How does one make a resistojet rocket?

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'allo!

I am currently a 10th grader trying to make a resistojet for a science project in an attempt to find out which heating filament geometry makes the resistojet most efficient (if changing the shape changes performance at all). A resistojet is a mode of propulsion that uses resistive heating (thus, the resisto- part) to heat up a filament that heats up a gas flow to generate thrust.

What materials are needed and what safety precautions do I need to remember? So far, I'm thinking of using aluminium as the material for the structure and using water as the propellant. No idea what to use for the heating filament. Probably a copper coil.

Thanks!
 
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berkeman

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'allo!

I am currently a 10th grader trying to make a resistojet for a science project. What materials are needed and what safety precautions do I need to remember?

Thanks!
Welcome to the PF. :smile:

We prefer that you show lots of effort here, especially on school projects. So please summarize for us what a Resistojet is and what makes it unique. I know I could spend a few minutes reading the link, but I'd prefer if you saved us all a lot of time and summarized the concept for us.

Also, since you have presumably read a lot about them, please tell us what you know about the materials and construction techniques that you are considering so far.

And good for you that you asked about safety precautions. Please tell us your initial thoughts on those as well. Thanks.
 
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already edited for easier reading (I hope!)

About the safety concerns, should I be concerned about stuff suddenly blowing up due to high voltages and currents? Should I ready a fire extinguisher? A bucket of water? Hell, should I even hide behind a wall when firing up the engine?
 

berkeman

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already edited for easier reading (I hope!)
But I don't want to have to read anything. It's your project, so you should be able to post pictures and diagrams and explain how the whole thing works...
 
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But I don't want to have to read anything. It's your project, so you should be able to post pictures and diagrams and explain how the whole thing works...
I agree. One learns much about a subject by explaining it to others.
 

berkeman

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Thread closed temporarily...

@yrjosmiel -- Start a Conversation with me to show what you are going to post next in this thread to show your efforts to answer this question on your own. You need to show your efforts as a student here. We do not help you cheat on your schoolwork...

Click on my avater to start that conversation.
 

berkeman

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Thread re-opened for the OP to post more information about their project.
 
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Baluncore

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These days, thermal inkjet printers use the technique.
https://en.wikipedia.org/wiki/Inkjet_printing#Thermal_DOD

A jet may be dangerous if you get anywhere near because it can inject material under the skin. That can result in a tattoo, or an embolism followed by stroke.

The failure of industrial hydraulic systems can produce invisible high pressure jets of oil. Some mechanics make the mistake of trying to find an invisible leak with their hand. Contact with the oil jet blocks blood flow and results in gangrene. https://en.wikipedia.org/wiki/Gangrene

Water jets are used to cut hard materials such as glass. Think traumatic amputation.

You need to be aware of the dangers of jets. Only then can you experiment safely.
 
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Since I can't edit the OP anymore after my absence, I'll just add the additional information and explanations here for those who want a tl;dce:

A resistojet is a method of propulsion used in spacecraft. It generates thrust by heating up the exhaust fluid (that is typically non-reactive). This heating is done by running a current through a resistor. Think of hair dryers, but much hotter and faster.
resistojet.jpg

Something like this.

What I'm planning to do in my experiments is changing the geometry of the heater. I'm thinking that different heater shapes will provide different levels of thrust. I am hypothesising that if the surface area of the heater is higher, given a constant heat power output by the heater, the thrust of the resistojet will increase.
 

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berkeman

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It generates thrust by heating up the exhaust fluid (that is typically non-reactive).
Does it heat it enough to generate a phase change? (like boiling water) How does just heating generate thrust? What are the typical fluids tht are used, and why are they chosen? :smile:
 
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The exhaust fluid is a gas. The typical propellants used are hydrazine, ammonia, hydrogen, and nitrogen. However, I have seen another project that used water. In this project, it does produce enough heat to generate a phase change. These propellants are used as they are fairly common and have a low molecular mass, which translates to higher exhaust velocities. The heating of the resistojet rocket is there to help increase the exhaust velocity of the gases. I suppose one can still generate thrust without the heating portion, but that would not be very efficient as it has a lower exhaust velocity, thus lower efficiency.
 

Baluncore

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The heating of the resistojet rocket is there to help increase the exhaust velocity of the gases. I suppose one can still generate thrust without the heating portion, but that would not be very efficient as it has a lower exhaust velocity, thus lower efficiency.
Without the electrical heater it would not be a resistojet.

If you oxidised a fuel near a resistive element the element would surely melt because you could not cool it.

Your propellant could be water but it takes a lot of energy to boil water, is there not another liquid that could be boiled with less energy, to produce a greater pressure, volume of exhaust or impulse?
 
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If it's the amount of energy required to boil the liquid propellant is the issue, then I think a liquid that is cheap, has a low specific heat capacity and has low heat of vaporisation would be ideal in this situation.

Acetic acid comes to mind first, as it has a lower specific heat capacity than water (2.043 J/g*K vs 4.18 J/g*K) and lower heat of vaporisation (23.7 KJ/mol vs 40 KJ/mol) but I have no idea how to obtain such in high concentrations and how much it would cost. Unless there is a cheap and easy way to get acetic acid from vinegar, this may not be an option. It also has a high molar mass, but this does not matter in my experiments much as the independent variable in my experiments would be the shape of the heating element. Poor performance is okay, as long as it is consistent.
 

Baluncore

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I don't think glacial acetic acid would be a good choice for the fluid. It boils at a higher temperature than water, is more expensive, and has a nasty habit of irritating partly digested experimenters at the end of their life's work. Document your own experiment, don't leave it to the Coroner.

You might think water is a safe inert propellent fluid, but if the water is under pressure and close to boiling, or your element has a high thermal mass, you can expect problems. https://en.wikipedia.org/wiki/Steam_explosion

It would be easy to accurately regulate the temperature of the feed water to something like say 70°C. That would be safe and would make the flash boiling process in the chamber more predictable.

I would expect to find it difficult to regulate the heater element temperature if fluid flow varied. The heating element could have a high thermal mass and be temperature regulated to say 200°C.

By using a fixed displacement pump, maybe a peristaltic pump with a silicon rubber tube, to regulate the water injection to the hot element, you would be safely in control of the critical experimental parameters.

Keep the experiment modular and small to reduce costs and construction time.
You may be building a prototype steam cleaner. https://en.wikipedia.org/wiki/Vapor_steam_cleaner
 
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By using a fixed displacement pump, maybe a peristaltic pump with a silicon rubber tube, to regulate the water injection to the hot element, you would be safely in control of the critical experimental parameters.
What made you specially mention peristaltic pumps? Also, would gear pumps be acceptable?
 

Baluncore

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Gear pumps are expensive when built accurately with materials that are chemically stable. A gear pump will have an unknown variable leakage back past the gear tooth tips. Gear pumps are better used for hydraulic oil pressure pumps or for bidirectional oil transfer pumps.

A peristaltic pump will deliver a continuous and predictable flow. The fluid flows through a chemically inert tube. PPs are available on eBay for less than $10. Take a look at;
https://www.ebay.com.au/itm/DC-Micro-Self-priming-Mute-Peristaltic-Liquid-Pump-for-Lab-Dosing-Analytical/232435413467?hash=item361e3af1db:m:mjJMaBja2BcXEMg7pmKV7fg

I expect you need to get the experiment going quickly and then show a good correlation between the numbers predicted by theory and the measured parameters, electrical energy and fluid flow, water temperature and the thrust developed. That is why you should keep it small, so you can run the prototype in a sink, with hot water on tap. Keeping it small will make it transportable and low cost.
 
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with hot water on tap.
Heh, that's not a thing here. Perhaps another heating coil in the tubes to pre-heat it before going in the chamber would be a decent substitute?
 
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I think the first problem you face is that resistojets are not really 'rockets'. The expected propulsive force from such project is pretty low.
Also, the expected 'power' is around a few hundred watts at most.

By my humble opinion the starting point should be to think up a way to accurately measure the thrust - or the increase in thrust when you switch on the coil.

To mix a phase change into this is a kind of overthinking, not related to the stated goal of the project.
You should keep focused.
 
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I think the first problem you face is that resistojets are not really 'rockets'.
Why so? Or are you saying that because it is technically not a "rocket" vehicle since we are just referring to the engine itself?

Also, I've been thinking of measuring the exhaust velocity of the jets with some anemometers. The max rotational speed of the anemometer during the testing should be proportional to the exhaust velocity. From the exhaust velocity, with a given mass flow rate, I can calculate the thrust.
 
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I'm saying that it won't produce anything what's expected from a rocket so it's a mistake to imagine it as a rocket.
It won't lift anything and you should be proud if you can make it so that it produces easily measurable thrust (or: increase of thrust).
 
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won't produce anything what's expected from a rocket
And what level of thrust should be expected from rockets, exactly? A resistojet rocket's typical thrust range is around 10-2 to 10 N.
 
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And what level of thrust should be expected from rockets, exactly? A resistojet rocket's typical thrust range is around 10-2 to 10 N.
That's a very good starting question for this project of yours.

To put it in some context: based on the stats of the first stage of Saturn V, 166GW power could produce 35100kN thrust.
So, with a pack of sophisticated engines it's around 0.2N for 1kW. (If I did the google and the math right, of course.)

I think you should keep this in the range of hair dryers. That would mean thrust (or thrust increase) below 0.2N.
 
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That's a very good starting question for this project of yours.

To put it in some context: based on the stats of the first stage of Saturn V, 166GW power could produce 35100kN thrust.
So, with a pack of sophisticated engines it's around 0.2N for 1kW. (If I did the google and the math right, of course.)

I think you should keep this in the range of hair dryers. That would mean thrust (or thrust increase) below 0.2N.
I have read about a resistojet that produces around 0.2N of thrust for 500 W:
xy5RoAt.png

source
Also, is it possible that I would need cooling for the nozzle itself or are the temperatures low enough to not be a problem? Does the heating element ever need cooling in any kind of resistojet?
a
Also, how would I design the optimal nozzle for the resistojet?
 

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Why so? Or are you saying that because it is technically not a "rocket" vehicle since we are just referring to the engine itself?

Also, I've been thinking of measuring the exhaust velocity of the jets with some anemometers. The max rotational speed of the anemometer during the testing should be proportional to the exhaust velocity. From the exhaust velocity, with a given mass flow rate, I can calculate the thrust.
As for specific heat, https://en.wikipedia.org/wiki/Heat_capacity#Specific_heat_capacity

From that chart it looks like the best one to use is helium, clocks in about 1/4 that of water, is lighter and therefore would give more thrust pound for pound. Or Kilogram for kilogram if you wish:)
 

Tom.G

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Also, how would I design the optimal nozzle for the resistojet?
The simple requirement is that the nozzle ends at the point that the exhaust gases reach ambient pressure. The specific shape is beyond my knowledge, but a Google search for 'rocket nozzle' finds over a million results! Many of those are for 'design' or for a 'calculator'.

Sounds like a fun project, please keep us updated on progress and results.

Cheers,
Tom

EDIT: No need for additional cooling of the heating element, you control the temperature with the applied power and the fuel flow rate.
 

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