Endothermic / Exothermic Propellant?

[prothwell]

I am interested in producing a solid propellant rocket that is heat-neutral.  What endothermic chemical reaction might produce a high volume of gas when used in conjunction with (consuming heat from) an exothermic propellant?  Could such an endothermic reaction be triggered by the heat of the burning exothermic propellant?  (avenues of inquiry welcomed)

[scooke]

Dear Philip,

We do often use the energy released from one reaction to provide the energy needed to sustain another one.  However, in this case you run into two realistic problems.  The first one is that I have not found any examples of an endothermic gas producing reaction.  They are all exothermic.  The second is that thermodynamically, entropy will increase - usually by heat losses as well as the gas production - and that means that in order to "sustain" the desired endothermic reaction (IF one could be found) would require continuing the exothermic reaction.  Getting past the activation energy is just the first part.  An endothermic reaction requires continuous heat input to continue reacting.  Thus, a "heat neutral" propellant defined at the point of reaction does not look possible.

As a side note, the "ion drive" concept is one that does not use released heat energy as a direct driving force.  It may be recycled in the power generation system used to propel the ion beam, but heat exhaust from the propellant is not a driver.

Best regards,

Steven Cooke

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[prothwell]

Steven, thank you very much for your reply!

I was thinking the solution to this problem might be as simple as finding a material that will vaporize at (relatively) low temperatures.  If nitrocellulose were used as the exothermic propellant, this evaporation would have to happen below 3,300 degrees Fahrenheit.  I would imagine tables have been published somewhere online for this sort of thing, but I've not been able to find them.

As for the issue of sustaining the exothermic reaction, I imagine the propellant charge could contain a cylindrical core of exothermic propellant surrounded by the evaporating material.  I suspect that this core could burn and be consumed faster than the surrounding material (while maintaining its ignition temperature), and that the remaining surrounding material downstream could continue to evaporate, consuming heat and lowering exhaust temps below ignition temps.  In the case of nitrocellulose, ignition temperature is around 330 deg. F, so even if this temperature can't be broached, it still represents a 90% reduction in exhaust temps.

Does any of this sound feasible, and how might I conduct research?

I really appreciate your help,

--Philip

[scooke]

Dear Philip,

There was a very interesting article on "endothermic fuels" in Chemical and Engineering News 30 April 2018 edition.  However, the gist of the article is that while the fuel system may be utilized in cooling, the propulsion itself is exothermic.  You need to play around with the Gas Laws a bit to see the flaw in your proposal.  Recall that ultimately we are always looking at some net ENERGY required to do WORK.  Creating a gas volume (or pressure) by an exothermic reaction, just to cool it down by expansion is what already happens with a rocket.  BUT, if you mean to cool the exhaust BEFORE leaving the rocket it will already have lost the energy (in terms of pressure, volume and temperature) that would be required to provide any propulsion force.

In short, it is a "perpetual motion" machine disallowed by the Laws of Thermodynamics.  In terms of your suggestions, yes, a  simple propulsion rocket can be made entirely from compressed gases!  That is in fact what is used in space for personal EVA movement.  Similarly, solid carbon dioxide flash-vaporized by an electric trigger makes an excellent explosive!  But I think that you are considering a larger, longer-duration and more powerful type of engine.  Look at the energy equations and usage, and then the mass balance for the system.  The propulsion force is the ultimate energy benchmark.  From that, you can calculate how that energy can (or cannot) be obtained from energy sources.  ANY energy removed from the source for any reason before it becomes the propellant reduces the available output energy.

Best regards,

Steven