Author Topic: Crewed Dragon V2 flight with Kestrel in trunk extension  (Read 15941 times)

Offline sevenperforce

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Over on the trunk modification speculation thread, and I got to thinking about the possibilities of turning the Dragon V2's trunk into something more like a true service module by adding an engine and propellant, not for on-orbit maneuvering, but for launch. I speculated that by doing so, the entire Falcon 9 second stage could be eliminated while preserving first-stage reuse with plenty of margin on crewed Dragon V2 flights.

Ran the numbers, and it looks promising.

The pressure-fed Kestrel engine on the Falcon 1 weighed in at only 52 kg and nominally launched with around 3.5 tonnes of fuel to execute orbital insertion. It was a combination ablative+radiative nozzle with helium-loop heat exchanger to improve pressurant efficiency. The second stage tankage and body together (less engine) massed 9.1% of the fuel mass. Let's suppose, just for the sake of argument, that SpaceX decided to dust off the Kestrel.

The extended trunk available for the Dragon 1 has an unpressurized volume of 34 cubic meters. Currently, the Dragon V2 only comes with the standard trunk, but it's not unrealistic to suppose that the standard trunk could be modified to match the extended trunk available with the Dragon 1.

A few numbers: assuming 95% volume utilization and propellant subcooling densification, the extended trunk could carry 32.3 tonnes of kerolox with an additional tankage mass of 2.94 tonnes, plus the 52 kg Kestrel engine. There would be substantial mass savings from using the trunk as the body and using Al-Li alloy for the tankage rather than the Al used for the Falcon 1, but since I don't know the added structural mass of the trunk extension, I'll just assume the same mass.

The Dragon V2 masses 6.4 tonnes plus 1.4 tonnes of onboard propellant; I will assume "payload" of 1.05 tonnes for a crew of 7. This works out to a "second stage" mass of 11.78 tonnes plus 32.3 tonnes of propellant. Given that the Kestrel's chamber pressure is about as high as the Merlin 1D's and that the Merlin is a GG cycle, I see no reason why an updated Kestrel with a longer niobum-alloy radiative nozzle extension could not match the MVac's 348s specific impulse. This would give our new "second stage" a dV of 4.5 km/s, not including any of the Dragon's onboard hypergol propellant.

The Kestrel is not a very big engine. It would have significantly improved thrust from propellant densification and a greater specific impulse, but it would still take a bit to get up to speed, so I will deduct 200 m/s for gravity drag.

Can the Falcon 9 first stage lift a 44.1-tonne modified Dragon V2 to 3.5 km/s with enough reserve propellant to return safely?

Yep.

I'll use the JCSAT-14 mission as a benchmark. MECO took place at 2,321 m/s with an estimated 2.7 tonnes of reserve propellant and a second stage plus payload massing 117.95 tonnes. This means the effective specific impulse from launch to 2,321 m/s is 175.8 s, a reduction which reflects gravity and aerodynamic drag losses. The rocket equation tells us that with a reduced "second stage" massing only 44.1 tonnes rather than 117.95 tonnes, 2,321 m/s would be reached with 57.38 tonnes of propellant remaining.

An additional 1,178 m/s of dV (to get up to 3.5 km/s) at an estimated 315 s vacuum Isp (the Merlin 1Ds are now even more overexpanded due to thrust increases, driving up pressure thrust and driving down thrust-specific fuel consumption) results in a vehicle mass of 84.4 tonnes at the new MECO.

The modified Dragon separates, leaving a first stage at 40.35 tonnes. Burning retrograde to immediately drop the first stage back down to the JCSAT-14 MECO speed of 2,321 m/s costs 12.8 tonnes of fuel, leaving us with 5.4 tonnes of reserve propellant for re-entry and landing burns, twice as much as the first stage had on the JCSAT-14 launch. And that's without any improved launch margins due to lower gravity drag.

Decreased launch costs from skipping the second stage and replacing it with a Kestrel stage inside the extended Dragon trunk will be left as an exercise for the reader...but it should be substantial. Granted, this won't work if you want to carry cargo to the ISS along with your crew. But for crew-only launches it would work well, and you'd end up with less waste.

Offline Lar

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #1 on: 05/17/2016 04:02 pm »
You're basically building a new kind of second stage...

Rockets are not LEGO elements... :)
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Offline Moderas

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #2 on: 05/17/2016 04:07 pm »
A big problem with modifying the trunk in to a second stage for crewed operations is that a rocket stage would not be allowed near the ISS. Reboosts for ISS are done by engines on the docking capsules and all current suppliers drop their rocket stages and rely on built in thrusters for the maneuvers to ISS. Dragon can not fly without its trunk, so you couldn't drop it early.

Offline Jim

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #3 on: 05/17/2016 04:10 pm »
I don't understand the need for it.  What is the point?
« Last Edit: 05/17/2016 04:10 pm by Jim »

Offline RDMM2081

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #4 on: 05/17/2016 04:15 pm »
I've been rolling this exact idea around for awhile, thanks for running some numbers on it!  What does the final orbit of the Dragon 2 look like after the trunk/stage separates?

Mostly my thoughts on why to do this were:
1)  Extra falcons laying around needing a "payload" to fly
2)  Extra Dragon 2 will end up existing somewhere in the CCtCap milestones
3)  This would be a great way to test Dragon 2 propulsive EDL for super cheap

The cons:
1) No evidence that Kestrel can simply be "dusted off"
2) Engineer/Develop some new mutant trunk/stage with somewhat limited applications
3) If we want to go so far as launching a Dragon 2 directly from the first stage for "reasons", could we accomplish the same result with a totally different second stage, with re-entry/recovery capabilities built in?  Wouldn't this stage have more potential uses than a mutant trunk/stage, for example, launching smallsats in fully recoverable mode?

Anyways, love the idea, thanks for sharing!

Offline dunderwood

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #5 on: 05/17/2016 04:25 pm »
Quote
Given that the Kestrel's chamber pressure is about as high as the Merlin 1D's

Woah there.  Do you have a source for that assumption?  I think you're off by an order of magnitude. 

Offline Moderas

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #6 on: 05/17/2016 04:29 pm »
Quote
Given that the Kestrel's chamber pressure is about as high as the Merlin 1D's

Woah there.  Do you have a source for that assumption?  I think you're off by an order of magnitude.

Indeed. Kestrel had a chamber pressure of 930 kPa vs. 9.7 MPa (9700 kPa) of the Merlin 1D based on the wiki page. Numbers may not be exact as it is wiki but the scale of the difference is enough.
« Last Edit: 05/17/2016 04:31 pm by Moderas »

Offline sevenperforce

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #7 on: 05/17/2016 04:33 pm »
You're basically building a new kind of second stage...

Rockets are not LEGO elements... :)
Well, it works in KSP!

But sure, this would effectively be a new stage. It wouldn't be entirely wasted development; they could use it to significantly extend the Dragon's range for BLEO missions too. Updating the Kestrel would not be prohibitively expensive; the existing Falcon 1 second stage architecture would only require moderate modifications.

Cost savings would be substantial.

A big problem with modifying the trunk in to a second stage for crewed operations is that a rocket stage would not be allowed near the ISS. Reboosts for ISS are done by engines on the docking capsules and all current suppliers drop their rocket stages and rely on built in thrusters for the maneuvers to ISS. Dragon can not fly without its trunk, so you couldn't drop it early.
But...Space Shuttle?

I mean, I know that the Shuttle dropped its external tank. But the OMS actually did the orbital insertion. I imagine that would be a fairly inconsequential issue...if NASA really did fuss about it, they could potentially put in pusher bolts to jettison the Kestrel after its burn.

I don't understand the need for it.  What is the point?
Reduction in otherwise-fixed marginal costs. After all the recent upratings, the Falcon 9 second stage is woefully overpowered for ISS missions. If SpaceX can retain the same first stage and replace the second stage with a service-module trunk, they go from losing the entire second stage to just losing a Kestrel. I don't know how much a Kestrel would cost, but the Falcon 1 launch price was going to be 6M and that included the cost of the Merlin 1C and the first stage, which are comparable to the second stage.

Plus, if Red Dragon goes well and SpaceX wants to increase the Dragon V2's BLEO range, stacking this on top of a Falcon Heavy would definitely do the trick. 44 tonnes is 81% of FHE's payload to LEO, meaning that you could have a Dragon V2 in orbit with 4.5 km/s of BLEO dV with 2/3 reuse of the FH boosters.

If we want to go so far as launching a Dragon 2 directly from the first stage for "reasons", could we accomplish the same result with a totally different second stage, with re-entry/recovery capabilities built in?  Wouldn't this stage have more potential uses than a mutant trunk/stage, for example, launching smallsats in fully recoverable mode?

Anyways, love the idea, thanks for sharing!
Of course! It was something I had been tossing around for a while.

Going full-reuse on a different second stage doesn't seem likely. Doing a re-entry at orbital speed would require an entirely new transatmospheric launch-re-entry vehicle (which is what MCT will be). And reserving braking propellant is equally problematic; the only reason the above concept works is by using a very small upper stage to give the first stage a lot of braking propellant. A larger upper stage with enough braking propellant for re-entry would put too much of a penalty on the first stage and you would end up losing both.

Quote
Given that the Kestrel's chamber pressure is about as high as the Merlin 1D's

Woah there.  Do you have a source for that assumption?  I think you're off by an order of magnitude.

Indeed. Kestrel had a chamber pressure of 930 kPa vs. 9.7 MPa (9700 kPa) of the Merlin 1D based on the wiki page. Numbers may not be exact as it is wiki but the scale of the difference is enough.
Whoops! I must have been thinking of the SuperDraco in comparison to the Merlin, or something. Hmm. Wonder what kind of isp an updated densified-propellant Kestrel could push.

Offline Jim

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #8 on: 05/17/2016 04:34 pm »

1)  Extra falcons laying around needing a "payload" to fly
2)  Extra Dragon 2 will end up existing somewhere in the CCtCap milestones
3)  This would be a great way to test Dragon 2 propulsive EDL for super cheap

The cons:
1) No evidence that Kestrel can simply be "dusted off"
2) Engineer/Develop some new mutant trunk/stage with somewhat limited applications
3) If we want to go so far as launching a Dragon 2 directly from the first stage for "reasons", could we accomplish the same result with a totally different second stage, with re-entry/recovery capabilities built in?  Wouldn't this stage have more potential uses than a mutant trunk/stage, for example, launching smallsats in fully recoverable mode?

Anyways, love the idea, thanks for sharing!

more cons

1.  What "extra falcons laying around needing a "payload" to fly"?  And if so, why not just the regular upper stage?
2.  So?  What is the point?  If they want to fly it, just use a regular F9?
3.  How is this "super cheap"?  It would be a new development which costs $$$.  What is the problem with a regular F9?  and for high speed entry, FH?

Offline Jim

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #9 on: 05/17/2016 04:37 pm »

But sure, this would effectively be a new stage. It wouldn't be entirely wasted development; they could use it to significantly extend the Dragon's range for BLEO missions too. Updating the Kestrel would not be prohibitively expensive; the existing Falcon 1 second stage architecture would only require moderate modifications.

Cost savings would be substantial.


Cost savings for what?  For what requirement?  They don't need a new stage.  There is no business case for it.   FH is enough to extend the "Dragon's range for BLEO missions". 

Offline envy887

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #10 on: 05/17/2016 04:37 pm »
...an additional tankage mass of 2.94 tonnes, plus the 52 kg Kestrel engine.
...
Decreased launch costs from skipping the second stage and replacing it with a Kestrel stage inside the extended Dragon trunk will be left as an exercise for the reader...but it should be substantial.

The Merlin vac stage only weighs 3960 kg dry, so the cost savings would be minimal - less than the cost of one Merlin. I could see this as viable for cheap-ish (few million per seat) orbital or suborbital joyrides, if Falcon and Dragon eventually are highly reusable. Other than that, the payload is vastly reduced so useful mission profiles almost non-existent.

Doesn't every ISS crew launch include cargo?


Offline Jim

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #11 on: 05/17/2016 04:44 pm »

Reduction in otherwise-fixed marginal costs. After all the recent upratings, the Falcon 9 second stage is woefully overpowered for ISS missions. If SpaceX can retain the same first stage and replace the second stage with a service-module trunk, they go from losing the entire second stage to just losing a Kestrel. I don't know how much a Kestrel would cost, but the Falcon 1 launch price was going to be 6M and that included the cost of the Merlin 1C and the first stage, which are comparable to the second stage.



wrong.

a.  It does not provide any deduction in any type of costs.
b.  The second stage is not "woefully overpowered".  It is throttled per mission requirements.
c.  They lose the ability to carry hardware in the trunk
d.  Kestrel doesn't have the performance to replace the Merlin
e. this increases cost by having to maintain another configuration with its engineering, manufacturing and operational costs.


Plus, if Red Dragon goes well and SpaceX wants to increase the Dragon V2's BLEO range, stacking this on top of a Falcon Heavy would definitely do the trick

Then just lengthen the existing second stage tanks vs adding a stage.
« Last Edit: 05/17/2016 04:46 pm by Jim »

Offline cscott

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #12 on: 05/17/2016 04:49 pm »
If the usual second stage really *were* "woefully overpowered" (a big if), SpaceX would be using the extra margin for second-stage recovery experiments.  Not developing a new, smaller, second stage.
« Last Edit: 05/17/2016 04:50 pm by cscott »

Offline envy887

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #13 on: 05/17/2016 05:05 pm »
Indeed. Kestrel had a chamber pressure of 930 kPa vs. 9.7 MPa (9700 kPa) of the Merlin 1D based on the wiki page. Numbers may not be exact as it is wiki but the scale of the difference is enough.
Whoops! I must have been thinking of the SuperDraco in comparison to the Merlin, or something. Hmm. Wonder what kind of isp an updated densified-propellant Kestrel could push.

Merlin vac also has 135 times as much thrust as Kestrel. 6.9 kN is woefully underpowered for a manned orbital second stage.

Double check your numbers here: http://www.silverbirdastronautics.com/LVperform.html The Kestrel 6.9 kN, 317s with a 2.95t second stage gets less than 200kg to ISS orbit per this model.
« Last Edit: 05/17/2016 05:05 pm by envy887 »

Offline ArbitraryConstant

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #14 on: 05/17/2016 05:09 pm »
Seems like if the Dragon 2 trunk were to grow extra propulsion it would be a SuperDraco with vac nozzle so they could have a service module with storable propellant.

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #15 on: 05/17/2016 05:12 pm »
Seems like if the Dragon 2 trunk were to grow extra propulsion it would be a SuperDraco with vac nozzle so they could have a service module with storable propellant.

That would be needed for moon landing if that is possible. For orbit to orbit propulsion Draco are adequate, just put extra tanks into the trunk. Maybe a set of Dracos but not sure if that would make sense.

Offline sevenperforce

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #16 on: 05/17/2016 05:26 pm »
What "extra falcons laying around needing a "payload" to fly"?  And if so, why not just the regular upper stage
With an expendable second stage and a recoverable first stage, first stages will start to accumulate since not everyone is going to bite at the cost reduction of a reused stage. Not right away anyhow. Having a smaller, cheaper second stage in an already-expendable trunk would be good.

It wouldn't be wasted development; they could use it to significantly extend the Dragon's range for BLEO missions too. Updating the Kestrel would not be prohibitively expensive; the existing Falcon 1 second stage architecture would only require moderate modifications.

Cost savings would be substantial.

Cost savings for what?  For what requirement?  They don't need a new stage.  There is no business case for it.   FH is enough to extend the "Dragon's range for BLEO missions".
FH extends Dragon's BLEO, but with either very limited payload or by flying expendable.

There are multiple instances where this would be useful for cost reduction:

1. For low-payload LEO missions (crew ferry or independent crewed flight), you pay for a Kestrel instead of paying for a Merlin.
2. For large-payload missions on F9 or FH, you can get ELV performance while still preserving first-stage reuse...so you get to keep your first stage(s) effectively for the price of a Kestrel.

The second stage is not "woefully overpowered".  It is throttled per mission requirements.
Overpowered in terms of dV, not in terms of thrust.

Quote
They lose the ability to carry hardware in the trunk
For this particular mission profile, yes. But the trunk will sometimes fly empty.

The Merlin vac stage only weighs 3960 kg dry, so the cost savings would be minimal - less than the cost of one Merlin. I could see this as viable for cheap-ish (few million per seat) orbital or suborbital joyrides, if Falcon and Dragon eventually are highly reusable. Other than that, the payload is vastly reduced so useful mission profiles almost non-existent.

Doesn't every ISS crew launch include cargo?
The independent orbital flight approach is a good one. As far as ISS launches are concerned, all include some sort of pressurized cargo, but not all include unpressurized cargo. In those launches, as in crew-only independent orbital flights, the volume of the trunk is entirely wasted space.

If the usual second stage really *were* "woefully overpowered" (a big if), SpaceX would be using the extra margin for second-stage recovery experiments.  Not developing a new, smaller, second stage.
It's quite overpowered for small launches but not so much that it has sufficient margins for recovery. And the engine and tankage is already developed; it just needs to be updated and extended.

Merlin vac also has 135 times as much thrust as Kestrel. 6.9 kN is woefully underpowered for a manned orbital second stage.

Double check your numbers here: http://www.silverbirdastronautics.com/LVperform.html The Kestrel 6.9 kN, 317s with a 2.95t second stage gets less than 200kg to ISS orbit per this model.
The Kestrel 2 was planned to have significantly higher thrust with lower weight...you could even go with two Kestrels without significant losses. At the same time, gravity drag can be made lower due to the first stage's high amount of reserve propellant and reduced launch mass/increased TWR. 317 s is really low, too; performance on launch was actually 330 seconds. Plus a larger extended bell could bring it closer to the Merlin 1D's isp, especially because no gas generator losses are involved.

Think about this less as "replacement second stage" and more as "beefier orbital maneuvering and insertion service module using Falcon 9 first stage as a near-SSTO".

Seems like if the Dragon 2 trunk were to grow extra propulsion it would be a SuperDraco with vac nozzle so they could have a service module with storable propellant.

That would be needed for moon landing if that is possible. For orbit to orbit propulsion Draco are adequate, just put extra tanks into the trunk. Maybe a set of Dracos but not sure if that would make sense.
This would be my primary preference, but then you need vac nozzles on the SuperDracos (which makes them useless in-atmo and potentially problematic on launch) and pressure-fed crossfeed, which is a nightmare.
« Last Edit: 05/17/2016 05:28 pm by sevenperforce »

Offline MrHollifield

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #17 on: 05/17/2016 05:32 pm »
How would launch abort work in this scenario?

Dragon 2 aborts with an empty trunk attached. The Kestrel-equipped trunk described here would be too heavy, so it would have to be left behind on launch abort. AIUI, this would mean Dragon would start tumbling as soon as the Super Dracos cut off, decreasing apogee and down range distance, while complicating parachute deployment.

Doesn't seem like the safest option for the crew on Dragon.

Online ncb1397

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #18 on: 05/17/2016 05:35 pm »
This idea has more merit if you replace the kestrel with a super-draco IMO. No point in replacing an expendable LOX-RP1 Merlin 2nd stage with an expendable LOX-RP1 Kestrel 2nd stage. Superdraco uses more storable propellants and thus would make a better service module used in a mode of operation akin to Apollo CSM or Orion.

Offline Jim

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Re: Crewed Dragon V2 flight with Kestrel in trunk extension
« Reply #19 on: 05/17/2016 05:57 pm »

What "extra falcons laying around needing a "payload" to fly"?  And if so, why not just the regular upper stage
With an expendable second stage and a recoverable first stage, first stages will start to accumulate since not everyone is going to bite at the cost reduction of a reused stage. Not right away anyhow. Having a smaller, cheaper second stage in an already-expendable trunk would be good.


there is no logic in that statement. It is still cheaper to use the existing second stage.   Flying the same but larger second stage over and over is cheaper than flying two different stages.  The actual material cost differences between the two stages is minor.


There are multiple instances where this would be useful for cost reduction:

1. For low-payload LEO missions (crew ferry or independent crewed flight), you pay for a Kestrel instead of paying for a Merlin.
2. For large-payload missions on F9 or FH, you can get ELV performance while still preserving first-stage reuse...so you get to keep your first stage(s) effectively for the price of a Kestrel



No, it doesn't any cost reduction and in fact increases over all costs due to maintaining additional configurations

1.  No real savings due to limited kestrel production.   Also, likely will cause abort black zones due to low thrust.

2.  How does adding a 3rd stage for save money?  With major mods to erector and pad infrastructure.  and maintaining a seldom used configuration.


There is no merit in this idea.  It goes against Spacex MO.  One (or two) size fits all.   Cost efficiency overrides vehicle efficiency.   So what if the second stage is overpowered for LEO, they just keep cranking them out, keeping them all the same.  Don't have to pause to adjust for a different one, don't have to engineer a different one, don't have to have different analyses for a different configurations.


Ask ULA what the costs in maintaining the Delta IV M+ (5,2) vs the  Delta IV M+ (4,2)
« Last Edit: 05/17/2016 06:10 pm by Jim »

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