General Discussion => Advanced Concepts => Topic started by: qraal on 01/30/2022 10:38 am

Title: Propellant Mix on Titan
Post by: qraal on 01/30/2022 10:38 am
I first learned this one from "Ignition" - Irvin Glassman proposed a rocket engine that uses acetylene and hydrogen, getting surprisingly high performance for an almost totally oxidiser free reaction:

Method of producing thrust by hydrogenation of an acetylenic hydrocarbon (

Of course acetylene is a nasty, nasty monopropellant by itself. It tends to go "bang!" at the worst time. Except it can be made to behave. Of relevance is this study:

Inhibition of Spontaneous Decomposition of Acetylene by Hydrocarbon and Hydrogen (

One of the authors kindly provided the paper. Of interest is that adding butane or hydrogen will inhibit acetylene's self-decomposition.

On Titan acetylene and butane are formed from the abundant methane in the atmosphere. The same photolytic reactions that make acetylene also make free hydrogen, so there's condensed acetylene and butane on the surface, as evidenced by spectrographic data. Additionally acetylene is predicted to form a co-crystal with butane and together the two will be concentrated by methane/ethane flows, probably forming aggregations around the lakes, temporary or permanent.

So given the materials, I'm wondering if there's decent software to work out the behaviour of the propellants at the expected equilibrium temperature and pressure of a reaction chamber? Realistic gas behaviour is tricky and I've learned enough about the various models for realistic gases to know when I need some software.
Title: Re: Propellant Mix on Titan
Post by: stefan r on 02/02/2022 11:42 pm
On Titan there is no afterburner.  Where does the hydrogen come from?  The atmosphere has less then 0.1% hydrogen. Are you distilling it or splitting water?  There might be a chunk of acetylene that you can paddle your kayak to.  That will have a bunch of other hydrocarbons contaminating it.  Is this compressed hydrogen or liquid?

On Titan you have a lot of competition.  The air is extremely dense, so hot air balloon/dirigible is easy.  Thick air is easy to propel with a propeller.  The drag is higher which limits the range and effectiveness of rockets.  The Titan mission needs a nuclear power source.  Nuclear turbofan engines are sexy.  Liquid oxygen and liquid methane are not cryogenic on Titan, outside air is typically 97K.  The wet part of the lake is mostly methane.  What is wrong with using a raptor engine?  How did you get to Titan if you do not have Raptor engines (or something better) sitting around?  If it is a small scout vehicle why not use a battery quadrotor drone?  You can buy drones for $80 and they weigh a fraction of compressor.

Many asteroids and dwarf planets have accumulated tholins.  In a vacuum rockets have capabilities that cannot be achieved with rotors or balloons.  Maybe useful on Enceladus or Oberon. 
Title: Re: Propellant Mix on Titan
Post by: qraal on 04/07/2022 06:51 am
Hi Stefan R

On Titan it's 0.1% H2, not *lower*. It's a level at which H2 is regularly concentrated from gas streams with similar levels on Earth. Cracking it from water negates the benefit of it being in a mild disequilibrium in gaseous form.

The acetylene-butane co-crystal forms preferentially and will exist in concentrated form around the lakes, at least according to the laboratory research so far. Of course we want to sample those concentrates and a whole load of other features around the lakes etc.

Since my focus is on a sample return, that's what I'm comparing it against. A NIAC proposal by Geoffrey Landis and colleague focused on LNG/LOX as the mix, since there's 5% CH4 in Titan's troposphere. The problem with that - which a non-oxidising mix of C2H2/C4H10/H2 avoids - is making the LOX from cryogenic H2O ice. That's really energy intensive, even if storing LOX and LNG is really easy on Titan at the ambient temperature. Happily the latest electrolyser designs are 95% efficient, but the energy cost is 16 MJ per kg of O2 produced.