Author Topic: SSTO Thread  (Read 182256 times)

Offline tnphysics

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Re: SSTO Thread
« Reply #520 on: 01/20/2009 11:50 am »
Current flight rates don't support the use of RLV's

Well, more correctly current flight rates combined with current ways of doing things don't support the use of RLVs.  There are actually ways that you could get enough flight rate to support an RLV even with only existing demand.  But it would take some change in the way some things are done.

~Jon

What are those methods? PDs or putting everything on one launcher?

Offline jongoff

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Re: SSTO Thread
« Reply #521 on: 01/20/2009 03:35 pm »
What are those methods? PDs or putting everything on one launcher?

One idea presented by a friend of mine out in UK was that most of the mass on orbit of a satellite bound for GEO is the propellant to boost it there.  The "beginning of life" mass--the mass of a satellite after the circularization burn is complete--of the most massive GEO birds today is only around 5mT, with many of them being much smaller than that.  However, the total mass needed in LEO to get that much mass into GTO is typically 3-4x that, once you include the kick stage and all the propellants.  His initial thought he presented was to prelaunch 2-4 "propulsion modules", dock them together, and then launch the actual satellite itself on the last launch, dock it with the stack in LEO, and then send it off to GEO.  Each of the pieces could fit on a small RLV (5mT to orbit would cover pretty much the biggest stuff on the market).  By capturing only 1/4-1/2 of the existing GEO market, his theory was you could get flight rates high enough to make that work--and the kicker is that doing it that way wouldn't require any changes to the actual satellite other than not loading the circularization burn propellants during payload processing.

Now, there are better methods (small depot tanks up the kick stage instead of doing tinkertoys modules, eventually having a reusable kick stage, etc) but that's the basic concept.  The big challenge is that GEO sat customers are really conservative.  While they definitely will go with lower launch cost options once they are proven (people still launch on Protons and Zenits, both of which have worse records than some other more expensive but less used launchers), they're sticklers about not being the first or second customer for something.  Now, if NASA did the smart thing, and acted as an anchor tenant for such services by using them to send a probe or two to the Moon or Mars, that might pave the way...

But there's a lot of work between here and there--both propellant transfer and RLVs.  Personally, I find depots to be the less daunting of the two (though my day job is working on the other problem).

~Jon

Offline kevin-rf

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Re: SSTO Thread
« Reply #522 on: 01/20/2009 03:48 pm »
If you have to throttle down the engine in the atmosphere (for powered landing for instance), FSC techniques allow you to do that without sacrificing expansion ratio for your in-space operations, or requiring separate landing engines.

With the ejectable ring doesn't that require inserting a new ring in the engine before coming back for a landing? ...
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Offline jongoff

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Re: SSTO Thread
« Reply #523 on: 01/20/2009 04:26 pm »
With the ejectable ring doesn't that require inserting a new ring in the engine before coming back for a landing? ...

I wasn't suggesting using that method.  There are other approaches for flow separation control that can potentially allow you to turn the flow separation control on and off.

~Jon

Offline A_M_Swallow

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Re: SSTO Thread
« Reply #524 on: 01/21/2009 12:56 am »
{snip}

Now, there are better methods (small depot tanks up the kick stage instead of doing tinkertoys modules, eventually having a reusable kick stage, etc) but that's the basic concept.  The big challenge is that GEO sat customers are really conservative.  While they definitely will go with lower launch cost options once they are proven (people still launch on Protons and Zenits, both of which have worse records than some other more expensive but less used launchers), they're sticklers about not being the first or second customer for something.  Now, if NASA did the smart thing, and acted as an anchor tenant for such services by using them to send a probe or two to the Moon or Mars, that might pave the way...

But there's a lot of work between here and there--both propellant transfer and RLVs.  Personally, I find depots to be the less daunting of the two (though my day job is working on the other problem).

~Jon
Depots and RLV are not incomparable.

The probe's LV upper stage could be refuelled by the depot allowing a single reuse as a kicker stage.  This saves launching the mass of an engine, tanks and heat shield for a kicker stage.

The tanker to fill the depot could be launched on an RLV.  Since the tanker does not need its solar panels exposed for the next 5 years it may be able to keep is faring.  The faring could then be used as part of the heat shield, if a suitable material can be found.  Alternatively the faring could be the outer wall of the fuel tank.

Offline cheesybagel

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Re: SSTO Thread
« Reply #525 on: 01/25/2009 03:25 pm »
What about borane fuels? They were supposed to offer the performance of hydrogen at the density of kerosene, but I believe that their was a problem.

The rockets proposed at the time used Fluorine as oxidant and Borane as fuel. The problems were toxicity, corrosion and clumping IIRC. There are ways of improving the ISP of kerosene rockets slightly. e.g. the Russians pre-chilled kerosene to make it denser, and used sintin (synthetic kerosene) which allegedly is only made of some specific hydrocarbon compound instead of the mix-mash that is regular kerosene. Then there are the middle ground hydrocarbons between kerosene and hydrogen. Proposals to use liquid propane or methane are often bandied around.

As for LOX/LH2 you can increase thrust at the cost of ISP by, varying the mixture ratio, adding more LOX. LOX has a much higher density than LH2 so you get higher overall propellant density. Having a variable mixture ratio LOX/LH2 engine reduces a lot of the advantage of going for a LOX/LH2/LCH4 tri-propellant engine for example.

Offline William Barton

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Re: SSTO Thread
« Reply #526 on: 01/25/2009 04:02 pm »
What about borane fuels? They were supposed to offer the performance of hydrogen at the density of kerosene, but I believe that their was a problem.

The rockets proposed at the time used Fluorine as oxidant and Borane as fuel. The problems were toxicity, corrosion and clumping IIRC. There are ways of improving the ISP of kerosene rockets slightly. e.g. the Russians pre-chilled kerosene to make it denser, and used sintin (synthetic kerosene) which allegedly is only made of some specific hydrocarbon compound instead of the mix-mash that is regular kerosene. Then there are the middle ground hydrocarbons between kerosene and hydrogen. Proposals to use liquid propane or methane are often bandied around.

As for LOX/LH2 you can increase thrust at the cost of ISP by, varying the mixture ratio, adding more LOX. LOX has a much higher density than LH2 so you get higher overall propellant density. Having a variable mixture ratio LOX/LH2 engine reduces a lot of the advantage of going for a LOX/LH2/LCH4 tri-propellant engine for example.


Are there any advantages to LNG as a fuel? It's easy to come by, relatively cheap, and a large manufacturing base exists, including fleets of specially equipped tankers. It wouldn't cost that much to put a sea delivery terminal/pipe offshore from Canaveral or Wallops.
« Last Edit: 01/25/2009 04:03 pm by William Barton »

Offline kevin-rf

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Re: SSTO Thread
« Reply #527 on: 01/25/2009 06:54 pm »
It wouldn't cost that much to put a sea delivery terminal/pipe offshore from Canaveral or Wallops.

I would be willing to bet natural gas (methane) is available at both the cape and wallops. At most all you would need to do is install a chiller to chill it into it's liquid form.

I think it is more an issue of no one has actually flow a Methane rocket (yet).
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Offline Nascent Ascent

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Re: SSTO Thread
« Reply #528 on: 01/26/2009 02:19 am »
You could also generate relatively easily Hydrogen from the Methane too.

Offline Proponent

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Re: SSTO Thread
« Reply #529 on: 01/26/2009 09:51 am »
I think it is more an issue of no one has actually flow a Methane rocket (yet).

Actually, the very first liquid-fuel rocket in Europe (about 75 years ago) burned methane and lox!  But certainly no current system uses methane.

What I wonder, though, is why everybody is so excited about methane and not propane.  According to Bruce Dunn and others, propane generally outperforms methane, and it's less cryogenic to boot.

The one place where I can see methane being preferable is for in situ fuel production, because it's probably easier to manufacture methane than propane.  This consideration, however, is irrelevant for earth launch and is far in the future for any other system, except maybe on the moon.
« Last Edit: 01/26/2009 09:52 am by Proponent »

Offline kevin-rf

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Re: SSTO Thread
« Reply #530 on: 01/26/2009 12:33 pm »

Actually, the very first liquid-fuel rocket in Europe (about 75 years ago) burned methane and lox!  But certainly no current system uses methane.

What I wonder, though, is why everybody is so excited about methane and not propane.  According to Bruce Dunn and others, propane generally outperforms methane, and it's less cryogenic to boot.

The one place where I can see methane being preferable is for in situ fuel production, because it's probably easier to manufacture methane than propane.  This consideration, however, is irrelevant for earth launch and is far in the future for any other system, except maybe on the moon.

Though people are playing with methane and propane as we type...

I would argue if you are going to use LOX (I can not see a chemical SSTO without it) you have to handle Cryo fuels anyway. Methane does have a slightly higher ISP but lower density than propane so we it is as always a tradeoff.

Not to pull us off topic, but the only "in situ" methane production I can think of is mars. Both Carbon and Hydrogen are scare on the moon, so it makes sense to use the H2 directly. At this point we have detected methane releases on mars. Meaning we (big unknown what form it really is in and how it is locked up) might be better off drilling for natural gas on mars. Big drilling rig vs. big plumbers wet dream ;)
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Offline jongoff

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Re: SSTO Thread
« Reply #531 on: 01/26/2009 02:50 pm »

Actually, the very first liquid-fuel rocket in Europe (about 75 years ago) burned methane and lox!  But certainly no current system uses methane.

What I wonder, though, is why everybody is so excited about methane and not propane.  According to Bruce Dunn and others, propane generally outperforms methane, and it's less cryogenic to boot.

The one place where I can see methane being preferable is for in situ fuel production, because it's probably easier to manufacture methane than propane.  This consideration, however, is irrelevant for earth launch and is far in the future for any other system, except maybe on the moon.

Though people are playing with methane and propane as we type...

I would argue if you are going to use LOX (I can not see a chemical SSTO without it) you have to handle Cryo fuels anyway. Methane does have a slightly higher ISP but lower density than propane so we it is as always a tradeoff.

Not to pull us off topic, but the only "in situ" methane production I can think of is mars. Both Carbon and Hydrogen are scare on the moon, so it makes sense to use the H2 directly. At this point we have detected methane releases on mars. Meaning we (big unknown what form it really is in and how it is locked up) might be better off drilling for natural gas on mars. Big drilling rig vs. big plumbers wet dream ;)

An interesting thing to note is that "sub-cooled" propane (propane cooled down to just above LOX temperature) has a density greater than Kerosene, has almost as good of Isp as methane, is an amazing coolant, and is cold enough that the merkapten stenchants all fall out during chill-down.  With the differences in mixture ratio to get the best Isp, a LOX/sub-cooled propane rocket ends up having tanks with the exact same proportions as a LOX/Kero rocket.

And there was at least one company here in Mojave firing big ~20-24klbf LOX/Propane (not subcooled) rockets up until recently...

~Jon

Offline relyon

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Re: SSTO Thread
« Reply #532 on: 01/26/2009 08:42 pm »
... has almost as good of Isp as methane ...

I understand everything except this statement. The C/H mass ratio of propane (~36:4) is higher than methane (~12:4) and that of the exhaust products correspondingly similar. With consistent mixture ratios across a range of inlet fuel temperatures, I'd expect Isp to stay relatively constant. Is the mixture ratio in "sub-cooled" propane engines altered to make use of the different thermal properties of the fuel (cooling) or is there something else I'm missing?

Bob
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Offline khallow

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Re: SSTO Thread
« Reply #533 on: 01/26/2009 09:36 pm »
Hmmm, if ethane isn't too hard to produce from methane, then that's another Martian fuel choice. It has somewhat better handling properties and density for a touch less isp. And of course, from ethane you can make ethylene, base stock for the plastics industry.
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Offline yinzer

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Re: SSTO Thread
« Reply #534 on: 01/26/2009 09:40 pm »
... has almost as good of Isp as methane ...

I understand everything except this statement. The C/H mass ratio of propane (~36:4) is higher than methane (~12:4) and that of the exhaust products correspondingly similar. With consistent mixture ratios across a range of inlet fuel temperatures, I'd expect Isp to stay relatively constant. Is the mixture ratio in "sub-cooled" propane engines altered to make use of the different thermal properties of the fuel (cooling) or is there something else I'm missing?

If you burn stoichiometrically, the average molecular weight of the exhaust is 29.1 for propane (3 CO2 @ 44, 4 H2O @ 18) and 26.7 for methane (1 CO2 @ 44, 4 H2O @ 18), and 31 for RP-1 (1 CO2, 1 H2O).  For LH2/LOX it's 18.

If you run fuel-rich to leave some unburned hydrogen the numbers change a bit, but not a huge amount.  Propane is roughly halfway between methane and kerosene, and all are a lot higher than liquid hydrogen.
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Offline Lampyridae

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Re: SSTO Thread
« Reply #535 on: 01/26/2009 10:36 pm »
Propylene is another one, it also has similar characteristics to RP-1 when you subcool it. Trouble is, you start getting plastics forming in your turbopumps and cooling channels! Garvey Aerospace has been messing around with propylene and aerospikes.

http://www.astronautix.com/engines/garspike.htm

This one uses ethanol.

http://www.csulb.edu/colleges/coe/mae/views/projects/rocket/news_2008/aerospike06252008.shtml
« Last Edit: 01/26/2009 10:48 pm by Lampyridae »

Offline relyon

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Re: SSTO Thread
« Reply #536 on: 01/27/2009 04:19 am »
If you burn stoichiometrically, the average molecular weight of the exhaust is 29.1 for propane (3 CO2 @ 44, 4 H2O @ 18) and 26.7 for methane (1 CO2 @ 44, 4 H2O @ 18), and 31 for RP-1 (1 CO2, 1 H2O).  For LH2/LOX it's 18.

If you run fuel-rich to leave some unburned hydrogen the numbers change a bit, but not a huge amount.  Propane is roughly halfway between methane and kerosene, and all are a lot higher than liquid hydrogen.

Precisely what I was thinking. Higher exhaust velocities could potentially offset the [average] molecular mass of the combustion products, but I don't see that being possible given the energy potentials of the fuel/oxidizer combinations being discussed (see http://www.engineeringtoolbox.com/adiabatic-flame-temperature-d_996.html) for some comparative adiabatic flame temperatures, albeit at room conditions).

Bob
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Offline jongoff

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Re: SSTO Thread
« Reply #537 on: 01/27/2009 04:55 am »
... has almost as good of Isp as methane ...

I understand everything except this statement. The C/H mass ratio of propane (~36:4) is higher than methane (~12:4) and that of the exhaust products correspondingly similar. With consistent mixture ratios across a range of inlet fuel temperatures, I'd expect Isp to stay relatively constant. Is the mixture ratio in "sub-cooled" propane engines altered to make use of the different thermal properties of the fuel (cooling) or is there something else I'm missing?

Honestly, I'm not a propellant chemist, but most numbers I've seen only give LOX/Methane a ~2-3% higher Isp than LOX/propane.  Which in my mind is really not worth it when you look at how much worse bulk density LOX/Methane has.

~Jon

Offline jongoff

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Re: SSTO Thread
« Reply #538 on: 01/27/2009 04:58 am »
If you burn stoichiometrically, the average molecular weight of the exhaust is 29.1 for propane (3 CO2 @ 44, 4 H2O @ 18) and 26.7 for methane (1 CO2 @ 44, 4 H2O @ 18), and 31 for RP-1 (1 CO2, 1 H2O).  For LH2/LOX it's 18.

If you run fuel-rich to leave some unburned hydrogen the numbers change a bit, but not a huge amount.  Propane is roughly halfway between methane and kerosene, and all are a lot higher than liquid hydrogen.

Precisely what I was thinking. Higher exhaust velocities could potentially offset the [average] molecular mass of the combustion products, but I don't see that being possible given the energy potentials of the fuel/oxidizer combinations being discussed (see http://www.engineeringtoolbox.com/adiabatic-flame-temperature-d_996.html) for some comparative adiabatic flame temperatures, albeit at room conditions).

I guess it just depends on how loosely you use the term "almost".  To me, 10s worth of Isp is not a lot of difference.  You can easily have a situation where you could get a LOX/Propane engine with better performance than a LOX/CH4 one just because of design details.  To me that's "almost as good of Isp".

~Jon

Offline Proponent

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Re: SSTO Thread
« Reply #539 on: 01/27/2009 01:09 pm »
[T]he only "in situ" methane production I can think of is mars. Both Carbon and Hydrogen are scare on the moon, so it makes sense to use the H2 directly

We do know that neutron spectrometer on Lunar Prospector produced evidence of hydrogen at the lunar south pole.  People tend to tie that result Clementine and Arecibo data suggesting the presence of water ice, but I wonder whether the hydrogen might not be in the form of organic (carbon-bearing) molecules.  This isn't my idea, BTW, but I don't remember where I saw it.

But, I agree with you that in situ propellants are still pretty far in the future.
« Last Edit: 01/27/2009 01:09 pm by Proponent »

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