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#20
by
Downix
on 18 Jan, 2011 21:50
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Could someone talk a little bit about the tradeoffs among solid propellants, or point me to a source? Shuttle SRBs use PBAN, I think, but there's HTPB which has been talked about as an alternative. What are the tradeoffs between them (and potentially other solid fuels?)
They're binding agents, the polymers used to hold the fuel in place. HTPB is the most common, used in over 75% of the solid motors out there. PBAN has a slower curing time, but gives slightly higher performance.
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#21
by
tu8ca
on 18 Jan, 2011 22:36
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Could someone talk a little bit about the tradeoffs among solid propellants, or point me to a source? Shuttle SRBs use PBAN, I think, but there's HTPB which has been talked about as an alternative. What are the tradeoffs between them (and potentially other solid fuels?)
There's also the China Lake Compound 20.
Interesting ... the 1.1 class propellants are supposed to have +4% isp over the standard composites, and the CL-20 a 14% increase over the 1.1 class. So if you had a stage with 280 isp, 1.1 class would bump it up to 291, Cl-20 to 332.
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#22
by
Malderi
on 18 Jan, 2011 23:27
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That's a pretty significant performance boost... There's got to be some major downside to it!
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#23
by
Downix
on 18 Jan, 2011 23:49
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
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#24
by
tu8ca
on 18 Jan, 2011 23:59
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
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#25
by
Downix
on 19 Jan, 2011 00:05
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
The casing is lighter, but the fuel itself is heavier. It's 20% heavier per cubic cm, but only gives 11% more performance. This means you still have a rocket which is roughly 9% heavier than comparable. Add to that the cost, then it becomes less appealing. It is, however, becoming popular for bullets, as it's density is not an issue but the extra kick is welcome.
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#26
by
alexw
on 19 Jan, 2011 00:11
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How many times have you seen liquid engines cut off right after ignition because something wasn't right? Kinda hard to do with solids. Even harder to offload fuel to pull back into the hangar to fix something.
You don't need to cut solid engines right after ignition because they are much simpler and therefore problems are much less likely. The Challenger disaster was due to a design flaw and redesigned SRBs are safe.
You may well need to cut solid rockets after ignition if there is an abort initiated for any reason -- solids, liquids, guidance, capsule, etc. See the Orion LAS controversy. Gotta unzip that booster case, because the now-lightened solid is going to keep going up like the proverbial bat from the hot place, and its plumage is a bad place for the (manned) payload to be.
-Alex
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#27
by
tu8ca
on 19 Jan, 2011 00:20
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
The casing is lighter, but the fuel itself is heavier. It's 20% heavier per cubic cm, but only gives 11% more performance. This means you still have a rocket which is roughly 9% heavier than comparable.
Maybe I'm missing something here ... performance is measured with respect to fuel mass, not density.
So if you have two fuels with identical performance, but one is ten percent denser, they'll still have identical performance if the propellant load is the same mass. But the denser fuel can have a smaller casing. right?
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#28
by
Downix
on 19 Jan, 2011 01:16
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
The casing is lighter, but the fuel itself is heavier. It's 20% heavier per cubic cm, but only gives 11% more performance. This means you still have a rocket which is roughly 9% heavier than comparable.
Maybe I'm missing something here ... performance is measured with respect to fuel mass, not density.
So if you have two fuels with identical performance, but one is ten percent denser, they'll still have identical performance if the propellant load is the same mass. But the denser fuel can have a smaller casing. right?
Semi correct. You also have to deal with burn rate. Smaller casings also mean in many cases smaller burning surface, which can then harm performance. They may overcome this with better chamber designs, but it takes time to develop. Give it time for them to optimize the design. Once they have this, then it should indeed give better performance.
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#29
by
baldusi
on 19 Jan, 2011 02:18
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It is, however, becoming popular for bullets, as it's density is not an issue but the extra kick is welcome.
I thought that bullets are limited by chamber pressure, and usually have longer that 18 months of storage requirements. How do they overcome those issues?
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#30
by
Downix
on 19 Jan, 2011 02:32
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It is, however, becoming popular for bullets, as it's density is not an issue but the extra kick is welcome.
I thought that bullets are limited by chamber pressure, and usually have longer that 18 months of storage requirements. How do they overcome those issues?
Depends on the bullet. Some anti-tank shells have short lives anyways.
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#31
by
baldusi
on 19 Jan, 2011 15:40
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Semi correct. You also have to deal with burn rate. Smaller casings also mean in many cases smaller burning surface, which can then harm performance. They may overcome this with better chamber designs, but it takes time to develop. Give it time for them to optimize the design. Once they have this, then it should indeed give better performance.
Why don't you simply make a bigger bore and keep the casing (if it supports the extra pressure)?
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#32
by
Downix
on 19 Jan, 2011 17:11
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Semi correct. You also have to deal with burn rate. Smaller casings also mean in many cases smaller burning surface, which can then harm performance. They may overcome this with better chamber designs, but it takes time to develop. Give it time for them to optimize the design. Once they have this, then it should indeed give better performance.
Why don't you simply make a bigger bore and keep the casing (if it supports the extra pressure)?
Then you may not have the pressure to get sufficient thrust. Or your burn-rate could be too fast or too slow. It's not simple by any measurement.
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#33
by
baldusi
on 19 Jan, 2011 17:26
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Then you may not have the pressure to get sufficient thrust. Or your burn-rate could be too fast or too slow. It's not simple by any measurement.
I was under the impression that a circular central bore gave a (relatively) constant pressure.
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#34
by
tu8ca
on 19 Jan, 2011 17:41
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Semi correct. You also have to deal with burn rate. Smaller casings also mean in many cases smaller burning surface, which can then harm performance. They may overcome this with better chamber designs, but it takes time to develop. Give it time for them to optimize the design. Once they have this, then it should indeed give better performance.
Why don't you simply make a bigger bore and keep the casing (if it supports the extra pressure)?
Then you may not have the pressure to get sufficient thrust. Or your burn-rate could be too fast or too slow. It's not simple by any measurement.
If you have one fuel denser by 10% over another and go with a smaller case to save weight, there are lots of variables that might avail themselves to tweaking. Grain shape first - lots of ways to add back grain surface area and control thrust profile. Of course this was just an illustrative comparison ... denser solid fuel saves case weight.
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#35
by
Downix
on 19 Jan, 2011 20:40
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Semi correct. You also have to deal with burn rate. Smaller casings also mean in many cases smaller burning surface, which can then harm performance. They may overcome this with better chamber designs, but it takes time to develop. Give it time for them to optimize the design. Once they have this, then it should indeed give better performance.
Why don't you simply make a bigger bore and keep the casing (if it supports the extra pressure)?
Then you may not have the pressure to get sufficient thrust. Or your burn-rate could be too fast or too slow. It's not simple by any measurement.
If you have one fuel denser by 10% over another and go with a smaller case to save weight, there are lots of variables that might avail themselves to tweaking. Grain shape first - lots of ways to add back grain surface area and control thrust profile. Of course this was just an illustrative comparison ... denser solid fuel saves case weight.
Precisely. This is a new fuel source. They are still working on designs to optimize use of the fuel. Once they have that done, I expect several systems to use it beyond the particular fast-decay tank shells it's already found in.
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#36
by
tnphysics
on 19 Jan, 2011 20:47
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
The 1.1 props are FAR more dangerous. 1.1 means detonable. Thus, a failure could cause turn the rocket into a bomb of the same size. I suspect that this is an ABSOLUTE no-no for most applications.
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#37
by
Downix
on 19 Jan, 2011 21:54
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That's a pretty significant performance boost... There's got to be some major downside to it!
More toxic, heavier due to higher density, decomposes more rapidly. While traditional solids are good for decades, CL-20 decomposes at a faster rate, and needs to be used within 18 months.
The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
The 1.1 props are FAR more dangerous. 1.1 means detonable. Thus, a failure could cause turn the rocket into a bomb of the same size. I suspect that this is an ABSOLUTE no-no for most applications.
I thought CL-20 was a 1.3 class, but I'm not certain.
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#38
by
tu8ca
on 19 Jan, 2011 21:58
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The 1.1 class propellants are more dangerous. CL-20 is probably a lot more expensive (guessing).
But a doesn't a denser propellant mean a smaller case, thereby saving weight?
The 1.1 props are FAR more dangerous. 1.1 means detonable. Thus, a failure could cause turn the rocket into a bomb of the same size. I suspect that this is an ABSOLUTE no-no for most applications.
I thought CL-20 was a 1.3 class, but I'm not certain.
Yes, I think that's right;
http://www.navair.navy.mil/techTrans/index.cfm?map=local.ccms.view.aB&doc=crada.13
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#39
by
usn_skwerl
on 15 Feb, 2011 04:21
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I apologize if this is the wrong thread, but I'm wondering if anyone can give a link comparing Ares I-X liftoff weight with what Ares I would have had. I heard George Diller say I-X was 200,000 lbs lighter, but can't seem to find the video where he says it. I thought it was during the launch, but apparently not.
Any help would be appreciated. Thanks.