Common Fuels for Combustion Spudguns

So, you've glued together some hunks of PVC, installed an ignition system and bought a bag of russets. What should you use for fuel to get the most oomph into that hunk of starch? There are many ways to judge the best fuel for a combustion spud gun. How easy is it to use? How expensive is it? And, perhaps most important to the average spudder, which fuel will launch the spud at the highest speed?

You Must Use The Correct Amount Of Fuel

Before discussing the various fuel options we'll start with the most important aspect of fueling a combustion spudgun.

You Must Use The Correct Amount Of Fuel.

An inexperienced spud gunner might think that adding more fuel to the chamber will increase the power of their gun. This is almost always incorrect. For all common fuels too much fuel won't ignite. All fuels have a property called the "combustion limits". The combustion limits is the range of fuel concentration (usually expressed as the volume percent of fuel in the chamber) that will actually ignite. For most hydrocarbon fuels (propane, butane, flammable aerosols etc.) the combustion limits is about 3% to 9% by volume. If you inject less than ~3%, or more than ~9% of the chamber volume in fuel then the gun will not ignite.

If you've built a combustion spudgun and you can't get it to fire the most likely problem is that you are using too much fuel.

So, your first challenge is to get your fuel in the 3~9% range so that the gun will fire. Your second challenge is to try to get the fuel as close as possible to the stoichiometric fuel ratio. The stoichiometric fuel ratio is the ratio of fuel to oxygen that allows all the fuel to burn and leaves no oxygen left over at the end of combustion. The stoichiometric fuel ratio will get you very close to the maximum power the fuel is capable of.

The stoichiometry is determined by the fuel you are using. For example, the balanced chemical equation for the complete combustion of propane (C₃H₈) in air (~21% oxygen, O₂) is;

C3H8 + 5O2 = 3CO2 + 4H2O

For every molecule of propane we need 5 molecules of oxygen. Combustion produces 3 molecules of carbon dioxide (CO₂) and 4 molecules of water. Gases have the handy property that the number of molecules is proportional to the volume at a constant pressure so the volume ratio of fuel to air will be the same as the ratio of molecules. Since air is only ~21% oxygen we can calculate that for every unit of volume of the chamber we need (1C₃H₈/5O₂)*(0.21) = 0.042 volumes of fuel. 0.042 volumes of fuel is the same as 4.2% of fuel. (There are some subtleties about calculating the actual volume percent that'll be discussed below.)

Common Fuels for Combustion Spud Guns

Heats of Combustion

The "heat of combustion" of a fuel is a measure of the amount of energy released when the fuel is burned. This is an important, but not the only, factor affecting the performance of a fuel. The table below lists the heats of combustion along with other parameters for a variety of pure fuels.

Heats of combustion can be measured in several different ways and this makes it difficult to find a consistent set of values for various fuels. Web based sources list both "high heat" values (which are obtained assuming the water produced condenses to liquid) and "low heat" values (which assume the water is present as steam) as well as thermodynamic heats of combustion. For this reason, the table below lists multiple values for most of the fuels. To obtain a consistent set of values for comparison purposes, I have used the red ones which were calculated using the method at [1].

The "Displasive Volume Percent" column gives the stoichiometric volume of fuel required if the fuel displaces some of the air in the chamber when it is injected, for example when fueling with the "squirt and screw" method or using a syringe. The "Additive Volume Percent" gives the amount of fuel required when the fuel does not displace air from the chamber, for example when using a pressurized meter system.

The key value for comparing two fuels based on their heats of combustion is not the actual heats of combustion. Instead, the "Heat per mole Oxygen" should be used since the amount of energy in the combustion chamber is limited by the amount of oxygen present in the chamber. Fuel is added to match that amount of oxygen. As you can see from the table, there is relatively little difference between the various fuels based on their "Heat per mole Oxygen" values. The only two fuels with significantly higher values are hydrogen and acetylene, both of which give 10~15% more energy than the other fuels.

Fuel	Heat of Combust. (Kcal/mole)	Combustion Limits in Air (Vol%)	Boiling Point,C (F)	Mole O2 per mole Fuel	Heat per mole Oxygen (Kcal)	Displasive Volume Percent(1)	Additive Volume Percent(1)	Molecular Weight (g/mol)	Combustion Equation
Hydrogen	68.3 62.5 59.7	4.0 - 74.2	-253 (-423)	0.5	119	29.5	41.9	2	2H2 + O2 = 2H2O
Methane	192 191 212 215	5 - 15	-162 (-259)	2	108	9.48	10.5	15	CH4 + 2O2 = CO2 + 2H2O
Ethane	373 340 371	3 - 12.5	-89 (-184)	3.5	106	5.65	5.99	30	2C2H6 + 7O2 = 4CO2 + 6H2O
Ethylene (Ethene)	315 337 337	2.8 - 28.6	-103.7 (-155)	3	112	6.53	6.98	28	C2H4 + 3O2 = 2CO2 + 2H2O
Acetylene	322 301	2.5 - 80	-81 (-119)	2.5	120	7.73	8.38	26	2C2H2 + 5O2 = 4CO2 + 2H2O
Propane	525 487 530 527	2.37 - 9.5	-42.1 (-44)	5	105	4.02	4.19	44	C3H8 + 5O2 = 3CO2 + 4H2O
Butane	684 633 687 683	1.86 - 8.41	-0.5 (31)	6.5	105	3.12	3.22	58	2C4H10 + 13O2 = 8CO2 + 10H2O
iso- Butane	683	1.86 - 8.41	-11.7 (11)	6.5	105	3.12	3.22	58	2C4H10 + 13O2 = 8CO2 + 10H2O
MAPP(2)
Methyl Acetylene			-23.2 (-9.0)	4		4.98	5.24	40	C3H4 + 4O2 = 3CO2 + 2H2O
Propadiene	457		-34.5 (-30)	4	114	4.98	5.24	40	C3H4 + 4O2 = 3CO2 + 2H2O
Propylene(3) (propene, MAP/Pro)	491	?	-47.6 (-54)	4.5	109	4.7	4.9	42	2C3H6 + 9O2 = 6CO2 + 6H2O
Diethyl Ether	647	1.85 - 36.5	34.5 (94)	6	108	3.37	3.49	74	C4H10O + 6O2 = 4CO2 + 5H2O
Dimethyl Ether	329	3.0 - 18.6	-25 (-13)	3	110	6.53	6.98	46	C2H6O + 3O2 = 2CO2 + 3H2O

Table Notes:

1. Additive and displasive volumes based on air containing 20.95% (volume) oxygen.
2. MAPP gas is a proprietary mixture of various hydrocarbons, principally methyl acetylene, propadiene and propane.
3. MAP/Pro gas is a proprietary mixture of propylene and 0.5% propane.

References:
http://members.nuvox.net/F~on.jwclymer/Frq/Fhoctable.html
http://home.fuse.net/clymer/rq combustion calculator
http://www.nrc.gov/freading-rm/fdoc-collections/fnuregs/fstaff/fsr1805/ffinal-report/f15_explosion_calculations.xls

"Traditional" Fuels

RightGuard

RightGuard, one of the traditional spudgun fuels, has undergone various changes in composition over the years. Some types of RightGuard are still usable as a spudgun fuel. If the label has a warning about flammability and/or the ingredient lists includes things like butane, isobutane, or propane then it is useable as a fuel. From the Heats of Combustion table above you can see that there is relatively little difference between these three ingredients. Even the optimum volume percents are fairly close to each other. Since RightGuard is generally used in the "squirt and screw" fueling mode, the actual percent of fuel is difficult to control. It seems likely that the other ingredients in the can make up a fairly low percentage and probably have a relatively minor affect on the energy of the fuel.

Aquanet

Aquanet, and similar hair sprays, have many of the same characteristics as RightGuard. If the label warns that it is flammable, and if the ingredient includes things like butane, isobutane, or propane then it is probably usable. As with RightGuard, the other ingredients in the can, which may or may not be flammable, probably constitute a fairly low percentage of the gas. There is the possibility that the other ingredients may gum up the spark gap, fan or cleanout plug threads after several uses.

Starter Fluid

By definition, automotive starter fluid is combustible. There are no doubt a couple of different formulations. One common formulation includes 10~30% (weight) dimethyl ether and a propane propellant. Most formulations should behave about the same as pure propane.

Other Fuels

Pretty much any combustible compound that evaporates significantly at ~70F can be used as fuel in a combustion spudgun. Methanol, ethanol, isopropanol (rubbing alcohol), gasoline, acetone (nail polish remover), paint thinner etc. can all be used as fuel. The challenge with these liquid fuels is getting them to evaporate in the gun in a reproducible fashion. The actual energy content of this type of fuel is fairly irrelevant since the shot to shot reproducibility is so poor. It really doesn't matter, with this type of fuel, whether the predicted muzzle velocity is 10% great with one fuel versus another since the shot to shot variability is probably more like 50%. (Even with precisely metered propane, the shot to shot variability in muzzle velocity for shooting spuds is typically in the 10 to 20% range.)

One thing that you should keep in mind with liquid fuels is the potential for weakening the PVC. Take a look at the ingredients list on your cans of PVC primer and glue. Anything listed on those cans should probably be avoided as fuels. Acetone and tetrahydrofuran (THF) in particular are probably not the best idea for fuel since they soften PVC.

Other Factors

The heat of combustion is not the only factor affecting the power of a particular fuel. The rate at which the fuel burns and the maximum temperature and pressure obtained from the fuel also affects the performance of the gun.

Flame Speed and Power Law

Gaseous fuels burn at widely varying rates. For a combustion spudgun, the faster the fuel burns the better the gun will perform. Unfortunately, there is not a lot of information available on the burn rates of various fuels. The table below lists a few fuels of interests (values from http://www.ub.uib.no/elpub/2004/h/404003/Hovedoppgave.pdf). The flame front speed is how fast the flame moves through the mixture at ambient conditions. In an actual combustion spudgun the flame front accelerates as the temperature in the chamber rises. The flame front speed as a function of temperature and pressure can be estimated using;

Flame speed_i = (Flame speed₀)*(T_i/T₀)^alpha*(P_i/P₀)^beta

Where Flame speed₀, alpha and beta are the values shown in the table below and Flame speed_i is the speed at temperature T_i and pressure P_i.

Fuel	Flame Front Speed, meters/sec (FPS)	Maximum Explosion Pressure (ATM)	Adiabatic Flame Temperature, C (F)	alpha	beta
Methane	0.45	8.6	2591 (4696)	2.0	-1.5
Propane	0.43	9.2	2633 (4771)	2.13	-0.17
Hydrogen	3.25	7.9	2755 (4991)	1.26	0.26
Acetylene	1.55	9.6	2918 (5284)	2.0	-0.06

As the table above shows, simple hydrocarbons such as methane and propane, behave similarly, with relatively slow flame front speeds and similar peak pressures and temperatures. Hydrogen and acetylene are substantially different. These two fuels burn much faster, 3 to 8 times faster at standard conditions, than does propane.

Hydrogen and acetylene have another characteristic that differentiates them from fuels such as propane of butane. Under certain conditions, hydrogen and acetylene will detonate (explode) instead of deflagrate (burn). When a fuel detonates it releases all of its energy essentially instantaneously. The flame front speed in a detonation event is at hyper-mach speeds (mach 6 to 7), roughly 4,000 times faster than the laminar flame front speed (Mach ~0.001). Because of the very high burn rate in a detonation, the gun is subjected to a tremendous shock force. Most spudders believe that this level of stress is unsafe and that hydrogen and acetylene are unsafe fuels for a gun constructed from PVC.

Latke's Propane vs. MAPP Study

Burnt Latke did a detailed study ( [2]) comparing the muzzle velocities of propane and MAPP</a>. Using a 1.5"D riffled barrel shooting spuds, Latke found that MAPP out performed propane with muzzle velocities of 444 (+/-34) FPS for MAPP and 398 (+/-34) FPS for propane. So MAPP gave muzzle velocities that were about 12% faster than propane. This increase in performance is greater than what you would expect based solely on the "Heat per mole Oxygen" values for the two fuels. If the muzzle velocity scales as the square root of the ratio of the "Heat per mole Oxygen" values, then MAPP would be expected to give muzzle velocities about ~4% higher than propane. It is possible that the 12% increase Latke observed is not statistically different from the expected ~4% increase. Alternatively, MAPP may burn slightly faster than propane. Two of the components of MAPP, methyl acetylene and propadiene, would be expected to have burn speeds that are more similar to acetylene than propane.

So, What's the Best Fuel?

There really isn't a "best fuel" for a combustion spud gun. If you want to get the maximum muzzle velocity from a gun the difference between the various fuels is relatively small. A typical combustion spud gun has so much shot to shot variation in velocity that the relatively small difference between fuels is not particularly relevant. Latke ([3]), as good of a spudder as there is, has done several studies using an accurate chronometer to measure muzzle velocities. In all of his studies with spuds as projectiles, using precisely metered fuels, the shot to shot variability in muzzle velocity is typically in the 10 to 20% range. In studies where Latke used the same round for several shots the shot to shot variability is still in the 5% range. So it really doesn't matter very much if one fuel has 5% more energy than another fuel.

The "best fuel" is the one that the gunner is most comfortable with, that is easiest to obtain and use, and is cheap. Beyond that there is really no significant difference between fuels.

There are other ways to get the best performance out of a combustion spudgun. Here is my list, from most important to least important;

• Mix the fuel well using a chamber fan (like shown at [4]).
• Accurately measure the fuel ([5] describes a cheap way, another at [6] way, a way with much more "bling" is at [7]).
• Properly size the barrel to the chamber (or vice versa)
• Increase the number of sparks

Any of these four will give much greater increases in performance than will changing from, for example, propane to MAPP.

Originally posted at SpudTech.com on Fri Feb 09, 2007 --Jimmy101 13:53, 15 May 2007 (EDT)