SRB metal doping, uranium question.

[kahlzun]

Hi guys!

I understand that there is a method of boosting the efficiency of solid rockets by doping them with powdered metals, such as aluminium, to increase the Mass•.

I have been offhandedly working on a 'everything nuclear' mod program, and I was wondering why aluminum has been chosen for this.. Would it work more effectively with a heavy material, such as uranium or tungsten or something?

Uranium is flammable in a powdered form, and very dense, would this work just as well, more poorly, or better than the Aluminium doping process?

[drakesdoom]

Aluminum is actually a fuel not just dead mass. Not sure if there would be a better option but for an initial launch motor like an SRB cheap is better just add more.

[kahlzun]

My concern was mostly would uranium work as a doping material for an SRBs, replacing the aluminium? It would oxidise less violently, but the extra mass• would increase efficiency? Or thrust?

How does the density of the doping material affect efficiency in a solid motor?

[agentexeider]

more importantly is that doping with uranium would create radioactive exhaust that would kill every living thing within 2 miles of the launch site and irradiate countless others when the particulates get into the jet stream. more over uranium is one of the rarest of the rare earth metals, so expense would be huge.

[Brotoro]

I don't think Aluminum is added to solid propellant to add mass. The Aluminum is part of the fuel (it can make up to 21% of the propellant, so I also wouldn't call this "doping") and it burns very energetically, more than Uranium would. You also generally don't want heavier elements in your fuel because a high exhaust speed is more efficient, and lighter molecules give you higher exhaust speeds. Plus, Aluminum is much cheaper than Uranium. And although Uranium is not highly radioactive, spreading it around in big clouds is not going to make you any friends. And nothing about this would be "nuclear" so there's no reason to include it in an "everything nuclear" mod program.

[K^2]

Extra mass does not improve efficiency. By itself, it makes it worse. Aluminum is added because it releases a lot of energy when oxidized. That energy allows the fuel to burn hotter, increasing velocity of some lighter molecules in the exhaust. This increases the overall I_SP of the rocket. Adding uranium would make your SRB perform way worse. All it will do is steal the thermal energy away from the light compounds which are the ones that give you the most thrust.

[Rune]

Extra mass does not improve efficiency. By itself, it makes it worse. Aluminum is added because it releases a lot of energy when oxidized. That energy allows the fuel to burn hotter, increasing velocity of some lighter molecules in the exhaust. This increases the overall I_SP of the rocket. Adding uranium would make your SRB perform way worse. All it will do is steal the thermal energy away from the light compounds which are the ones that give you the most thrust.

That should read: " All it will do is steal the thermal energy away from the light compounds which are the ones that give you the most impulse." If you are after thrust, increasing the density of propellant is not a bad idea. If it is isp what you are after, then that's another story, and it goes like you say. Minor nitpicking, I know.

But yeah, in solid fuel some metals are added to make the chemical reaction more exothermic. In fact, boron additives have also been considered for liquid fuels, and there's always the crazy theoretical combo of hydrogen, oxygen, and fluorine. That's supposed to get like 700s out of the water vapour exhaust... if you can handle the fluorine and contain the hellish chemical reaction.

Rune. Think kinetic energy equation: speed is more important than mass.

[jwenting]

more importantly is that doping with uranium would create radioactive exhaust that would kill every living thing within 2 miles of the launch site and irradiate countless others when the particulates get into the jet stream. more over uranium is one of the rarest of the rare earth metals, so expense would be huge.

uh, no. Use depleted Uranium, which is a waste product from Uranium refining and has a half life of several million years. The radioactivity released would not increase the background radiation noticeably, and if you aim them over water it gets diluted in that to the point where you wouldn't know it exists at all.

What's more important is that Uranium Oxide is rather toxic (as are all heavy metal oxides, I've worked with lead and mercury oxide during my graduation work, the safety precautions were rather extreme), and Uranium of course tends to oxidise rather well at rocket exhaust speeds (heck, most anything does).

[falofonos]

If you want nuclear boosters then perhaps http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket would provide.

Then again, perhaps not.

[Fuzzy Dunlop]

Uranium would significantly increase the thrust, but it would also make the boosters much heavier. So specific impulse will be much lower, and thrust to weight may or may not increase.

[Brotoro]

If you want nuclear boosters then perhaps http://en.wikipedia.org/wiki/Nuclear_salt-water_rocket would provide.

Then again, perhaps not.

An open-cycle nuclear rocket engine like the Nuclear Salt Water Rocket would not be good for use as boosters in a planetary atmosphere. The exhaust from that type of engine actually is very radioactive (it WOULD be what a lot of people incorrectly seem to think the exhaust from NERVA-style engines is like) because it expels the fission products along with the reaction mass. It would be more efficient than a NERVA engine, however, and if you only use it out in space and be careful where you point it, its exhaust would not be a problem because the exhaust travels faster than solar escape velocity, so it would leave the solar system.

[kahlzun]

Of course, since Kerbals seem to be immune to radiation, the above are moot points for KSP..

[Themohawkninja]

Since this is about aluminum in combustible/explosive materials, I have a question.

Why is it that aluminium is used in combustible/explosive materials like rocket engines, or thermite?

[bivouac]

Why is it that aluminium is used in combustible/explosive materials like rocket engines, or thermite?

Short Answer: It's very reactive and predictable.

Long Answer:

While not a rocket scientist, the chemist in me feels compelled to answer. This has everything to do with the formation of aluminium oxide. We utilize it as a propellent not just because it's flammable, but because it has a MASSIVE enthalpy of formation, ∆H_ƒ. That is, when we form aluminium oxide from pure aluminium, the process of forming the product releases a whole lot of energy. Uranium was mentioned in previous pages as a propellent, but its efficiency and I_sp would be mitigated due to its intense mass and the fact that its enthalpy of formation is weaker than that of aluminium by several hundred kJ/mol. With that being said, using aluminium allows for a lighter fuel that releases more energy than would uranium. I really can't understate just how heavy and dense uranium is. Consider that you average golf ball weighs just over one and a half ounces. That same gold ball made out of uranium would weigh just over one and a half pounds. If we use a heavier material to generate thrust, we subsequently increase the amount of thrust needed to propel the rocket. By using a lighter material, we can produce that same amount of thrust and use the saved weight towards a spacecraft rather than fuel weight compensation.

It was mentioned that aluminium may, "react violently". That's precisely what you want when making a rocket engine. A lightweight substance that combusts/oxidizes predictably. Remember that essentially, all rocket engines are doing is regulating a controlled explosion. (Which is why they're extremely dangerous to fly and operate.) You want your fuel to produce a violent reaction for thrust while maintaining a predictable nature. While other materials can be used, we should also think about that cost of those materials. Aluminium is hands-down one of the single most abundant elements on the planet. It also happens to make a decent rocket fuel. Think about it like this. You would be paying 90% more money for a 10% (or less) increase in propulsion/efficiency. The cost-benefit ratio tips towards the cheaper material.

[falofonos]

Aluminium reacts enthusiastically and exothermically when given the chance. It has a low mass, it's relatively stable and it's cheap.

In explosives, it is mainly there to oxidise and produce heat which accelerates the chemical reaction of the explosive.

Thermite wouldn't work very well without aluminium, as it is just a mixture of powdered aluminium and another metal oxide (usually iron), once ignited the aluminium steals the oxygen atoms from the other metal and gives off heat. So much heat that the mixture melts, providing more opportunities for aluminium to strip oxygen from unreacted metal oxide in the mix.

[philly_idle]

Also, you need light elements (more precisely a low molecular mass) to get a high exhaust velocity.

. Skip to 01:30 where he starts talking about I_sp and how it's related to temperature and mass.

[Brotoro]

Aluminum is also useful in the solid propellant because it helps reduce combustion instabilities. Before the aluminum burns, it's in the form of tiny droplets of molten metal, and these droplets can absorb acoustic vibrations in the combustion chamber and dampen the instabilities these cause. I don't know if uranium would act the same way.