TMI this way than via a single shot on a VBFR.

Offline sanman

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Re: Lots of little Raptors
« Reply #131 on: 01/31/2015 07:57 am »
That window is for Trans-Martian Injection. You have two yeas to assemble and transfer fuel to a modular MCT somewhere between LEO and L1 before sending the thing on its way to Mars. Is it better to have a smaller rocket that flies a number of times each cycle to the rendezvous point, or a gigantic rocket that sits in a hangar for two years at a time between short flights? The argument could be made that you could send a much larger assembly through TMI this way than via a single shot on a VBFR.

Well, I kind of said something not so dissimilar to that in the MCT thread, and they said the problem is that you can't do it for $500K per person.

http://forum.nasaspaceflight.com/index.php?topic=35424.msg1323147#msg1323147

If you had a Mars-cycler (as per Aldrin's suggestion), and you shuttle people between it and the ground, then your interplanetary vehicle never has to be designed for EDL.
Have a purpose-built vehicle that travels from ground to orbit and back. Have another purpose-built vehicle that travels between the planets. That interplanetary vehicle probably might experience less wear-and-tear, given that it only lives in the vacuum of space, and so that helps its reusability lifetime.

Meanwhile, the ones that go from Earth-to-orbit get more servicing, and some of them even hitch a ride to Mars to be used for ground-to-orbit over there, and then eventually get sent back to Earth again for more servicing.

Offline MP99

Re: Lots of little Raptors
« Reply #132 on: 01/31/2015 07:58 am »


If your Mars launch window is limited, then wouldn't it mean that bigger rocket is better, to carry as much as possible thru limited window?

That window is for Trans-Martian Injection. You have two yeas to assemble and transfer fuel to a modular MCT somewhere between LEO and L1 before sending the thing on its way to Mars. Is it better to have a smaller rocket that flies a number of times each cycle to the rendezvous point, or a gigantic rocket that sits in a hangar for two years at a time between short flights? The argument could be made that you could send a much larger assembly through TMI this way than via a single shot on a VBFR.

Yup.

BFR won't really be safe to fly on if it only flies every two years. That assumes the crew launch on MCT. Even if not, losing MCT during launch could have impacts to anyone already on Mars.

BTW I raise your L1, and throw L2 into the pot. But either of these allow methalox to be stored with negligible station keeping and zero boiloff via passive cooling (sunshields, etc). This does allow a trickle of prop launches during the quiet period between windows.

Cheers, Martin

Offline MP99

Re: Lots of little Raptors
« Reply #133 on: 01/31/2015 03:35 pm »
I don't know that I agree on such a less ginormous BFR eliminating FH though.  I could see it eliminating a fully expendable FH, or maybe FH that needs the central core expended.   Although those payloads would probably be rare.  But I just don't see how even a less ginormous BFR could launch from LC-40, SLC-4E, or Boca Chica.  And I think FH-R with all 3 cores RTLS will be of a performance level (23mt to LEO, 6-7mt to GTO) that it will probably have a relatively high flight rate. 
Part of the advantage of F9/FH is the ability for it's stages to be road transportated around, where any sort of BFR will be limited to barge ocean transport.

Quote from: http://www.parabolicarc.com/2015/01/06/highlights-elon-musks-reddit-session/
The Mars transport system will be a completely new architecture. Am hoping to present that towards the end of this year. Good thing we didn’t do it sooner, as we have learned a huge amount from Falcon and Dragon.

Quote from: http://www.parabolicarc.com/2015/01/06/highlights-elon-musks-reddit-session/
Actually, we could make the 2nd stage of Falcon reusable and still have significant payload on Falcon Heavy, but I think our engineering resources are better spent moving on to the Mars system.


F9 v1.1 (and therefore FH) are the first stab at a reusable architecture, but you would expect a follow-on system to learn a huge number of lessons, and be much more mature at it. (Falcon will evolve as much as it can from lessons-learnt, but I'm assuming there are limits to what you can tweak vs architectural changes.)

Also, those Raptors are a whole step forward in terms of power density and pushing the materials (much higher pressures), but again, they will be the product of a team that has several years experience flying / reusing / ground handling M1D & Falcon.

The question is whether the cost/kg is low enough when you only have to prep one core and can reuse the upper stage, so that it can carry the overhead of extra prop and/or heatshields to actually recover that upper stage.

But, I agree on the road transport thing. Taking the largest CommSats to SSO may require FH core to land on a barge, which then entails a long road journey back from Florida to Texas. BFR would probably need both core & upper stage to RTLS to make the economics work out.

cheers, Martin

Offline MP99

Re: Lots of little Raptors
« Reply #134 on: 01/31/2015 04:20 pm »
One other element to consider...

Assume BFR-US has a single Raptor-vac and its thrust scales with Isp (500 klb / 320s * 380s) = ~600klb.

At 50% throttle, that would hit 6g for ~23t, which leaves ~17t for upper stage, heatshield, payload adaptor, landing legs/system, recovery prop, etc (assuming a 6t CommSat).

This might be one of the justifications for a smaller Raptor - that it can carry smaller payloads without having silly G's at burnout.



I'm also wondering whether the nozzle on Raptor-vac could survive the sort of tail-first reentry burn that F9S1 goes through today? I'm sure it's fine if it's thrusting, but what about turbulent aero forces just after engine shutdown?

If not, might that 17t include carrying the PLF all the way back to the ground so it can be reused, but also repurposed as a heatshield for reentry?

I'd envisage it being in four petals, which would keep the payload enclosed through to SECO1 (or maybe post-GTI), then open out to release the payload.

By rotating 180 degrees, the PLF could then close around the stage to provide a heatshield for the reentry from GTO.

If the petals open up during the atmospheric descent, they could reduce the terminal velocity dramatically (sort of a parachute), and perhaps act like huge grid fins.

And, finally, they could provide the legs for the stage to land on. They'd want to undertake the transition from "parachute" to legs late in the landing burn, to minimise aero effects from the transition - much like the late opening of the legs on F9's core.



So, question is - presuming Raptor-vac uses a similar nozzle to M1D-vac, could it survive a tail-first reentry, and the subsequent aero forces throughout descent?

cheers, Martin
« Last Edit: 01/31/2015 04:23 pm by MP99 »

Offline Mariusuiram

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Re: Lots of little Raptors
« Reply #135 on: 02/01/2015 07:24 am »
Isn’t the fact that the Upper Stage could be part of the MCT a relevant point for calculations?

If it was designed for in-space reuse, the current F9 could put 13.5 mT of cargo + the full US (empty), correct? The US can reach orbit? What is its empty weight? I’d imagine upwards of 10-15k mT?

Now a refillable, reusable US will probably require more effort than the current one and weigh more, but you are still adding to your mass to orbit capability. Shouldn’t this have a substantial impact on people’s calculations? The alternative being that the 2nd stage jettisons the MCT then the MCT uses a smaller engine from LEO to do TMI or other BEO maneuvers. So your mass goal in comparison to F9 is just the cargo compartment + cargo, while the engines, tanks, etc come up as the 2nd stage (minus the fuel). Then you could have a ~20-40 mT MCT structure + ~100 mT of cargo with the depleted 2nd stage as your booster for TMI.

What’s the issue? Would the US be oversized for TMI needs and thus be wasteful? I assume I am misunderstanding something. Because based on that model, you are putting to TMI the same as you put up in LEO at the cost of an additional launch (of propellant) and some sort of in-orbit fueling mechanism.

Considering Elon quotes, this would also address his comment that full reusability will be possible with the BFR. He says the next generation vehicle will have full reusability. Which is different than saying they will land the 2nd stage after launch. Its probably cheaper mass-wise to design the second stage for in-orbit refueling & reuse than re-entry after ejecting the cargo.

Offline Lars-J

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Re: Lots of little Raptors
« Reply #136 on: 02/01/2015 08:05 am »

That window is for Trans-Martian Injection. You have two yeas to assemble and transfer fuel to a modular MCT somewhere between LEO and L1 before sending the thing on its way to Mars. Is it better to have a smaller rocket that flies a number of times each cycle to the rendezvous point, or a gigantic rocket that sits in a hangar for two years at a time between short flights? The argument could be made that you could send a much larger assembly through TMI this way than via a single shot on a VBFR.

Well, I kind of said something not so dissimilar to that in the MCT thread, and they said the problem is that you can't do it for $500K per person.

http://forum.nasaspaceflight.com/index.php?topic=35424.msg1323147#msg1323147

If you had a Mars-cycler (as per Aldrin's suggestion), and you shuttle people between it and the ground, then your interplanetary vehicle never has to be designed for EDL.
Have a purpose-built vehicle that travels from ground to orbit and back. Have another purpose-built vehicle that travels between the planets. That interplanetary vehicle probably might experience less wear-and-tear, given that it only lives in the vacuum of space, and so that helps its reusability lifetime.

Meanwhile, the ones that go from Earth-to-orbit get more servicing, and some of them even hitch a ride to Mars to be used for ground-to-orbit over there, and then eventually get sent back to Earth again for more servicing.

I'll never understand the fascination with 'cyclers'. They really only give you ONE thing. More living space during the transit. That's all. You still have to bring ALL the supplies with you (no mass savings), you still have to accelerate to TMI (no propellant savings), and for what? To save on habitable volume? Remember that this is living space volume AND cargo volume that will be needed on Mars when you get there.

And as a cherry on top, you have a very narrow launch window, and add another failure more if you fail to rendezvous.

Cyclers are just a bad idea, and Aldrin arguing for them doesn't make it any better.

Offline Burninate

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Re: Lots of little Raptors
« Reply #137 on: 02/01/2015 08:17 am »

That window is for Trans-Martian Injection. You have two yeas to assemble and transfer fuel to a modular MCT somewhere between LEO and L1 before sending the thing on its way to Mars. Is it better to have a smaller rocket that flies a number of times each cycle to the rendezvous point, or a gigantic rocket that sits in a hangar for two years at a time between short flights? The argument could be made that you could send a much larger assembly through TMI this way than via a single shot on a VBFR.

Well, I kind of said something not so dissimilar to that in the MCT thread, and they said the problem is that you can't do it for $500K per person.

http://forum.nasaspaceflight.com/index.php?topic=35424.msg1323147#msg1323147

If you had a Mars-cycler (as per Aldrin's suggestion), and you shuttle people between it and the ground, then your interplanetary vehicle never has to be designed for EDL.
Have a purpose-built vehicle that travels from ground to orbit and back. Have another purpose-built vehicle that travels between the planets. That interplanetary vehicle probably might experience less wear-and-tear, given that it only lives in the vacuum of space, and so that helps its reusability lifetime.

Meanwhile, the ones that go from Earth-to-orbit get more servicing, and some of them even hitch a ride to Mars to be used for ground-to-orbit over there, and then eventually get sent back to Earth again for more servicing.

I'll never understand the fascination with 'cyclers'. They really only give you ONE thing. More living space during the transit. That's all. You still have to bring ALL the supplies with you (no mass savings), you still have to accelerate to TMI (no propellant savings), and for what? To save on habitable volume? Remember that this is living space volume AND cargo volume that will be needed on Mars when you get there.

And as a cherry on top, you have a very narrow launch window, and add another failure more if you fail to rendezvous.

Cyclers are just a bad idea, and Aldrin arguing for them doesn't make it any better.
They potentially give you two things:
*More living space during transit
*Much, much more radiation protection during transit

The asteroid redirect mission could just as easily redirect the rock into heliocentric cycler orbit, divide it into sandbags, and strap the sandbags to the exterior of a habitat.

That said, a cycler in the context of the bullet points associated with the MCT, does not make much sense.  It's not remotely in the critical path.
« Last Edit: 02/01/2015 08:21 am by Burninate »

Offline MP99

Re: Lots of little Raptors
« Reply #138 on: 02/01/2015 12:52 pm »


Isn’t the fact that the Upper Stage could be part of the MCT a relevant point for calculations?

If it was designed for in-space reuse, the current F9 could put 13.5 mT of cargo + the full US (empty), correct? The US can reach orbit?

I believe that BFR will have a basic Upper-Stage structure, which will then be customised into three variants:-

1) A basic Upper-Stage which carries a standard payload under a standard PLF. Elon has said that he will be deferring Upper-Stage reuse to BFR, and I believe this will be used for GTO payloads while carrying a large mass penalty for reuse.

2) The same basic structure, but integrated into MCT. This may involve a barrel stretch or shrink of that U/S structure. The reentry of variant (1) at Earth will allow the design to be optimised, which will increase the maturity / safety of EDL at Mars, and is just one of the justifications for (1).

3) A stretched version of (1) which will be used as a tanker. No payload. No PLF. Just some sort of ogive-like tank top, like Shuttle's ET.



BFR can't fly only once every two years at Mars windows, so variant (1) is required to make it safe to fly while being commercially viable.

Variant (3) is a *relatively* trivial evolution of (1), and comparable to ACES-41 & ACES-71.

Cheers, Martin

Offline guckyfan

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Re: Lots of little Raptors
« Reply #139 on: 02/01/2015 01:04 pm »
I believe that BFR will have a basic Upper-Stage structure, which will then be customised into three variants:-

1) A basic Upper-Stage which carries a standard payload under a standard PLF. Elon has said that he will be deferring Upper-Stage reuse to BFR, and I believe this will be used for GTO payloads while carrying a large mass penalty for reuse.

2) The same basic structure, but integrated into MCT. This may involve a barrel stretch or shrink of that U/S structure. The reentry of variant (1) at Earth will allow the design to be optimised, which will increase the maturity / safety of EDL at Mars, and is just one of the justifications for (1).

3) A stretched version of (1) which will be used as a tanker. No payload. No PLF. Just some sort of ogive-like tank top, like Shuttle's ET.

I believe there will be no version 1)

There will be versions 2a) and 2b)

2a) The manned version of MCT

2b) The cargo version of MCT. With large payload doors to allow transfer of big infrastructure items. This version will do all transport functions. Including satellites to LEO, GTO or whereever. No fairing version. Unless someone comes up with a payload that needs every bit of performance. Unlikely IMO and then he would have to pay for fairing development.

Offline GORDAP

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Re: Lots of little Raptors
« Reply #140 on: 02/01/2015 03:20 pm »
I'll never understand the fascination with 'cyclers'. They really only give you ONE thing. More living space during the transit. That's all. You still have to bring ALL the supplies with you (no mass savings), you still have to accelerate to TMI (no propellant savings), and for what? To save on habitable volume? Remember that this is living space volume AND cargo volume that will be needed on Mars when you get there.

And as a cherry on top, you have a very narrow launch window, and add another failure more if you fail to rendezvous.

Cyclers are just a bad idea, and Aldrin arguing for them doesn't make it any better.

Lars-J, don't cyclers potentially give quite a bit more than the "One thing" (extended living space) than you say? 

I think you can more easily design them to give variable artificial gravity by spinning opposing habitat modules, and architect those habs to more easily provide a 'storm shelter' against solar flares.  But the big thing they give you is the ability to use low thrust, high ISP propulsion (ion, nuclear thermal, VASIMR, etc.).  This gives you either faster transit times or greater mass ratios (or a combination of these two).

Of course there's no question that the MCT will not be a cycler - Musk has been very clear about that.  But I just think there are too many advantages in the long run for having crafts that are optimized for their jobs:  Launchers/Landers for moving people and cargo to/from orbit (needing high thrust, streamlined form factor, TPS and landing gear, and very little life support) and planet-to-planet craft (high ISP, high degree of life support including spin gravity and radiation mitigation, unconstrained form factor, and no TPS/landing legs).  I suspect that if we do enter an era in the next few decades where multiple hundreds of folks are going to Mars each synod, I'd bet it'll be on some craft that'll never touch dirt or regolith. 

Note:  Your comment re restricted launch windows gives me pause.  Hmm, maybe I'm misunderstanding 'cyclers'?  I envision a craft that's constructed in LEO, and transits to Mars orbit where it offloads cargo and people to a lander that came up from the surface, then it transits back to Earth orbit where it gets it's next batch of cargo and people.  Repeat ad infinitum.  Guess I'll read up on Aldrin's version.

Edit: O.K., now I understand 'Cyclers' - should have done that before posting. ::)  I see your point about no fuel savings.  So let me call what I'm thinking about something else.  Hmm, 'vacuum ships' or something.  Anyway, still think there would be an advantage to high ISP, unconstrained form factor craft going just from orbit to orbit (never touch the surface).
« Last Edit: 02/01/2015 03:49 pm by GORDAP »

Offline guckyfan

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Re: Lots of little Raptors
« Reply #141 on: 02/01/2015 03:46 pm »
Note:  Your comment re restricted launch windows gives me pause.  Hmm, maybe I'm misunderstanding 'cyclers'?  I envision a craft that's constructed in LEO, and transits to Mars orbit where it offloads cargo and people to a lander that came up from the surface, then it transits back to Earth orbit where it gets it's next batch of cargo and people.  Repeat ad infinitum.  Guess I'll read up on Aldrin's version.

Yes, you do. A cycler is something that cycles between Earth and Mars on a free trajectory. Or almost free, it will need occasional course corrections.

Unless you have a whole ecosphere that can provide food and air to breathe, it indeed offers only better living space plus better radiation protection because it can be massive.

It requires more delta-v than direct flight because its trajectory is not an energy saving one. So it saves only if your vehicle can be substantially lighter. That may be the case for crew. Not with cargo. The cargo does not become lighter because you use the cycler for transport. So a cycler is worse than useless for cargo.


Offline GORDAP

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Re: Lots of little Raptors
« Reply #142 on: 02/01/2015 03:52 pm »
Yup, now I see.  See my edit above.

But wouldn't there still be other advantages for using high ISP, unconstrained form factor craft to transit from orbit to orbit only.  I think this would go for cargo as well.  (Sorry - I shouldn't have confused this with 'Cyclers'.)

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