Author Topic: Clusters of small chemical tugs  (Read 2311 times)

Offline Joel

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Clusters of small chemical tugs
« on: 09/12/2012 11:16 PM »
I am looking into a concept of deploying a large number of small cooperating chemical tugs that are able to cluster together in order to handle different payload sizes. I imagine tugs with thrust of around 30 kN that are refilled in LEO. Example uses would be transporting comsats of various sizes from LEO to GTO: clusters of 2-by-2 or 3-by-3 would correspond to a Centaur upper stage. A 5-by-5 cluster (corresponding to a S-IVB) could be used for a manned mission to the moon, etc. Just to give an idea.

Now for my questions:
    - Would such a concept make sense and does anyone know if it already has been investigated?
    - Is 30 kN of thrust a tank diameter of around 1.5 meters a reasonable size? This would correspond roughly to the Falcon 1 upper stage.
    - Is it safe to cluster large numbers of upper stage engines together or are the risks for catastrophic failure too high?
    - Would clustering be an efficent method to limit the boil-off assuming that a fuel combination such as RP-1/LOX, LH2/LOX or CH4/LOX is being used? If there is propellant crossfeed, I imagine that it should be possible to empty the tanks sequentially and have the empty tanks shade the rest from the sun.

I'm looking at this problem as a possible example problem for my doctoral thesis. We have optimal control algorithms that are able to tackle large sets of cooperating dynamical systems like these. So now I am looking for an interesting scenario!

Thanks!
« Last Edit: 09/12/2012 11:34 PM by Joel »

Offline RocketmanUS

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Re: Clusters of small chemical tugs
« Reply #1 on: 09/13/2012 01:10 AM »
ACES thread
http://forum.nasaspaceflight.com/index.php?topic=27680.0

They do have some info on clustering stages for BEO.

Small to large payloads, for LEO to GTO smaller tug or clustering them for larger payloads does make sense. Would need a connecting device between the tugs. Only one tug would be in control in a cluster.
 
Only need cross feed for BEO, that would be using ACES stages. Would crossfeed then drop the empty stage(s) leaving the working stages for the rest of the journey.

Tugs could use a sun shade to lower boil off. Crossfeeding allows for lowering the mass of the propulsion stage for greater performance. Also allows to drop a stage that is not performing when clustered ( just send the propellant to another stage ).

Not having to make super large stage should keep cost down. Small ( Falcon 1 size ) and medium ( Centaur size ) sized stages that we have ( or up to 5 meter diameter ACES ).

Use of fuel depot or fuel transfer station ( tanker to tug ). That would depend on the type of mission. Would allow to store back up satellites in LEO till needed. Would help in lowering cost with reusable US.

Example for non cluster use of tug.

Launch F9 ( reusable 2nd stage ), payload to LEO or propellant.

Launch F9 ( tug as 2nd stage ) with payload ( gets tug to LEO while payload pays for launch ).

Tug is refueled in LEO and has payload attached to it there. After delivering payload to desired orbit it would return to LEO for reuse.




« Last Edit: 09/13/2012 01:20 AM by RocketmanUS »

Online douglas100

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Re: Clusters of small chemical tugs
« Reply #2 on: 09/13/2012 08:35 AM »
I know this is Advanced Concepts, but my bias is always toward whether an idea seems practical.

I think it makes more sense to use a single tug based on a currently available upper stage and load it with propellant required for the mission. A lighter payload would mean less prop would be loaded.

I believe that a number of small tugs will be more expensive than a single large tug for the same payload. Then there is the expense and complication of operating a number of small vehicles, assembling them, fueling them, adding the payload, etc. It's almost like building a mini ISS. And you're talking about refueling this squadron in LEO.

I suppose this might make an interesting theoretical exercise. But it seems to me to be needlessly complicated and impractical.
Douglas Clark

Online douglas100

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Re: Clusters of small chemical tugs
« Reply #3 on: 09/13/2012 08:47 AM »

Would such a concept make sense and does anyone know if it already has been investigated?

Here is a paper which investigates a similar idea.

http://mae.ucdavis.edu/faculty/sarigul/papers/AIAA-2008-7805.pdf

Douglas Clark

Offline Joel

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Re: Clusters of small chemical tugs
« Reply #4 on: 09/13/2012 09:23 AM »
Here is a paper which investigates a similar idea.

http://mae.ucdavis.edu/faculty/sarigul/papers/AIAA-2008-7805.pdf

Interesting link, thanks! Is it more feasible to stack tugs on top of each other instead of next to each other like they do in the paper? Will there still be enough thrust? And would it influence the Isp? I mean, the total nozzle area will be smaller. I guess you can also use a combination of parallel and vertical stacking.

I also find it a bit overly complicated to use a heterogeneous system with different kinds of tugs which need to be refueled with different kinds of fuel. I guess that will complicate the development a lot. It's hard enough to develop _one_ tug.

Online douglas100

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Re: Clusters of small chemical tugs
« Reply #5 on: 09/13/2012 03:49 PM »
Quote
Interesting link, thanks!

You're welcome.

Quote
Is it more feasible to stack tugs on top of each other instead of next to each other like they do in the paper?

I think so. It keeps the line of thrust through the centre of gravity of the stack no matter how many tugs are linked. And the assembly task is limited to automatic docking, which thanks to years of space station operations, is straightforward.

Quote
Will there still be enough thrust?

Yes. But the thrust to mass ratio will be fairly low, especially when the tugs at the bottom of the stack are working. It will probably need multiple burns at perigee to gradually raise the apogee and avoid gravity losses.

Quote
And would it influence the Isp? I mean, the total nozzle area will be smaller.

I don't understand. It doesn't matter whether you use engines in parallel or serially. The Isp for each engine in a vacuum depends on the design of the engine and the propellant combination and nothing else.

Quote
I also find it a bit overly complicated to use a heterogeneous system with different kinds of tugs which need to be refueled with different kinds of fuel.

I believe the paper I cited was talking about single use tugs.

You are proposing to use lots of small tugs and make them refuelable. I think this is far too complicated and completely unnecessary. A small tug won't be much cheaper than a larger one. It will still have to carry complex systems for rendezvous and docking and propellant transfer. And because small tugs means more tugs, the price of (say) 5 small tugs would be a lot more than one Centaur or ACES sized tug doing the same job. Also, a group of tugs would be much more expensive to operate than a single large tug.

As far as handling different sizes of payload is concerned, I think the flexibility you claim for using different numbers of small tugs is not correct. A single reuseble large tug can carry any size of payload up to its maximum limit, depending on the amount of propellant loaded. It would have all the flexibility you would need and need no orbital assembly.


Douglas Clark

Offline A_M_Swallow

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Re: Clusters of small chemical tugs
« Reply #6 on: 09/13/2012 03:56 PM »
If you want a methane/LOX tug the engine in the Morpheus lander can do 19 kN, ISP 321 seconds.

A cluster of Solar Electric Propulsion (SEP) tugs would have to ensure the solar panels do not block each others light.

The viability of bring the tug stages back needs demonstrating.  Exp(2) = 7.39 which is a lot of fuel to launch.

Online douglas100

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Re: Clusters of small chemical tugs
« Reply #7 on: 09/13/2012 04:10 PM »

The viability of bring the tug stages back needs demonstrating.  Exp(2) = 7.39 which is a lot of fuel to launch.

Yes, that's a point I didn't mention. Returning a tug to LEO for reuse would be very expensive in terms propellant launched from Earth (unless you used aerocapture--another undemonstrated technology).
Douglas Clark

Offline RocketmanUS

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Re: Clusters of small chemical tugs
« Reply #8 on: 09/13/2012 05:47 PM »
I know this is Advanced Concepts, but my bias is always toward whether an idea seems practical.

I think it makes more sense to use a single tug based on a currently available upper stage and load it with propellant required for the mission. A lighter payload would mean less prop would be loaded.

I believe that a number of small tugs will be more expensive than a single large tug for the same payload. Then there is the expense and complication of operating a number of small vehicles, assembling them, fueling them, adding the payload, etc. It's almost like building a mini ISS. And you're talking about refueling this squadron in LEO.

I suppose this might make an interesting theoretical exercise. But it seems to me to be needlessly complicated and impractical.
One large tug ( like size of Centaur ) for small payloads would end up with to much dry mass compared to a small tug ( like size of F1 ).

Use two sizes, one of each.

Here is a paper which investigates a similar idea.

http://mae.ucdavis.edu/faculty/sarigul/papers/AIAA-2008-7805.pdf

Interesting link, thanks! Is it more feasible to stack tugs on top of each other instead of next to each other like they do in the paper? Will there still be enough thrust? And would it influence the Isp? I mean, the total nozzle area will be smaller. I guess you can also use a combination of parallel and vertical stacking.

I also find it a bit overly complicated to use a heterogeneous system with different kinds of tugs which need to be refueled with different kinds of fuel. I guess that will complicate the development a lot. It's hard enough to develop _one_ tug.


For LEO to GTO could put them in series ( up to three tugs clustered max for LEO ). But for BEO were larger thrust might be wanted and the ability to cross feed, cluster in parallel ( DRA5 for Mars would not need as many stages with a cross feed system ).


The viability of bring the tug stages back needs demonstrating.  Exp(2) = 7.39 which is a lot of fuel to launch.

Yes, that's a point I didn't mention. Returning a tug to LEO for reuse would be very expensive in terms propellant launched from Earth (unless you used aerocapture--another undemonstrated technology).
Aerocapture would be the way to go for  GTO or high orbit missions. There are practical methods that can be used. t/Space was to use an aerocapture system for their Lunar CEV.


Payload attachment

A fuel depot could be used to attach the payload ( from it's deliver stage ) to the reusable tug. The payload would need to be on it's own power or the tug would need a universal power supply system



Offline DSS_jgarzik

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Re: Clusters of small chemical tugs
« Reply #9 on: 09/13/2012 07:40 PM »
One large tug is one large Single Point Of Failure, a concern with all the debris out there.

Offline Joel

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Re: Clusters of small chemical tugs
« Reply #10 on: 09/13/2012 07:44 PM »
Ok. Thanks for all the input.

Since I'm really only interested in reusable tugs, I guess I need to allow for aerobraking. I already have a model for coplanar orbital transfer (simulations appears to be working) so adding an altitude-dependent friction term to the equations should be straightforward. I assume that the atmosphere at those altitudes can be modelled as a rarified gas.

I think that I will allow for clustering in parallel for maximum generality. The result of the optimization might very well be to fire the tugs sequentally, which would be equivalent to a serial stacking.

Good idea to use the Morpheus specs to model the tug. Then I could also include some moon-landing missions to the optimization scenario... In addition to LEO-to-GTO and LEO-to-GEO trucking.

Thanks again!

Offline RocketmanUS

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Re: Clusters of small chemical tugs
« Reply #11 on: 09/13/2012 08:03 PM »
Ok. Thanks for all the input.

Since I'm really only interested in reusable tugs, I guess I need to allow for aerobraking. I already have a model for coplanar orbital transfer (simulations appears to be working) so adding an altitude-dependent friction term to the equations should be straightforward. I assume that the atmosphere at those altitudes can be modelled as a rarified gas.

I think that I will allow for clustering in parallel for maximum generality. The result of the optimization might very well be to fire the tugs sequentally, which would be equivalent to a serial stacking.

Good idea to use the Morpheus specs to model the tug. Then I could also include some moon-landing missions to the optimization scenario... In addition to LEO-to-GTO and LEO-to-GEO trucking.

Thanks again!
You will want to separate the tugs ( when in parallel ) before the reenter LEO to keep the reentry just one profile. If you have them reenter as a cluster then you would need to design for each type of cluster ( two or three, ect ). Each tug would be the same, with each having side attachments.

Offline JohnFornaro

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Re: Clusters of small chemical tugs
« Reply #12 on: 09/14/2012 02:06 PM »
I am looking into a concept of deploying a large number of small cooperating chemical tugs that are able to cluster together in order to handle different payload sizes.

Which is an excellent concept.  Probably, there's a way to make it work with comsats and such, as you initially propose.  But I would take it a step further

You start with one tug, sized so as to be readily filled by one Falcon 9.  Then, you build a triangular truss at the back of which you can fasten three tugs, and at the front of which you could fasten the payload.  You build the next three tugs, so at least one would be available for dedicated payloads.  A fair amount of payload could be transferred from LEO to LLO, EML-1 or EML-2.  This presupposes a landing system for the lunar surface for that option, but such a system will have to be built to launch propellant, and land habitat and factory modules anyway.  (Keep in mind that I'm assuming a long term picture of a manned lunar base, and an ice cracking facility)

The tugs could also be used to ferry materials and crew back and forth to a prop depot/lab/hotel at EML1 or 2.

Cluster in parallel, definitely. For maximum generality, as you point out.  But, aerobraking will be hard, since the truss must also withstand those temps and pressures.

When SLS comes on line, then you could fill up three or more tugs in one launch.  Other launches would load the truss; either these would be single SLS launches, or multiple F-9 launches.  For smaller payloads, say crew only, you could just use one tug.
Sometimes I just flat out don't get it.

Offline Hop_David

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Re: Clusters of small chemical tugs
« Reply #13 on: 09/14/2012 03:11 PM »
The viability of bring the tug stages back needs demonstrating.  Exp(2) = 7.39 which is a lot of fuel to launch.

e2? How'd you get that number?

Offline A_M_Swallow

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Re: Clusters of small chemical tugs
« Reply #14 on: 09/14/2012 11:08 PM »
The viability of bring the tug stages back needs demonstrating.  Exp(2) = 7.39 which is a lot of fuel to launch.

e2? How'd you get that number?

e = 2.71828
so e2 = 7.3890461584 or 7.39 to 3 s.f.

What number are you talking about?


p.s.  In this case it is a scaling factor.  There and back is twice the delta-v of a one way trip.
« Last Edit: 09/14/2012 11:16 PM by A_M_Swallow »

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