Friday, May 27, 2016

Rocket Post 13 - We are not alone! This exists and its name is "Resistojet".

What a difference a keyword makes!  I was wondering if it was a new idea to model a nuclear thermal rocket in miniature with an electric heater coil? What I named "SteamFlash Rocket" has a history as "Water Resistojet" going back to 1906. It is normal for it to provide a very low thrust. NASA experimented with it for many years as a possible small control thruster for satellites and spacecraft. Resistojets have been successful but with working fluids other than water. The one flying "Water Resistojet" I can find was on the experimental no-budget "STRaND-1" satellite where the working fluid was water with alcohol antifreeze added.

"STRaND-1" has a lot of "hitechfromlotech" in it. Including another idea we are interested in which is to use a cell phone to provide computing power for robotics.

Why did we miss this at first?
Excuses excuses! The word "Resistojet" does not appear in the Zuppero et al article[1]. The article does have the related term "Thermal Rocket" but that did not stand out for me because I thought all rockets were "Thermal" except the odd ion or laser experiment. Turns out "Thermal Rocket" means heat not coming from combustion - IMO not clear to newbies. The word "Resistojet" does not appear in the Wikipedia article "Steam Rocket". I am a Wikipedia member so adding it is on my TODO list. I did ask around model rocket enthusiasts about what I was doing and none of them had ever heard of it. One of my helpers said "they are just into whizz-bangs so they will not get this". The audience for this blog and related posts is about 300 and none of you said anything. It was only after getting a very low thrust result that I headed into Google for a long search session and eventually teased out "resistojet".

So is it a good thing to build and test one?
YES! Satisfying to discover that working from isolated innocence, our "resistojet" is remarkably close to the NASA experiments. Similar physical size, coil resistance, voltage and current used. I kept it simple so it has been quicker and more interesting to build and run this than to spend hours passively reading papers on the subject. We appear to have done something new in building a transparent plastic resistojet and videoing what is happening inside. We may have some useful findings coming out of that. Sigh of relief that I did not buy the expensive lithium battery and its expensive charger, safety and support gear. Going through static tests run by utility batteries was a good move. The biggest effort was the thrust measuring mount which is going to be useful for tests of other flight propulsion options like ducted fans.

Where are we at with STEM inspiration for High School students?
(or should I say "inspire STEM with Steam"?!)
Very unlikely that this can lift off a rocket from the ground. The only NASA proposal that is even close is Zuppero et al proposing lift off from The Moon with the advantages of low gravity and operating in a vacuum. Powering a model aircraft may be possible, otherwise this is a static test activity. Makers can go for an efficiency challenge - how many watts to produce 1N of thrust. Smaller is better, About 10000 so far for our Resistojet. About 2000 for equivalent NASA experiments [2]. To fly a model rocket needs about 200 so that design would need to be 10 times better than NASA's research results or taking a very different approach.

The fact that "resistojet" has a story and some interesting practical applications could be a plus for the target audience.

Where to from here?
Look again at the Robert Truax Steam Rocket concept, This works on a different principle of storing energy in superheated water under pressure. This beast gets us concerned about safety for the young target audience but a very small capacity unit may be safe using the same safety rules as educators use with solid fuel rockets. Refs:
Robert Truax – Patent US 3029704 A – Steam powered rocket and launcher therefor. 1959.
Good ideas and information here some of which could apply to a Resistojet.

AQUARIUS Hot Water Rockets. German Student Project. Similar to Truax 1959 with use of electric pre-heater.

Gets me thinking - Hybrid? Truax concept superheated water tank provides feed pressure into a Resistojet chamber where the resistance heater adds some extra energy to the wet steam just before it exits the nozzle.

Robert Truax and the NASA papers emphasise that nozzle design can make a big difference. The expansion bell, even as a simple cone, receives a lot of reaction force from the exiting wet steam. Our Resistojet currently has a 1mm exit hole. NASA's equivalents range from 0.15mm to 0.3mm which is going to build more pressure and give faster exit velocities. Greater temperature and pressure will demand a stronger and more temperature-resistant flash boiler.

An interesting option is the "Super Capacitor" for electrical energy storage. This stores less energy than the same weight lithium battery but it can release that energy in a much more rapid burst which means more power for a short time which is what a rocket or aircraft needs. The "Super Capacitor" also looks like a safer and lower cost option than lithium. A set of capacitors would have a similar cost to a lithium battery but would not need the special charger and safety electronic controllers. The capacitors do have a disadvantage in that the voltage drops as the current flows which may need something like a coil in sections that run in series to start with then switch to parallel during the run.

Where to from here also means looking away from a rocket as an attractive robotics application and finding other robotic activities to try with cell phone as robot brain.

Footnotes: Rocket Post 13 - We are not alone! This exists and its name is "Resistojet"

[1] Zuppero, A. Schnitzler, G. Larson, T. (1998). Nuclear-Heated Steam Rocket Using Lunar Ice.

[2] Result of 0.17 N from 330 W input:
Manzella D., Carney L. (1989) Investigation of a Liquid-Fed Water Resistojet Plume [NASA Technical Memorandum 102310 AIAA-89-2840] Retrieved from: