You are not logged in.
I've found this interesting article by Geoffrey Landis about a new rocket propelled by LOX and a mixture of acetylene and CO at 50%: it has an Isp about 350 s: the hydrogen needed is less than 1% the total propellant mass, and so the little amounth (0.03%) of atmospheric water vapor may be enough to drive the production, without bringing hydrogen from Earth.
http://www.uapress.arizona.edu/onlinebk … rces28.pdf
Last edited by Quaoar (2013-12-28 07:24:07)
Offline
Like button can go here
Acetylene is safest-stored dissolved in a suitable liquid. Most acetylene bottles used in welding have the gas dissolved in acetone, under roughly 10-20 atm pressure at Earthly temperatures. If you don't do something like this, you are almost guaranteed a tank explosion. Acetylene can be quite tricky to handle, more so than the article indicates. Rocket designers are not used to handling this stuff, it's the Earthly mechanical engineers dealing in compressed gases generally, who have the relevant experience. And none of them are used to creating high strength/weight structures.
GW
Last edited by GW Johnson (2013-12-28 14:14:18)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
Like button can go here
Do you think that dissolving the acetylene in CO at low temperatures would be enough to make it safe?
Landis thinks an acetylene-CO 50% mixture may be stable enough. I'm not an expert.
Offline
Like button can go here
I wouldn't be too surprised if it works at low temperatures. But I wouldn't know either. In any case I think meth-lox is a pretty strong contender, given that the chemistry of synthesis is less complex.
-Josh
Offline
Like button can go here
I wouldn't be too surprised if it works at low temperatures. But I wouldn't know either. In any case I think meth-lox is a pretty strong contender, given that the chemistry of synthesis is less complex.
You're right. Propylene/CO-LOX has the only vantage to be synthetyzed with the little water vapor of Mras atmosphere, without bringing hydrogen form home. Probably it will be more simple to land in a place where we know there is ice, and use it to synthetyze LOX-LCH4.
Offline
Like button can go here
The paper linked says carbon monoxide boiling temperature us -190°C, however the gas encyclopedia at http://www.airliquide.ca/ says it's boiling temperature is -191.5°C at 1 atmosphere pressure. Methane is -161.48°C, and LOX is -182.96°C. These are all soft cryogenics, meaning they have to be kept cold to stay liquid, but not as cold (difficult) as hydrogen or helium. Colder temperature requires more active refrigeration to store, but not a lot. They're all colder than Mars.
What is solubility of acetylene in CO at different temperatures? Acetylene sublimates at -84.7°C, so at temperature of liquid CO it's a solid. It would have to be dissolved like salt in water. That also means it won't effervesce (off-gas). Acetylene would have to warm much more than the boiling temperature of CO before it sublimates. Obviously that will occur in the combustion chamber of a rocket engine. If any acetylene does come out of solution, it will precipitate, not effervesce. Ok, so it is safe for Mars. On Earth you wouldn't want a large tank of liquid CO stored in a vessel not capable of pressure necessary to contain liquid at outdoors temperature. The CO boil-off would be highly toxic. But it would work on Mars.
Another concern is we have LOX/LCH4 rocket engines right now. Engines that use this new fuel would have to be developed.
Offline
Like button can go here
This paper says H2/LOX has Isp up to 470s, hydrocarbon fuels about 375s, and CO/LOX about 280s. That makes CO better than I thought, but still pretty poor. As comparison, the Space Shuttle Main Engine, known as SSME, manufacturer designation RS-24, had Isp at sea level of 363s, and in vacuum of 453s. There are LOX/LH2 engines with higher Isp, but they're optimized for vacuum only. AEC aka RS-44 had Isp of 481s. CO/LOX is about the same as a solid rocket: Shuttle SRB was 269s in vacuum before the Challenger dissaster, RSRM after the redesign had 235s at sea level or 267s in vacuum.
Where does the figure for acetylene/CO come from?
Offline
Like button can go here
I imagine that dunnspace has figures for it on their website. I doubt that any engine has ever been fired with acetylene but an approximation on the Isp can be calculated based on the gas laws, if you know the flame temperature of acetylene in pure oxygen, which we do.
-Josh
Offline
Like button can go here
I've found this interesting article on extraction of atmospheric water ( http://www.lpi.usra.edu/publications/re … ington.pdf ).
If this technology will work, a LOX-Acetylene/CO rocket may be a very good option.
Last edited by Quaoar (2014-03-16 07:59:06)
Offline
Like button can go here
Bumping this thread as I think it's prudent to keep the rocket and water systems discussions separate.
While I myself implied an operational link between the techs, I'm beginning to doubt that either one actually requires the other.
For any Hydrogen source having better hydrogen:propellent leveraging is desirable and an Acetylene or other fuel with low Hydrogen content is perfectly justifiable on it's own merits regardless of how we are obtaining Hydrogen.
Conversely no matter what use we are making of Hydrogen (including non-propellent uses), the system that can make the most at the lowest cost/mass/risk is the best.
When you start combining two techs and say that A makes B possible/feasible in-place of C, what were really saying is that A is going to change the SCALE of the second tech such that were crossing over an efficiency hump in which B and C are understood to be dominant on either side of. But Acetylene/CO offer at most a 3/4 reduction in Hydrogen needs vs Methane and only ~2/3 compared to some other rather obvious Hydrocarbons that should in all rights be the default choice rather then Methane. That doesn't seem to me to be enough to cross thouse kinds of humps which usually take several orders of magnitude.
Offline
Like button can go here
Sometimes I wish people would forget about comparing things on Isp. Isp varies all over the map, vacuum or otherwise, depending upon what chamber pressure and expansion ratio you think you can use. And not everybody agrees about those things.
I wish people would compare characteristic velocity c* instead. That's the true function of thermochemistry and chamber pressure. Everything else, including Isp, depends upon that, and a whole lot more (expansion ratio, specific heat ratio (implicit in c*), nozzle kinetic energy efficiency, and chamber to ambient pressure ratio, not to mention separated-bell flows if your pressure ratio is too low. But, that requires folks to learn elementary interior ballistics. It's not hard. But it's not one simple calculation, either. But, it is repeatable and all can agree upon it.
Most of the high vacuum Isp's I see quoted depend upon large (but unspecified) expansion ratios, and do not include the nozzle kinetic energy efficiency. They also assume very high chamber pressures, that most manufacturers do not yet build to. For example, shuttle vac Isp is for 3000 psia chamber pressure. That's the only engine ever made that operates that high. Most everybody else is at or under 1000 psia, which makes a huge difference.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
Like button can go here
Acetylene is a chemically unstable monopropellant. On Earth it is stored with ceramic foams in an attempt to limit the rate of reaction in the vevent of exposure to fire or mechanical shock. I would imagine that this would dent the mass ratio if C2H2 were used a propellant. Ethylene may be safer, but it too is vulnerable to catalytic decomposition if contaminated with tiny amounts of free oxygen. Those carbon-carbon double bonds have a nasty habit of unzipping and forming themselves into long chains with explosive consequences. Same problem with acetylene, but even more unstable.
Offline
Like button can go here
I know nothing about "ceramic foams". I don't think acetylene is a monopropellant, though. It never burns for me without air or oxygen. Explosive? YES!
I do know the 50-pound steel acetylene bottle on the welding cart in my shop holds around 5 lb of acetylene dissolved in around 5 lb of acetone. When full and for most of its useful "life" welding and cutting, bottle pressure is near 250 psig. That's awfully low compared to most rocket motor chamber pressures (500-1000 psig). Isp's are quite crappy when figured at 250 psig, no matter what you assume about the nozzle.
Once withdrawn from solution in the bottle, you NEVER use acetylene at more than about 10 psig. PERIOD. Your torch will blow up and kill you if you try. That makes the rocket chamber pressure mismatch even worse. There is no usable Isp at 10 psig chamber pressures in a rocket engine.
Check the inert weight fraction for 5 lb acetylene in 50 lb worth of bottle (90%). Now scale THAT up to 1000's of lb of acetylene on some rocket design. Not so attractive, is it?
The lesson here is things that look "theoretically" good from a chemistry standpoint are often quite worthless in practical terms. I think you can safely forget acetylene as a rocket fuel.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
Offline
Like button can go here
No one is proposing pure acetylene being used, either we find a mixture that makes a viable liquid-feed engine or we don't use acetylene in any form.
The question is what kind of mixture might work, I don't think any of us are sufficient in chemistry to really tell. Something with a lower burn temperature is logically what's needed, and it needs to dissolve the Acetylene fully and produce a dense fluid.
Last edited by Impaler (2015-01-06 20:15:29)
Offline
Like button can go here
what kind of mixture might work, I don't think any of us are sufficient in chemistry to really tell.
Hmm. Challenge. My knowledge of chemistry is limited, but all this Mars stuff is causing me to study and learn. Here's a simple principle: hydrazine is hypergolic, but adding a methyl group stabilizes it. Mixing pure hydrazine with the stabilized form is enough. So what do we get if we add a methyl group to acetylene? Carbon bonds certain ways, so don't expect adding a simple methyl group to work. Acetylene is C2H2, represented as H-C=C-H. Adding a -CH3 group to one of the carbon atoms of acetylene? The bonds would be unbalanced. You have to cut one of the three bonds between the central carbon atoms to bond to the methyl group. What does the other carbon bond to? This line of reasoning leads to acetone. Oh, wait! That's what GW Johnson already said.
Acetone is flammable, chemical formula (CH3)2CO. Would an acetylene/acetone fuel mixture burned with LOX be useful?
Offline
Like button can go here
On the plus side, acetylene has an exceptionally low minimum ignition energy in air (0.007mj compared to 0.28mj for methane), so adding it to the fuel mix should improve ignitability. It may even self-ignite with some oxidisers or in the presence of a catalyst. It also has a high laminar burning velocity (1.55m/s versus 0.448m/s for methane), which improves combustion efficiency and may help avoid instabilities. Hydrogen offers the same benefits, but is a deep cryogen, an embrittling agent and is therefore more difficult to store.
Offline
Like button can go here
Acetone, the simplest ketone. As a partially oxygenated fuel it has lower energy density and therefore offers a lower ISP than a pure hydrocarbon fuel of the same weight. This suggests to me that an acetylene/acetone mix is unlikely to offer ISP advantages over any pure hydrocarbon fuel. It may improve reaction kinetics, which may be valuable in engine design, but it is not going to increase ISP.
How about using the sabatier reactor to produce longer chain hydrocarbons? Propane is a liquid at room temperature at 10bar. If you can keep the propellant tank warm it will be self pressurising and has similar combustion properties to methane. On Mars, with <1% earth sea level pressure, combustion chamber pressures can be relaxed without impacting ISP too much. This suggests to me that a low-cost pressure fed engine system using high energy density liquid fuels offers a lot of advantages.
Offline
Like button can go here
So long as the mixture is matching the ISP of Methane and won't coke the engine it's a viable substitute, improved hydrogen leveraging is the main goal with ANY other benefits as bonus, Landis and others have looked at CO because it's Hydrogen-less and compounds leverage even further. The Acetone looks like it may have potential, any idea what kind of ratio it would be at to the Acetylene and then the final Hydrogen:Propellent leverage would look like? Also what are the chemical synthesis requirements of Acetone?
Offline
Like button can go here
Following my same chemical argument, you could use propylene. That's just acetylene with the one of the three carbon bonds broken, one bonded to methyl, the other to a hydrogen atom. That gives you a pure hydrocarbon. But I don't know if would stabilize acetylene. Propylene boils at -47.6°C at standard pressure. Acetylene melts at -80.8°C; it doesn't have a boiling point because its triple point is at 1.27 atmospheres pressure.
Its most singular hazard is associated with its intrinsic instability, especially when it is pressurized: under certain conditions acetylene can react in an exothermic addition-type reaction to form a number of products, typically benzene and/or vinylacetylene, possibly in addition to carbon and hydrogen. Consequently, acetylene, if initiated by intense heat or a shockwave, can decompose explosively if the absolute pressure of the gas exceeds about 200 kPa (29 psi). Most regulators and pressure gauges on equipment report gauge pressure and the safe limit for acetylene therefore is 101 kPa gage or 15 psig. It is therefore shipped and stored dissolved in acetone or dimethylformamide (DMF), contained in a gas cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling. Copper catalyses the decomposition of acetylene and as a result acetylene should not be transported in copper pipes. Brass pipe fittings should also be avoided.
So the objective is to keep it liquid, while keeping pressure below 101 kPa above Earth ambient pressure. Would acetylene dissolve in liquid propylene chilled below -47.6°C? If so, then that would work.
Would be interesting if it works. That can be maintained in space with just a sunshade. Would really have to watch the pressure. The Shuttle external tank operating pressure for LH2 was 32–34 psi (220–230 kPa) (absolute), for LOX 20–22 psi (140–150 kPa) (gauge). So pressure of this acetylene mixture would have to be maximum 29 psi (200 kPa) (absolute). A safety valve would have to vent at that pressure.
Offline
Like button can go here
Well, I thought it good that I was able to come up with this. Solids will dissolve more when liquid temperature increases, but gasses dissolve less. So to dissolve a lot of acetylene, you need to keep propylene cold. Normally fuel is kept just below boiling so it doesn't take much heat to boil. You want liquid in tanks to keep pressure down, so tanks can have thin/light walls. But you want fuel gas in the engine to minimize energy lost to cold fuel. You want exhaust gas hot so the gas expands as much as possible for maximum pressure, maximum thrust. And higher pressure results in higher exhaust velocity, so higher Isp. But, you may have to chill propylene further to dissolve significant quantity of acetylene. Since acetylene melts at -80.8°C, that's the temperature you probably want. But that's still warmer than LH2 or LCH4.
I started by taking acetylene, and modifying it for a stable solvent. That maximize chance that acetylene will dissolve. But more directly, acetylene is a bi-polar molecule. Electrons will be held strongly by carbon, less so by hydrogen. That makes the two hydrogen ends positive, and the carbon central body negative. You need a polar solvent to dissolve that. Propylene has the same double carbon body, but with a double bond instead of triple, so not quite as strongly charged. But propylene is not symmetric, it has a methyl group at one end. That will make it easier for acetylene to dissolve.
Offline
Like button can go here
Well, I thought it good that I was able to come up with this. Solids will dissolve more when liquid temperature increases, but gasses dissolve less. So to dissolve a lot of acetylene, you need to keep propylene cold. Normally fuel is kept just below boiling so it doesn't take much heat to boil. You want liquid in tanks to keep pressure down, so tanks can have thin/light walls. But you want fuel gas in the engine to minimize energy lost to cold fuel. You want exhaust gas hot so the gas expands as much as possible for maximum pressure, maximum thrust. And higher pressure results in higher exhaust velocity, so higher Isp. But, you may have to chill propylene further to dissolve significant quantity of acetylene. Since acetylene melts at -80.8°C, that's the temperature you probably want. But that's still warmer than LH2 or LCH4.
I started by taking acetylene, and modifying it for a stable solvent. That maximize chance that acetylene will dissolve. But more directly, acetylene is a bi-polar molecule. Electrons will be held strongly by carbon, less so by hydrogen. That makes the two hydrogen ends positive, and the carbon central body negative. You need a polar solvent to dissolve that. Propylene has the same double carbon body, but with a double bond instead of triple, so not quite as strongly charged. But propylene is not symmetric, it has a methyl group at one end. That will make it easier for acetylene to dissolve.
A discovery of great importance, I would say. In fact, provided the extra reaction steps do not reduce the energy efficiency of fuel synthesis by a large margin, the use of energy dense liquid fuel that is indefinitely storable under Mars ambient conditions without refrigeration, opens up the possibility of entirely new mission architecture.
Using methane/oxygen propellant, the need for heavy refrigeration means that each mission is limited in the range of Martian surface that it can cover. This prompts a Mars direct architecture based upon multiple independent missions to fresh sites. However, if propellant is easily storable, then it may make more sense to choose a single base site from day 1 and to land a number of refuelling posts at intervals of perhaps 500km across the Martian surface at which the rover can refuel. Sabatier reactors of a similar size to those proposed for the Mars sample return would be suitable and could be mass produced and delivered to Mars using a series of heavy lifts. This gives the rovers a truly global reach.
After delivery of the first hab, only 1 heavy lift would strictly be needed per mission, with the crew shipping out and returning in the same ERV. This halves the cost of ongoing Mars missions. The extra heavy lift allows stockpiling of equipment at a single base, thereby enhancing the capability of the mission as a whole and improving base habitability, allowing longer stays. It should also reduce the need for a nuclear power supply, given that the same initial solar power plant can manufacture propellant for successive ERVs.
Offline
Like button can go here
Well, I thought it good that I was able to come up with this. Solids will dissolve more when liquid temperature increases, but gasses dissolve less. So to dissolve a lot of acetylene, you need to keep propylene cold. Normally fuel is kept just below boiling so it doesn't take much heat to boil. You want liquid in tanks to keep pressure down, so tanks can have thin/light walls. But you want fuel gas in the engine to minimize energy lost to cold fuel. You want exhaust gas hot so the gas expands as much as possible for maximum pressure, maximum thrust. And higher pressure results in higher exhaust velocity, so higher Isp. But, you may have to chill propylene further to dissolve significant quantity of acetylene. Since acetylene melts at -80.8°C, that's the temperature you probably want. But that's still warmer than LH2 or LCH4.
If the two liquids have different partial pressure, then the composition of the boil-off gas will be dominated by a single constituent. That may mean the combustion properties of the gas changing over time, or (shock-horror) a tank full of concentrated actylene.
If you do inject the fuel and oxidiser as liquids, you gain a higher compression ratio, cool the cylinder head and maintain a constant fuel composition. On the other hand, since injection pressures are typically very high, you might be able to use the evaporating liquid to cool the engine prior to injection. The liquids would stay liquid so long as they remain beneath their critical point.
Both fuels are a little risky if you put them through complex plumbing, since even a trace of oxygen could trigger catastrophic polymerisation. Even Martian air contains some O2. Not sure how big a problem would end up being.
Offline
Like button can go here
In the original 'Case for Mars' Robert Zubrin reported the results of a study he had carried out concerning extraction of water from the Martian atmosphere using a compressor. An 8kWe fan was found to produce 90kg of water per day. This would yield 10kg H2 and 80kg O2 if electrolysed. The system could therefore yield the required 6tonnes of liquid hydrogen in 600 days.
Since we now have a fuel option that is even less hydrogen intensive and given that 8kWe is only 10% of the total energy budget assumed for mars direct, why bother shipping liquid hydrogen from Earth?
Offline
Like button can go here