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Modular Design Questions

Posted by: jak42 - Fri Mar 30, 2007 2:46 am
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Modular Design Questions 
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Post Modular Design Questions   Posted on: Fri Mar 30, 2007 2:46 am
The film was really inspiring, I think AA is really the only of the NewSpace companies besides SpaceX that is really flying vehicles.

I had some questions on the modular design concept. I've been a fan of this concept since the late '70's when Lutz Kaiser pioneered it with OTRAG.

The first question is whether you've looked at the mass ratio tradeoff between modular and integrated design and whether there is some sized module that optimizes the mass ratio? Some of the opponents of modular design claim that the mass ratio scales sublinearly with size, so that when you make the tanks and other structures bigger, you actually get better scalability than by stacking modules together.

The second question is whether you've decided on going with the serial staging concept shown at the end of the film or whether that was just for illustration purposes? As you probably know, OTRAG staging was parallel. The second stage was contained within the first and simply slid out of the first stage when the first stage fuel was exhausted and fell away as the second stage ignited (in theory, I don't think they ever managed to launch a multistage vehicle). The outer stage could even support the inner with wheels, such as T-space is using for exiting their air launched vehicle from the back of the plane. Thus, no need for any explosive separation or mechanical staging hardware to risk the kinds of problems at staging that SpaceX experienced recently.

One reason I can think of for not doing parallel staging is that if you are planning on flying back the first stage, then it will have propellant in it and you probably don't want to ignite the second stage surrounded by half full propellant tanks. It might be possible have some kind of cold gas thruster on the top of the vehicle that pushed the first stage down when the engines were turned off, then only igniting the second stage when it was clear.

The second reason I can think of for not doing it is that your modules are much wider than OTRAG's were. If I recall correctly, Lutz used very long but thin stainless steel oil pipe milled down very thin, which made sideways stacking much easier. Increasing the breadth of the vehicle might increase wind resistance unacceptably.

Parallel staging has gotten a bad rap due to incidents such as the Challenger disaster and other cases involving parallel staged solids, but in your case, the stages will be liquid fueled which could change the consideration.

jak


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Post    Posted on: Fri Mar 30, 2007 9:24 am
I know your questions are directed toward the Armadillo team but let me answer what I can from memory. (dubious as it is)

On modular vs scaling, I do remember them mentioning they may scale up their modules because of the exact thing you are saying. (still being modular however) At this time they are using the smaller modular design because at the moment they can make them cheaper and more reliable. John's example on the subject of reliability:
Quote:
If each module only had 99% reliability, an eight module system would have an 7.7% chance of having a failure of some kind (1.0 - 0.99^ 8 ). However, the configurations we will likely use would require two modules in opposite "banks" to both fail to bring the vehicle down, which would only have a (1.0 - 0.99^4) * (1.0 - 0.99^4) = 0.00155 chance of happening. I expect the real number to be much smaller than that, because I expect we will have better than 99% reliability per module. Each bank should have its own guidance electronics as well, so that will also be redundant.


As for how wide the vehicle will be John, said he had investigated flowformed tanks, but they were expensive so they went with other options. I think when the time is right Armadillo will likely use some flowformed type tanks for the superior aerodynamics, probably before they begin to scale too up much.

This is sorta the question I wanted to address. I don't think there is anything wrong with parallel stacking. You see the problems the shuttle had were do to foam falling off the shuttle and damaging the heatshield. IMO this is a problem with the foam and not a problem with the parallel stacking. If they were to remove the foam from the fuel tank and find a better solution (which didn't involve shedding matterial) to keep the liquids cold/ice from forming... case closed. Not sure of the solids cases you mentioned but yeah since Armadillo uses liquid fuels, the rocket engines themselves can be carefully, independantly throttled.


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Post More on Module Mass Ratio   Posted on: Tue Apr 17, 2007 1:58 am
Let me clarify a bit about my previous comment about the mass ratio of modules, it was somewhat cryptic.

Suppose you need a certain volume of propellant, and tank geometry is spherical as would be the case for a Pixel derived module.

As the first case, assume that, instead of modules, you're going to use a big, honkin' booster like Truax's Sea Dragon. Suppose the radius of the propellant tank is r, then the volume would be

4/3 * pi * r **3.

Now, suppose, instead, that you want the same amount of propellant, but you're going to distribute it over modules with radius one half the big, honkin' tank. The volume of each tank is

4/3 * pi * (r/2) ** 3 = 4/3 pi * (r**3)/8

In other words, it would take 8 tanks to match the same volume as the big, honkin' tank.

Now, the amount of mass in the tank shell scales as the area. Densities are typically in grams/cc of material, and cc can be calculated by multiplying the thickness of the shell by the surface area.

The surface area of the big tank is

4 pi r ** 2

The surface area of one little tank is

4 pi (r/2)** 2 = 4 pi (r**2)/4 = pi r**2

Since there are 8 tanks, the total surface area for the module-based machine is

8 pi r**2

In other words, the module based machine has twice as much tank surface area as the big, honkin one. Since the mass scales as the surface area, it would have about the twice as much mass too.

"About" because it is possible that the thickness of the big, honkin' tank would need to be more to hold the additional weight of propellant, so that might result improvement in the comparative mass, but it probably won't need twice as much thickness.

There are two counterarguments I can see:

1) Economies of scale make modules much, much cheaper, and the reduction in learning curve associated with making lots of them make them potentially more reliable too (though, I've heard that the classical learning curve has gradually disappeared as CAM techniques have become more common). So, from a cost per launch standpoint, even if you can't lift the same amount of payload, it is cheaper to use modules.

2) The tank material and manufacturing process is such that it is not possible to make a big, honkin tank out of an especially lightweight material, whereas it is possible to make smaller, module tanks. For example, if the tank is carbon fiber and needs to be autoclaved, but there isn't an autoclave big enough (nor would it be economical to build one) to bake the big honker.

Anyway, unless I've made a grevious error in my algebra, this is the argument that people use against a module architecture, or at least did w.r.t. OTRAG. Personally, I think some combination of 1) and 2) (most especially 1) is likely to prevail, which is why I'd be interested in seeing whether there's some kind of optimum possible when you figure in something other than just the geometry.

jak


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Post Re: More on Module Mass Ratio   Posted on: Tue Apr 17, 2007 2:36 am
jak42 wrote:
the amount of mass in the tank shell scales as the area.
Only if the wall thickness does not increase.
jak42 wrote:
... but it probably won't need twice as much thickness.
Yes, it needs twice. According to this source:
http://www.innovatia.com/Design_Center/FundRoc_4-8.htm
wall thickness scales as the radius. A tank with twice the diameter would have 4 times the surface area and 8 times the volume, but it would also have twice the wall thickness and therefore 8 times the mass. So the mass to volume ratio does not improve.


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Post    Posted on: Thu Apr 19, 2007 2:18 am
Great, thanx for the reference. Looks like the wall stress scales as the radius so it cause the tank thickness to scale as the radius as well.

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Post    Posted on: Thu Apr 19, 2007 4:27 am
For a given pressure, tank mass scales linearly with tank volume.

Interestingly, regardless of propellant density.

Which means a shuttle external tank could hold 3 times more RP-1 / LOX, than it does LH2 / LOX. With 7 RD-180's, and no SRB's, it would have a mass ratio over 20 and could put over 50 tons into LEO, as a SSTO.


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Post    Posted on: Thu Apr 19, 2007 7:45 am
WannabeSpaceCadet wrote:
For a given pressure, tank mass scales linearly with tank volume.

Interestingly, regardless of propellant density.

Which means a shuttle external tank could hold 3 times more RP-1 / LOX, than it does LH2 / LOX. With 7 RD-180's, and no SRB's, it would have a mass ratio over 20 and could put over 50 tons into LEO, as a SSTO.


Yeah this is why I'm very much in favour of LOX/Hydrocarbon and other higher density propellant combinations. Clearly LOX/LH2 wins in terms of performance but when you choose propellants you have to take into account a range of factors, not just ISP.

- Density, LOX/LH2 has a combined avg density of 0.33g/cm^3 in non-ideal ratios designed to increase density. LOX/Kerosene has an avg combined density of 1.02g/cm^3 ideal ratio, a 3.09 times advantage to the kerosene based combination and therefore 3 times the tank mass ratio for any given size.
- Cryogenics, LOX is a cryogen but it is common for both, kerosene is perfectly storable and so you don't need any tank insulation. Tank insulation on the shuttle is heavy (especially so considering the massively increased volume needed per unit mass LH2) and fragile (as evident by the huge delay due to hail damage), and has managed to cause the total loss of a shuttle and crew. A kerosene tank for a given volume would be lighter, simpler, hold many times the mass and be less susceptible to damage.
- Other component design. Yes we can make turbine systems run with LH2 but it's a VERY complicated thing to do. LH2 as woeful as its density is requires very large and powerful turbopumps compared to denser fuels such as kerosene. They also need to be made out of exotic materials and need a whole host of support systems to handle start/stop transients without freezing.

I thing for too long a time ISP has been the drive of government agencies but if ISP was all that mattered in a propellant combination we'd all be flying with liquid fluorine and liquid hydrogen gel with a suspension of beryllium particles.


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Post    Posted on: Thu Apr 19, 2007 8:14 am
Regarding cerosene I permanently have in mind that the ressources cerosene is refined off are going to be exhausted in a not that far away future. The only ways out seem to be Sintin which is expensive or a new technology which is a thread about in the technology section. That technology is based on solar power to turn CO2 into cerosene, gasolune and the like again.

Regarding tank mass I am wondering a bit - it can scale linearly with tank volume only if the thickness etc. are increased. Else tank mass should scale linearly with tank surface merely because of Geometrics.



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)


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Post    Posted on: Sat Apr 21, 2007 3:37 am
Andrew Burns wrote:
WannabeSpaceCadet wrote:
I thing for too long a time ISP has been the drive of government agencies but if ISP was all that mattered in a propellant combination we'd all be flying with liquid fluorine and liquid hydrogen gel with a suspension of beryllium particles.


Andrew,

What do you think about lithium aluminum hydride? I've seen a technical paper involving combining it with kerosene and using hydrogen peroxide for the oxidizer (with which it is apparently hypergolic) citing ISPs around 400, and one US patent where they combined it with some kind of polymer binder and got an ISP of around 420 (their experimental solid burned through a steel casing that would have worked fine with a typical aluminum perchlorate solid). None of this work was done by the guv'nment. It is stable when stored in oil-based liquid, but I think it may self combust (as lithium does) in air, or maybe just oxidize more or less quickly. I'm not sure if the combustion products are hazardous, certainly for aluminum they're not since that is used in the shuttle solids and other solids as well and the combustion product is aluminum oxide. I'm not sure about lithium oxide, though. I think it might be expensive to make, but I am not sure. It has also been studied as a possible material for hydrogen storage in vehicles and such, since it apparently freely absorbs and releases molecular hydrogen.

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Post    Posted on: Sat Apr 21, 2007 12:54 pm
I haven't heard much about lithium aluminium hydride so I just had a quick search around for info. It looks good, albeit fairly dangerous and reactive when on its own. Assuming the mixture with kerosene reliably (very reliably) passivates its pyrophoric tendencies it certainly has advantages. I have only two potential problems I could see with a biprop using LAH 'sweetener'.

1) Either the suspension has the be fine enough or the kerosene partially gelled so that the LAH doesn't settle out of the fuel during storage or even as the rocket sits on the pad.
2) Steps have to be taken to eliminate propellants drying on the insides of tanks and plumbing. It's unlikely using hydrocarbon fuels, but I wouldn't like the idea of some small film of fuel left in a tank as the bulk is used, that film drying out or separating from the LAH and having spontaneous combustion occur.

Certainly if these issues could be solved and some testing were to be done with it, it might be a nice work around the generally low ISP's associated with peroxide fuels. I definitely think it has way more going for it as a peroxide based hybrid additive though. If you could get 420sec ISP out of a peroxide based hybrid (a hybrid grain would effectively solve the settling a reactivity problems) you'd have a very competitive propulsion system, especially so counting hypergolic ignition and the ability to use decomposed peroxide as turbine/reciprocating pump working fluid. I can imagine you'd be able to build a booster orders of magnitude simpler than other motors of similar performance, and with huge performance advantages compared to solids of the same thrust level.


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Post    Posted on: Sat Apr 21, 2007 1:11 pm
All this gets us off the topic which is modularity, and whether bigger modules would be more efficient. The answer is: only slightly due to some components (e.g. electronics & sensors) not increasing in mass as the module increases.

Of course some labour requirements would decrease with fewer bigger modules, but others would increase with more difficult handling. Overall, starting with smaller modules makes sense for faster, cheaper (better? :wink: ) development & testing.

Explore the solution space, then scale up!


Last edited by WannabeSpaceCadet on Tue Apr 24, 2007 4:03 am, edited 1 time in total.



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Post    Posted on: Mon Apr 23, 2007 6:01 am
Scaling up in size may end up necessary where scaling up solely in numbers fails. Unfortunately as you use more and more modules, the stresses on them increase and so you may find out their limitation at an inconveniently late point in development.
e.g. Aerodynamic drag will be less significant with larger modules, in addition stresses due to acceleration are significant when there are a large number of modules in a stage and may become limiting. Reducing the number of modules within a stage reduces the stresses between the spheres somewhat both by reducing the stage width and increasing the height to something more favourable.


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Post    Posted on: Tue Apr 24, 2007 4:15 am
Armadillo have chosen a flight profile that will minimize aerodynamic effects. While they will pay a small penalty for this in delta V, at the same time it lessens many 'traditional' concerns, allowing then to go with spherical tanks, wider vehicles etc.

In a production system, the minimum stage size would determine the maximum module size, and ought to determine the minimum module size. Subject to manufacturing and transport limitations.


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