The best kind of fuel for model rockets?

[BadRocketsCo.]

So lately I have been thinking on building a model rocket. I wanted to ask you guys, what kind of fuel would be best for it? I have heard, that Saltpeter+sugar is a thing, but is there anything better?

[szputnyik]

Gasoline or gun powder.

[lajoswinkler]

Define "the best", and then we can talk seriously.

[Armchair Rocket Scientist]

So lately I have been thinking on building a model rocket. I wanted to ask you guys, what kind of fuel would be best for it? I have heard, that Saltpeter+sugar is a thing, but is there anything better?

If you mean you're building your own motor, sugar/KNO3 (commonly called Rocket Candy or R-Candy) is probably a good bet. If you're using a commercial motor, in the US black powder is commonly available up to "E" class and Ammonium Perchlorate Composite Propellant is used for anything larger.

I notice your profile says you're in Estonia. The first thing you need to do is look up all the relevant legal information. Among other things:

The sale and construction of motors and propellant may be restricted by fire protection agencies and whoever regulates firearms and explosives (APCP isn't actually an explosive, but here in the US it took a decade-long legal battle between our rocketry associations and the BATF to stop it being treated as one by the government. In other countries you may need explosive licenses to buy or store AP or AP motors).

Black powder isn't a very good propellant for making your own motors, but most rockets use it to eject their parachutes. It is a low explosive and its sale and storage may be regulated.

The sale of some GPS units capable of registering very high speeds and/or altitudes may be restricted on the basis that someone could use them to build a guided missile. However, if you were ready to build a rocket that would need that kind of equipment you would presumably already have the equivalent of a level 3 high power certification and not be here asking this question.

For larger and more powerful rockets whatever agency regulates air traffic (equivalent to the FAA in the US) will probably make you get permission for a launch, and not let you launch too close to an airport at all.

Finally, many countries have their own non-government organization that regulates the hobby. Among other things, these organizations provide liability insurance at official launches, handle a lot of the paperwork (e.g. by getting a blanket airspace waiver the entire day of the launch), and are responsible for high-power certification.

Also, while there are several rocketeers in the KSP community, you'd be much better off asking an actual amateur rocketry forum.

[QuesoExplosivo]

^Ninja'd

Sucrose and potassium nitrate is a viable rocket propellant. People also use sorbitol instead of sucrose for its lower melting point (so it's easier to cast). Ammonium perchlorate+aluminum+binder is used in high-power rockets.

[Beowolf]

Antimatter would be best, assuming you can acquire and contain it.

[lajoswinkler]

Antimatter would be best, assuming you can acquire and contain it.

Rocket that spits out gamma rays isn't exactly going anywhere, but anyone close to it will soon get to his funeral...

[SciMan]

I'm sure that at least a few hybrid rocket motors have flown on the bigger model rockets. The fuel combination is usually something like paraffin wax + nitrogen dioxide (laughing gas).

I know that sources exist for APCP motors in the same size as typical A-G rated BP motors, as I've held one in my hand before (high school model rocketry club headed by physics teacher)

They're lighter than a BP motor for the same specs, but they still use a black powder chute deploy charge just like the standard BP rocket motors do.

Also, they usually need a bigger electric match than the one used with BP rockets. It's still an electric match, but it spits out a bigger flame, is physically larger, and takes more current to fire. A car battery is more than good enough to fire off both types of electric match, so that shouldn't be a problem.

[Armchair Rocket Scientist]

I'm sure that at least a few hybrid rocket motors have flown on the bigger model rockets. The fuel combination is usually something like paraffin wax + nitrogen dioxide (laughing gas).

AFAIK, hybrids are considerable more difficult to work with than solid-fuel motors. Also, the smallest are still big enough that OP would almost certainly require some sort of certification.

I know that sources exist for APCP motors in the same size as typical A-G rated BP motors, as I've held one in my hand before (high school model rocketry club headed by physics teacher)

Yep. The smallest I know of that are currently on the market are Aerotech's 18mm Ds, which are about the size of a black powder C. There used to be smaller ones, but they seem to no longer be in production. This is most likely because composite motors are more expensive than BPs of the same size, and it's really hard to fit the igniter into a very small composite motor.

Back in the really old days (70s) I think there were some F size and larger black powder rockets in use. These are out of production for a different reason: the hard-packed black powder used in model rocket motors is very brittle compared to AP, and in larger motors the propellant grain is vulnerable to cracking during handling. A cracked propellant grain means more surface area, which means a much faster burn rate, which means a case overpressure and catastrophic motor failure (CATO) which usually means a damaged or destroyed rocket.

They're lighter than a BP motor for the same specs, but they still use a black powder chute deploy charge just like the standard BP rocket motors do.

Yep. Black powder motors only get an ISP of 80-100 s, while good AP motors tend to get around 200 s.

[BadRocketsCo.]

Thank you everyone! Which goes to the rocket fuel, for my first model I will probably use KNO3+sorbito+Fe2O3. Which goes to making the fuel, is it even safe to put KNO3 into boiling water with the rest of the stuff? (yes, I know, the boiling method is a whole hour long, but sounds like the safest and most fool-proof.)

[BadRocketsCo.]

Define "the best", and then we can talk seriously.

"Best" as if in "a first time model rocket builder can make it and it is efficient"

Edited July 16, 2014 by BadRocketsCo.

[BadRocketsCo.]

Antimatter would be best, assuming you can acquire and contain it.

Hey, if I had the budget of dozens of countries and builders/scientists of the whole world, it wouldn't be such a bad idea!

[Streetwind]

Thank you everyone! Which goes to the rocket fuel, for my first model I will probably use KNO3+sorbito+Fe2O3. Which goes to making the fuel, is it even safe to put KNO3 into boiling water with the rest of the stuff? (yes, I know, the boiling method is a whole hour long, but sounds like the safest and most fool-proof.)

You don't put it into boiling water. You put it into cold water and then heat it up carefully and evenly. Regardless of whether or not there are actual serious risks, this is a chemistry best practice that anyone working with reactive substances should observe.

(Also, safety goggles and gloves and something more sturdy than a t-shirt.)

[PlonioFludrasco]

What about ammonium nitrate+sugar? Is it a viable option?

[Roastduck]

heat it up carefully and evenly

I'm assuming a pot on a stove wouldn't be careful or even enough. What's the best method of doing it?

[Streetwind]

No, that's actually perfectly fine. Modern cookware spreads heat very well, and stoves (especially electric ones) have very even heating areas. It's more important to not crank things up recklessly or forget to stir. Or, you know, accidentally dump the mix into an open flame.

You'll probably want a pan though, not a pot. Lots of surface area lets water evaporate quickly and avoids having overly thick layers of material (which can cause hotspots). It's also easier to monitor and to scrape your finished fuel out.

. It's not really meant to be fully instructional (get yourself an actual recipe and guide!), but it shows roughly what to expect. He's also doing it without water in one of several examples (apparently the outcome is slightly different than if using water). Edited July 16, 2014 by Streetwind

[Roastduck]

Okay, follow up question: Once you have a fuel mixed and ready to mold (I'm assuming in a cardboard tube, based on the model engines I've seen before), what do you use to mold a nozzle into the fuel, if at all?

[Moss]

This reminds me of when I was thirteen and decided to make thermite for some theatre special effects...

[Roastduck]

This reminds me of when I was thirteen and decided to make thermite for some theatre special effects...

oh my...

10char

[BadRocketsCo.]

This reminds me of when I was thirteen and decided to make thermite for some theatre special effects...

I can imagine your parents weren't very happy when they found out

[lajoswinkler]

"Best" as if in "a first time model rocket builder can make it and it is efficient"

Then sugar (sucrose), potassium nitrate, a pinch of iron(III) oxide and maybe a spoonful of honey or something similar, sugary and gooey like corn syrup. We don't have corn syrup around here so I used honey.

It can be a VERY powerful and very safe propellant, but its quality depends not only on the contents, but also on the timing, stirring, curing, all to avoid drying, cracks, etc.... It's a craft.

Just don't melt the stuff. Not only the contents don't make a homogeneous solution, but hotspots can turn your fun into a premature engine test while still in pot.

Use water. It takes longer, but the quality of the material is superb. It is hygroscopic, though, so the engines need air tight storage immediately after you make them.

What you want is something with the consistency of warm plasticine when loading and ramming it into the engine chamber, which cures as it cools down, forming somewhat brittle material.

What about ammonium nitrate+sugar? Is it a viable option?

No. The reaction rate is pathetic, the mixture is deliquescent and mostly produces just smoke. On longer storage, side reactions occur. Sugar degradation, ammonia release, cracks.

[BadRocketsCo.]

And yeah, a good question is, what is a good material for the nozzle?

[lajoswinkler]

And yeah, a good question is, what is a good material for the nozzle?

Gypsum is ok, but it's for single use. In rare occasions, two uses.

Concrete is tougher, but takes longer to cure. Steel requires a lathe and is heavy, so that's for serious, large rockets.

So fat gypsum is my favorite. If properly made (it can be reinforced using nails and wire) and dried, it's tough enough, it's lightweight and it can be machined using standard available borers. You can even make a crude de Laval nozzle.

Some people use bentonite clay and ram it with a hammer, but they get a porous plug that easily chips away and will probably be pooped out as schrapnels when a more serious engine starts working.

[Idobox]

What about aluminium? it's a soft and light metal that can be worked with tools made for wood (that will damage the tools though), and the melting point is over 600°C.

Copper is more expensive and heavy, but it has a much higher melting point, and is still relatively easy to machine.

[lajoswinkler]

What about aluminium? it's a soft and light metal that can be worked with tools made for wood (that will damage the tools though), and the melting point is over 600°C.

Copper is more expensive and heavy, but it has a much higher melting point, and is still relatively easy to machine.

Aluminium will react with moist nitrates over the time, especially with ammonium nitrate. That can even be dangerous if the metal is in a granular or powdery form.

For nozzles, you can expect corrosion upon longer storage (weeks, months).

If you can devise a system which screws the nozzles onto the engines before the launch, it will probably work fine. Expect destruction of the nozzle aperture after few launches.

Copper is heavy and might be corroded by such mixtures, too.

Unless you're planning on going towards serious heights and engine powers, dry and recinforced gypsum nozzles are perfectly fine and allow kilometres of flight.