Fuel Cell Rocket

[dbmorpher]

Hello I've had an idea circulating for a while now and I guess I just want to share it before I forget. Basically I was researching fuel cells for an idea I had for a car engine (car engine was a bust) but I did find out that a Solid Oxide Fuel Cell (SOFC) can reach 1000C which I also found out is the operating temperature of a pebble bed reactor. Pebble beds are also a form of reactor that can be used in an NTR and is a form of Very High Temperature Reactor as it can reach 1000C also. This heat should make the SOFC a suitable energy source for a thermal rocket.

Now I calculated a lot of the specs but the problem I had was with the actual dimensional design of the engine. I am not familiar with the mathematics enough as I am currently in Algebra II I tried NASA sources such as the GRC site and even tryied contacting NASA engineers themselves but I tried to get a simulator when really I should have tried an engineer. I also tried multiple rocket companies such as Aerojet Rocketdyne and SpaceX. I can give you my calculations if you want to see, just pm me but I don't really want to put up a download. Also please don't steal my idea if it works.

[GoSlash27]

dbmorpher,

I promise I won't steal your idea!

The problem here is that rockets don't work by generating heat. They work by ejecting exhaust molecules at a high velocity. eK= mv^2/2. The heat is usually a by- product of the process, not the source of the thrust. All you have to do is figure out how to channel that heat into accelerating particles and you have a rocket.

Best,

-Slashy

[dbmorpher]

dbmorpher,

I promise I won't steal your idea!

The problem here is that rockets don't work by generating heat. They work by ejecting exhaust molecules at a high velocity. eK= mv^2/2. The heat is usually a by- product of the process, not the source of the thrust. All you have to do is figure out how to channel that heat into accelerating particles and you have a rocket.

Best,

-Slashy

Slashy, thank you for not stealing but I am talking on something along the lines of an NTR which transfers heat from a fuel to the propellent. My design would feuture a tubular SOFC where LH2 is pumped down the middle and air or oxygen (stored in liquid for for travel) is used for oxidation. The fuel cell heats the Hydrogen not absorbed and also the Water byproduct which expands i the nozzle to produce thrust. Also the fuel cell produces electrical energy which powers ship systems. The turbopump could also be shutoff and the nozzle plugged to simply use the fuel cel to provide power and water for crew needs. My problem is that I am unfamiliar with rocket engine construction in the real world.

[sgt_flyer]

Mmh - NTR's exhausts is way hotter than 1000°C - KIWi-1 exhaust ran at 2683°K.

LH2-lox exhausts generally runs near 3000°K - SSME's even ran at higher temps.

What gives NTRs a better ISP than chemical, is that it don't use oxydizers.

As for the comparison between a nuclear thermal reactor and a nuclear thermal rocket, lies in the fact that the rocket's reactor is 'transferring' it's heat to the reaction mass - which is ejected out of the rocket, so it's relatively easy to keep it under control, whereas we work with closed loops on surface reactors - which could cause a runaway reaction if not cooled correctly. Heck - current reactor's fuel are not let to heat more than 600°C in the fuel rods themselves (or else you 'll damage the rods) - NTR's fuelrods are different than standard reactor fuel rods (and much higher in price too )

The concept behind your idea is neat - but i don't think it can match even chemical ISPs.

Heck, even a turbojet's combustion chamber runs hotter - to the point the exhaust is cooled down by air from intakes before it runs through the driving turbine.

Edited July 6, 2014 by sgt_flyer

[K^2]

Not going to work, sorry. It's an interesting idea, and almost had me going for a moment.

First, lets look at why NTR has such great I_SP. After all, any conventional rocket will have hotter temperatures in its combustion chamber. The trick is using H2 as propellant. It's a very light molecule. Your next best alternative is LH2/LOX bipropellant rocket. The exhaust is H2O, which is 9 times heavier. So you need 9 times more energy to get same I_SP. Everything else being the same, that's 9 times higher temperatures. And, in reality, the difference is closer to 4-5 times. Hence almost double the I_SP of a typical NTR.

Here is the kicker, though. NTR gets energy from nuclear fuel. You propose using chemical fuel. Fuel cell, in fact. Again, your best bet there is LH2/LOX to power your fuel cell. To get H2 to typical NTR temperatures, you can probably go 5:1. That is, for each H2O you produce in fuel cell, you'll accelerate 5 H2 molecules.

Now, for the moment, lets forget the efficiency. We'll pretend that everything is 100% efficient. For simplicity, lets say that H2O molecule gives you 45 units of energy. Mass of each H2 is 2 units, and each H2O is 18 units. Split between 5 H2 molecules, each gets 9 units of energy, and therefore, exits with 3 units of velocity. v_H2 = 3. In contrast, we could have just burned H2O in a rocket, get 45 units of energy to push H2O molecule, and get v_H2O = sqrt(5) = 2.24. So at a first glance, the exhaust velocity of H2 is better, so we expect better I_SP. But we did burn some LH2/LOX to get that. So we have to work out the total impulse and divide it by total mass of fuel/propellant.

In case of conventional rocket, everything is easy. It took 18 units of fuel to get 18 units of fuel moving at 2.24 units of velocity. That means I_SP is 2.24 units.

In case of thermal rocket, things are more complicated. We've used 18 units of LH2/LOX + 10 units of H2 propellant. But we only got 10 units of H2 moving at 3 units of velocity. So the total I_SP = 30 / 28 = 1.07.

As a result, you get much better I_SP from a conventional rocket than you get from a fuel cell thermal rocket even at 100% efficiency for both. In reality, of course, fuel cell will have additional losses, making it even less efficient. The reason this works out differently for an NTR is because every single uranium atom that has decayed and become a dead weight gives you energy to get millions of H2 molecules up to speed. So the I_SP works out to be just the exhaust velocity, and that makes NTR more efficient.

As a rule of thumb, if you are building a chemical rocket, you want to use all of the products of combustion moving. And the best way to do this is a conventional rocket. When you find an alternative power source, that's when you start looking at other options. Ion drive, NTR, etc.

[GoSlash27]

dbmorpher,

Unless you are planning on building it yourself, you don't really need to know the ins and outs of rocket construction. Although I guarantee you you could knock together a functioning rocket if you just wanted to. Not all that difficult. Making a *good* rocket, otoh, requires a whole lot of research, planning, money, and a good team of eggheads.

Are you looking for guidance on what to do with your idea?

-Slashy

[GoSlash27]

K^2,

My thinking on the subject was highly over simplified

You cannot convert energy from one form to another without efficiency losses, and the losses multiply through each step. Burn fuel--> exchange heat-->convert heat to pressure-->expel propellant through nozzle can never be as efficient as Burn fuel-->expel propellant simply because there's more steps.

Thanks for the more in-depth analysis; it's an eye-opener!

Best,

-Slashy

[dbmorpher]

Thanks for the help guys.

I went back and thought about the design. I know the fuel cell will not be entirely efficient but I believe the inefficiency is what drives the engine. You see the fuel cell, being cylindrical will never touch some H2. Supposing I make the cell conic it would absorb more but I don't want it to absorb all of it. Supposing there is a splitter from the H2 tank, some of the propellant stays propellant but a small part goes to the fuel cell. The propellant and the fuel can be blocked by quartz in the Combustion* Chamber so they don't mix but the heat is transferred because of quartz' thermal transparency. Therefore the exhaust does not have any water in it making the rocket's efficiency more than triple (hopefully).

Here is a sketch that might help

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P.S. the low oxygen requirement of an sofc compared to a combustion engine means that this engine could operate in the atmosphere without use of LOX

*It really isn't a combustion chamber because there is not combustion, just electrolyzation

[magnemoe]

Note that one idea is to use an huge parable to focus solar light onto an tungsten block you run hydrogen trough.this could give ISP similar to an NTR but obviously with lower trust

[GoSlash27]

I see what you're getting at, but 1) I don't think this can ever be as efficient as a run-of-the-mill chemical rocket and 2) there's nothing I can do to help you with it.

Good luck, though!

-Slashy

[K^2]

Note that one idea is to use an huge parable to focus solar light onto an tungsten block you run hydrogen trough.this could give ISP similar to an NTR but obviously with lower trust

That's actually pretty brilliant. It can benefit from all of the solar sail tech we've developed, but it allows for much higher thrust, with I_SP high enough to be very useful.

I'll have to run some numbers, but that definitely has potential.

[Tommygun]

Note that one idea is to use an huge parable to focus solar light onto an tungsten block you run hydrogen trough.this could give ISP similar to an NTR but obviously with lower trust

I'm guessing using Earth or spaced based lasers on the reflector could give a much more appreciable boost to the craft than it would to a traditional solar sail too?

[dbmorpher]

Yes magnemoe's idea is called a solar thermal rocket and its not really that new. The Wikipedia page is pretty big too. http://en.wikipedia.org/wiki/Solar_thermal_rocket