h a l f b a k e r y
I like this idea, only I think it should be run by the government.

meta:

account: browse anonymously, or get an account and write.

 user: pass:
register,

# liquid nitrogen brine rocket

heated brine and liquid nitrogen mixed together in a rocket chamber.
 (+10) [vote for, against]

This is a very low spec, but cheap and reusable booster rocket.

Brine is heated to about 110degC. This is inside a tank in a rocket. In another tank is a quantity of liquid nitrogen.

The rocket will not last long enough to leave the atmosphere so it will have petrol engine driven pumps to feed the liquid nitrogen and hot brine into a mixing chamber.

The hot brine will be cooled to -20 degC by the nitrogen, still above freezing point, and the the liquid nitrogen will be heated from -196degC to -20degC, turning to gas in the process.

This will produce lots of pressure which will force the mixture out of the nozzle producing lots of thrust, just not for very long.

 — NotationToby, May 22 2011

Thermite dry ice rocket Thermite_20dry_20ice_20rocket
similar scheme, but with the unfortunate potential of setting stuff on fire [bungston, May 23 2011]

Expansion ratio http://en.wikipedia...iki/Expansion_ratio
The expansion ratio of a liquefied and cryogenic substance is the volume of a given amount of that substance in liquid form compared to the volume of the same amount of substance in gaseous form, at a given temperature and pressure. [baconbrain, May 23 2011]

Negative glider Negative_20glider
Heavy sarcasm? Never! [bungston, May 23 2011]

Tsiolkovsky rocket equation http://en.wikipedia...sky_rocket_equation
Momentum, momentum, momentum ... [8th of 7, May 23 2011]

 So, a rocket powered by the vaporisation of liquid nitrogen in contact with hot brine?

 OK, two questions: 1) Why brine? (OK, it can get a little colder before it freezes, and why not brine.)

 2) How much oomph do you get from a litre of liquid nitrogen?

3) How much hot brine do you need to turn a litre of liquid nitrogen to vapour?
 — MaxwellBuchanan, May 22 2011

 The energetics of the system are doubtful, but it would be amusing to try this. The idea of using water as reaction mass isn't new. Using the energy impalance between LN and hot water is, however, intriguing.

 [+] for innovative thinking.

Now do the math ...
 — 8th of 7, May 22 2011

 OK.

 The latent heat of vaporization of liquid nitrogen is about 200kJ/kg. The heat capacity of brine is going to be roughly similar to that of water, about 1kJ/kg/°C

 The brine is cooling by about 130°C, thereby giving up about 130kJ/kg.

 So, you need something like 1.7kg of hot brine to vaporize 1kg of liquid N2.

That answers the third of my two questions, but what about the second one?
 — MaxwellBuchanan, May 22 2011

 Oh, and why not use sugar solution instead of brine? Add a little flavouring and your rocket would leave behind a trail of gently drifting sorbet.

[ ] pending answer to question 2.
 — MaxwellBuchanan, May 22 2011

 [MB] Yes, you've understood it.

 Brine as it has a fairly large liquid range, a little better than water 100deg compared to 130degC, and a high specific heat capacity.

Q2+Q3 not sure, could probably work it out given time and an internet connection.
 — NotationToby, May 22 2011

Hot brine is also intensely corrosive.
 — baconbrain, May 22 2011

 I don't think you're getting the best value out of your brine. You're having to put in a lot of energy to take the nitrogen through a phase transition, but your brine (which is providing that energy) is not itself going through a phase transition.

 You'd get better results if you allowed the brine to freeze. For water (not brine), this would give you an extra 334kJ/kg (ie, significantly more than the thermal transition alone).

 So, spray liquid nitrogen and hot brine into the chamber, and keep the ratio sufficient so that the brine not only cools, but also freezes (and then cools further to nitrogen's liquefaction point), thereby giving up as much of its energy as possible.

Additional small point: the liquefaction temperature of N2 will be significantly higher in the high pressure of the "combustion" chamber.
 — MaxwellBuchanan, May 22 2011

 okay lets guess its 10% efficient, perhaps pessimistic. t1 = 383k, t2=250k, ideally 133/383 about 25% efficient Its a while since school.

 130kj at 10%, 13 kj. 2.7 kg of mass about 5000j per kilogram. v = root(2E/M) = 100m/s

 sound credible for an exhaust velocity.

--------------------- edit goofed out by a factor of 3
 — NotationToby, May 22 2011

 I didn't want it freezing but, maybe at these energies a little ice in the exhaust mightn't do to much damage.

 Interesting though about letting it freeze and not vaporising the nitrogen. Might work better, solid brine particles in a high pressure liquid nitrogen pressure jet.

Like a cryogenic sandblaster.
 — NotationToby, May 22 2011

 // not itself going through a phase transition //

 Yes, but it remains a dense liquid, rather than a crystalline solid (which might block the mixing chamber nozzle).

Using a dense liquid as a reaction mass has advantages over a hot fluid (gas) as produced by conventional rocket motors. But the energy density of the overall system is going to be fairly poor ... liquid helium would give better energetics, but hydrogen would be acceptable, and is more readily available as a bulk liquid.
 — 8th of 7, May 22 2011

 // letting it freeze and not vaporising the nitrogen//

Uh, what would that accomplish?
 — MaxwellBuchanan, May 22 2011

 however, if it gives nearly 4 times as much energy up when frozen. The ratio of brine to nitrogen, changes from 1.7:1 to about 0.4:1. This assumes nitrogen vaporisation, though.

 Turning it into a giant brine snow cannon.

[[[edit ratios reversed
 — NotationToby, May 22 2011

 The nitrogen doesn't have to come out as a gas does it?

 Couldn't it just be squirted out by a small fraction of vaporised nitrogen. A bit like a blow-down vapak proper rocket engine.

This is back in sandblaster mode, not snow cannon.
 — NotationToby, May 22 2011

increasing the nitrogen fraction, when the nitrogen is left unvaporised, by allowing the brine to freeze, might make the difference between having a unmanagable sludge and a grainy liquid.
 — NotationToby, May 22 2011

 // it gives nearly 4 times as much energy up when frozen //

 No, that's a non-sequitur.

 The total energy of the system is a constant and is dependant on the transfer of energy from the brine to the nitrogen.

This isn't about enthalpies of combustion; it's about converting the thermal energy in a hot liquid (brine) and a cold liquid (LN2) to a cold liquid (brine) plus a cold gas under pressure (N2) which expels the cold liquid as reaction mass.
 — 8th of 7, May 22 2011

surely the energy removed from the brine must go somewhere?
 — NotationToby, May 22 2011

 I hate to say this in writing, but [8th] is ri....he's ri....he's rrr....he's notwrong.

 The expansion comes almost entirely from the liquid-to-gas transition. The energy comes almost entirely from the hot brine. So your best bang- for-buck will be to provide *just* enough energy to *just* vapourise the nitrogen, from the smallest possible amount of hot brine.

 If you achieve this, your exhaust would consist of brine-snow and gaseous nitrogen, both at somewhere close to the boiling point of liquid N2.

(You can always expand gaseous nitrogen to get a bit more push, but that's not a very efficient way to go in this system.)
 — MaxwellBuchanan, May 22 2011

 I'll have to sleep on this one. I need to understand why the brine snow doesn't contribute additonal energy to the system when freezing.

 Also I dont think we need to vapourise all the nitrogen.

When a steam rocket, the hot water type, not the hydrogen peroxide imposter, flashes to steam in the nozzle it is less than half of the water that changes to steam.
 — NotationToby, May 22 2011

Hmm. But is that a choice?
 — MaxwellBuchanan, May 22 2011

 — spidermother, May 22 2011

Surely the brine is needed to cure the ham and sausage needed for the long space flight.
 — DenholmRicshaw, May 22 2011

 Unvaporized nitrogen is fine. It would be as fine as unburnt gasoline exiting the exhaust pipe, which helps to generate the long tongue of flame out the back that one can use in the advertisement for the drag races. Somewhere around here is some stuff on rocket nozzle shapes, which apparently make a big difference.

 One already needs a pressure containment vessel for the nitrogen. Since you have one, why not two: supercritical hot brine (hey! you call this jug a containment vessel?) that flashes to steam on release, only to cool its salty ardor in the numbing embrace of the N sisters. Ideally in a phase change rocket like this one would use two "reactants" that would both be a gas when finished, since gases have the maximum volume/weight and so make better propellants.

some similar schemings in my linked dry ice / thermite rocket. the current idea is nifty in that no reactions occur, simply phase changes. I wonder which expands more - dry ice or liquid nitrogen?
 — bungston, May 23 2011

Strikes me, as things often do when I stand up to say things, that there is a lot of free heat around when launching a rocket. Maybe the LN could be vapourised by passing it through a pre-heater stage in the nose.
 — bigsleep, May 23 2011

 I am having to learn a lot before I can really participate in this discussion. (See linky for expansion ratio.) Thanks for all the exposition.

 I see brine as being very heavy to carry, and want to look at a LN rocket that uses air and air friction for the heat source. I am envisioning using the fins as radiators.

 I also notice that Liquid Oxygen has a greater expansion ratio than LN, and suggest using a liquid such as kerosene instead of brine, then setting the resultant mix on fire. Which gets us neatly back to the standard rockets that used LOX as coolant and as pump propellant before using it as oxidizer for the kerosene fuel. (I'll need to look them up again.)

 Neon has the best gas expansion ratio, by the way, and is very rare. Using that sounds Halfbakery to me.

 When I was a pup, a classmate had a Freon-powered rocket plane toy. (Thanks, Dougie, for fucking up the ozone layer. I hope you had fun.)

 =======

By the way, my Firefox browser puts a little formation of dots in the lower right of this comment window. I can click and drag on that and make the window frikkin' HUGE.
 — baconbrain, May 23 2011

/rocket that uses air and air friction for the heat source/ Such a rocket would work best, I think, if it started in orbit and pointed down, towards Earth or other gravitational body with an atmosphere. This would maximize the air friction encountered and consequent heat. One could make the case that gravity alone should be adequate to propel the rocket and if the case, the energy generated by the air and air friction could be used to power other aspects of the rocket, like scientific apparatus, bass amplifiers, and/or LASER BEAMS.
 — bungston, May 23 2011

[bungston] Are you proposing some kind of heavy sarcasm reactor ?
 — bigsleep, May 23 2011

 Is it still sarcastic if the end product is awesome?

I like the idea of a friction powered Stirling engine. It was my idea for how to make something fall faster than freefall absent internal energy inputs (link). Re the LASER BEAMS and bass amps, this is why I halfbake for a while after my AM coffeepot. I need to suppress/channel that flight of ideas.
 — bungston, May 23 2011

Makes me wish I was still an aerospace major so I could try this out! The propellant is cheap and easy to find, I could totally one-up yvess rossi with my salty rocket pack.
 — DIYMatt, May 23 2011

For the most part here, the brine is not serving any purpose. The nitrogen itself would be just as efficient a reaction mass, and atmospheric heat would serve to convert it to gas just as readily as the brine.
 — MechE, May 23 2011

 //want to look at a LN rocket that uses air and air friction for the heat source. //

That would be elegant. However, I have a horrible feeling that the fins needed to extract heat fast enough from air would impose a huge drag burden. The alternative, of using frictional heat, would only work at very high speeds, by which time you're in the upper atmosphere, and there must be a loss in available heating.
 — MaxwellBuchanan, May 23 2011

 // brine is not serving any purpose. The nitrogen itself would be just as efficient a reaction mass, and atmospheric heat would serve to convert it to gas //

 No, that's not quite right. The brine has a huge specific heat capacity, and transfers its energy to the nitrogen. The brine iteslf then acts as reaction mass, and a nozzle to handle a cold heavy liquid is much easier to design and build than one for hot gas; and it's much more re-useable too.

The main problem is that the overall energetics of the system are small compared with combustion.
 — 8th of 7, May 23 2011

 // a nozzle to handle a cold heavy liquid is much easier to design and build than one for hot gas; and it's much more re-useable too.//

 That is a very good point.

How bad is the energy difference (per metric pound of hot-brine-'n'-liquid nitrogen, compared to good ol' kerosene and LOX?
 — MaxwellBuchanan, May 23 2011

 The brine doesn't add anything as a reaction mass, if the pressure is all coming from the nitrogen. The impulse will be the same if the exhaust is nitrogen+brine or simply nitrogen.

I will acknowledge that the brine produces a very rapid heating of the nitrogen, but I would suggest that a properly designed fin structure in the rocket chamber could do the same thing with atmospheric heat.
 — MechE, May 23 2011

 // The impulse will be the same if the exhaust is nitrogen+brine or simply nitrogen //

 Newtonian reaction drives (i.e. rockets) work on conservation of momentum.

 The Tsiolkovsky rocket equation is your friend here, we think.

 — 8th of 7, May 23 2011

Maybe a grant could be obtained to start this Panacea Space Agency.
 — bigsleep, May 23 2011

Yes, and momentum is mv. The mass goes up if the brine is in the exhaust, but what happens to the velocity?
 — MechE, May 23 2011

Hmm. The lad may have a point. Suppose you had a rocket running on nitrogen only, and you just chucked loads of water into the exhaust plume - same end result, and the rocket wouldn't go any faster, I think.
 — MaxwellBuchanan, May 23 2011

Ooh ooh! How about a cloud-scooping rocket? OK, not much good for fair days or high altitudes. Forget I said anything.
 — MaxwellBuchanan, May 23 2011

 // cloud-scooping rocket //

Baked. Bussard ramscoop.
 — 8th of 7, May 23 2011

I thought that was all hydrogeny and interplanetary? I was thinking of scooping water and super-heating it as a blend with the rocket fuel. But then again, the same argument applies - greater reaction mass, smaller exhaust velocity. I think.
 — MaxwellBuchanan, May 23 2011

 // I think //

Is that a statement, or a question ? If a question, is it rhetorical ?
 — 8th of 7, May 23 2011

It was a rhetorical statement.
 — MaxwellBuchanan, May 23 2011

//I think.// At last, proof of [MaxwellBuchanan]'s existence.
 — mouseposture, May 23 2011

Maybe the friction-heated nitrogen would work if the rocket were fired from a gun first. Then it would be up to spead, hot from air resistance, and could dig into its frozen propellant.
 — bungston, May 23 2011

 //The brine doesn't add anything as a reaction mass, if the pressure is all coming from the nitrogen. The impulse will be the same if the exhaust is nitrogen+brine or simply nitrogen.// ... //Yes, and momentum is mv.//.

 But energy is 1/2 mv^2, so for a given amount of energy, increasing the reaction mass increases the impulse. This is a poor optimisation when what is desired is the highest impulse per weight, rather than per energy.

An obvious example of a useful, non-energetic, reaction mass is the water in a water rocket. Water rockets have less impulse than a binary liquid fuel combustion rocket, but more than a compressed air rocket (a water rocket without the water).
 — spidermother, May 24 2011

 The number you're looking for, [MB], in relation to Kero/LOX is known as the "specific impulse" (/sp)of a fuel combination.

/sp of Kero/LOX is about 250.
/sp of Ozone/Hydrogen is one of the highest, at about 355.
 — infidel, May 24 2011

///sp of Ozone/Hydrogen is one of the highest, at about 355//
I thought xenon ion was higher?
 — AbsintheWithoutLeave, May 24 2011

Yebbut ion drives don't really count, as the energy source is separate from the reaction mass, so you can't make a direct comparison.
 — spidermother, May 24 2011

ethanol might be interesting as it can go from 78 to -114 degC, a 192 degree change.
 — NotationToby, May 24 2011

 // you can't make a direct comparison //



 Actually, you can make a direct comparison, but it's meaningless.

</pedant>
 — 8th of 7, May 24 2011

 [annotate]

back: main index