Author Topic: Low cost second stage for LEO?  (Read 3853 times)

Online LouScheffer

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Low cost second stage for LEO?
« on: 04/26/2017 02:48 pm »
Assuming SpaceX can recover boosters and fairings, the largest remaining cost is the second stage.  If a Falcon costs $50m to build, and 70% of the cost is the booster, and 10% is the fairing, then 20% is the second stage, or about $10 million.

Recovering the second stage looks hard.  But what about reducing the cost?  A lower cost stage will have less performance, but particularly to LEO this may still result is very useful capacity.  SpaceX could reduce the cost in at least two ways - use a regular merlin 1D (not vacuum optimized, potentially used) as the engine, and use cheaper (but heavier) construction for the stage.

How much would this hurt performance?  SpaceX quotes 22.8 tonnes to LEO.  Using the usual assumptions of 4.5t empty mass and 111.5t of fuel, the second stage delta V is 348*9.8*ln((111.5+4.5+22.8 )/(4.5+22.8 )) = 5546 m/s.   If we reduce the ISP to 311, and assume double the empty mass, then we get a similar delta-V for a 12.5 tonne payload:  311*9.8*ln((111.5+9+12.5)/(9+12.5) = 5554 m/s.  12.5 tonnes of payload seems like plenty for most LEO missions.

I'm not a manufacturing engineer, but I'd have to imagine that with twice the mass allowed, you could simplify the construction considerably.  No need for aluminum-lithium, no careful shaving of wall thickness, no common bulkhead.  You could use argon in external metal tanks instead of helium in LOX-submerged COPVs.  (Enough argon to pressurize the second stage tanks masses about 300 kg).  Testing and inspection should be easier since you are not running so close to the margins.

Reducing the cost of the second stage by half would save another 10% of the cost of the rocket, about the same as fairing recovery.  Are there enough non-mass-limited LEO missions to make this worthwhile?  Probably not, if the LEO constellations have enough satellites in the same plane to fully utilize the upmass.  No other applications have the volume required.


Offline cppetrie

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Re: Low cost second stage for LEO?
« Reply #1 on: 04/26/2017 02:56 pm »
This would sacrifice all of the economies of scale of using the same 2nd stage for all missions and would require a new production line to service only the few LEO missions that are manifested. Doesn't seem likely to pay off.

Offline smfarmer11

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Re: Low cost second stage for LEO?
« Reply #2 on: 04/26/2017 03:13 pm »
Considering spacex's internet constellation plans in LEO and VLEO, that orbital regime will probably take up at least 75% of their yearly launches. So the economy of scale would be present.

Offline suncity

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Re: Low cost second stage for LEO?
« Reply #3 on: 04/26/2017 03:19 pm »
Using an already-flown Merlin could make sense. But it would be much better to add a nozzle extension to vacuum-optimize it.

Offline sevenperforce

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Re: Low cost second stage for LEO?
« Reply #4 on: 04/26/2017 03:25 pm »
Assuming SpaceX can recover boosters and fairings, the largest remaining cost is the second stage.  If a Falcon costs $50m to build, and 70% of the cost is the booster, and 10% is the fairing, then 20% is the second stage, or about $10 million.

Recovering the second stage looks hard.  But what about reducing the cost?  A lower cost stage will have less performance, but particularly to LEO this may still result is very useful capacity.  SpaceX could reduce the cost in at least two ways - use a regular merlin 1D (not vacuum optimized, potentially used) as the engine...
No comment on the rest of this, but using used Merlin 1Ds for the upper stage has already been discussed and it doesn't really make much sense. If you have a used Merlin 1D laying around, why not use it on a new booster?

Offline hkultala

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Re: Low cost second stage for LEO?
« Reply #5 on: 04/26/2017 03:34 pm »
Assuming SpaceX can recover boosters and fairings, the largest remaining cost is the second stage.  If a Falcon costs $50m to build, and 70% of the cost is the booster, and 10% is the fairing, then 20% is the second stage, or about $10 million.

Recovering the second stage looks hard.  But what about reducing the cost?  A lower cost stage will have less performance, but particularly to LEO this may still result is very useful capacity.  SpaceX could reduce the cost in at least two ways - use a regular merlin 1D (not vacuum optimized, potentially used) as the engine, and use cheaper (but heavier) construction for the stage.

How much would this hurt performance?  SpaceX quotes 22.8 tonnes to LEO.  Using the usual assumptions of 4.5t empty mass and 111.5t of fuel, the second stage delta V is 348*9.8*ln((111.5+4.5+22.8 )/(4.5+22.8 )) = 5546 m/s.   If we reduce the ISP to 311, and assume double the empty mass, then we get a similar delta-V for a 12.5 tonne payload:  311*9.8*ln((111.5+9+12.5)/(9+12.5) = 5554 m/s.  12.5 tonnes of payload seems like plenty for most LEO missions.


1)
22.8 tonnes is with EXPENDABLE first stage.

LEO Payload with first stage recovery is something around 17 tonnes.
Using your equation, this makes the "cheap second stage performance" only 7.7 tonnes.



Though the "double the weight stage" would probably be too much; atmospheric Merlin is lighter than vacuum merlin and I don't think they could save much by making the stage weight twice as much.

2)
Most payloads are going into GTO, not LEO.
And worse second stage hurts much more for GTO missions. Actually it would hurt so much that when reusing the first stage, it could not even reach GTO without ANY payload. (need payload weight of -1.8 tonnes with your equations, assuming 5.5 tonnes for normal GTO payload with recoverable 1st stage, or -1.3 tonnes assuming 6 tonnes normal GTO payload with recoverable 1st stage.
« Last Edit: 04/26/2017 03:36 pm by hkultala »

Offline gospacex

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Re: Low cost second stage for LEO?
« Reply #6 on: 04/26/2017 04:45 pm »
It might make sense to once in a while use an old sea-level Merlin. Presumably, Merlins would not be infinitely reusable. (<-- This comes from "rockets are legos" dept, which does not know whether MVac-to-stage plumbing is compatible with sea-level Merlins)

Making an entire new "cheaper heavier stage" adds too many complications such as needing to track two kinds of materials and differently manufactured stage barrel sections.

Offline cppetrie

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Re: Low cost second stage for LEO?
« Reply #7 on: 04/26/2017 05:53 pm »
Considering spacex's internet constellation plans in LEO and VLEO, that orbital regime will probably take up at least 75% of their yearly launches. So the economy of scale would be present.
They would gain more by being able to carry more sats per launch than a marginal gain in cost of 2nd stage by reducing payload capacity. The other manufacturing downsides already mentioned also make this an unlikely path forward. Also unless it shortened manufacturing time it doesn't solve the 2nd stage production issue which is likely a bigger problem than the cost of the stage if they are planning to launch 1000s of sats.

Offline Lars-J

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Re: Low cost second stage for LEO?
« Reply #8 on: 04/26/2017 06:11 pm »
It might make sense to once in a while use an old sea-level Merlin. Presumably, Merlins would not be infinitely reusable. (<-- This comes from "rockets are legos" dept, which does not know whether MVac-to-stage plumbing is compatible with sea-level Merlins)

M1D and M1DVac are not interchangeable. This idea comes up every now and then, but apparently it needs to be repeated: They are very different.

Image 1 - A 1:1 scale comparison to highlight the difference
Image 2 - A finished M1DVac
Image 3 - A finished M1D (very different)


Offline Zed_Noir

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Re: Low cost second stage for LEO?
« Reply #9 on: 04/28/2017 08:13 pm »
Maybe SpaceX should bring back the Kestrel with an electric pump and solar-electric power system.

Offline Nomadd

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Re: Low cost second stage for LEO?
« Reply #10 on: 04/28/2017 08:47 pm »
Maybe SpaceX should bring back the Kestrel with an electric pump and solar-electric power system.
And mount 9 of them on the stage?
Those who danced were thought to be quite insane by those who couldn't hear the music.

Offline guckyfan

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Re: Low cost second stage for LEO?
« Reply #11 on: 04/28/2017 11:25 pm »
M1D and M1DVac are not interchangeable. This idea comes up every now and then, but apparently it needs to be repeated: They are very different.

OK I believ we all know now that a M1D can not be converted to a M1DVac.

But that is not the point raised here. The idea is to install a M1D in place of a MVac. Are the mount points so different?

Offline sevenperforce

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Re: Low cost second stage for LEO?
« Reply #12 on: 04/28/2017 11:32 pm »
M1D and M1DVac are not interchangeable. This idea comes up every now and then, but apparently it needs to be repeated: They are very different.

OK I believ we all know now that a M1D can not be converted to a M1DVac.

But that is not the point raised here. The idea is to install a M1D in place of a MVac. Are the mount points so different?
.............................yes.

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