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A few questions

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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A few questions

Unread postAuthor: Jeeperforlife » Fri Jan 25, 2008 12:35 pm

I am planning on building a hybrid late winter early spring once I sell one of my Jeeps. I have a few questions about basic design.

Is there a advantage of Mapp gas over propane.

I am trying to come up with a "cleaner" looking way for filling. Is it possible to fill the chamber to Xpsi of fuel then Xpsi air? I guess what I am asking is it possible to not use a meter but to measure it out directly in the chamber using a gage showing how many psi of fuel you have in there?

I have read about chamber to barrel ratios, what is ideal, what is accectible?

When I run pure Oxygen and Hydrogen there is not a way to measure in Xpowers like when using air. What is a good pressure to start with and where is way beyond safe? I will be using a water electosallis machine, I have built several.

If I WANTED to achieve DDT and the gun was built for it what would be a good way.
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Unread postAuthor: DYI » Fri Jan 25, 2008 1:03 pm

You ask some interesting questions, Jeeper. I can answer most of them quite well, but some of the answers will require a bit more info and possibly SB15's or Rag's help.

1. MAPP gas burns hotter than propane, and has a slightly higher maximum combustion pressure. Performance gains in normal combustions have been calculated at around 11% over propane.

2. This is possible. To give you a general idea, an 8x mix has ~5.15 of fuel pressure before air is added. A grade 1A or better, 0-10 psig gauge should be adequate for this purpose, unless you intend on going over 10x or so. The hideously expensive gauging required pushes most people to the fuel meter route.

3. At 1x (atmospheric), ~0.8:1 is ideal. Essentially, if you double the pre-ignition pressure, you can double the barrel length. So for an 8x mix, a 0.1:1 C:B would be fairly efficient (although not enough research has been done into the subject to know for sure). The higher pressures you can generate, the higher the acceleration forces you can achieve. This means that while ideal barrel length increases in direct relation to pressure generated, you can actually use shorter barrels, because of the burst disk valve incorporated into the hybrid design.

4. To answer this question of safety, I need to know your chamber design. Atmospheric pressure is always a good place to start. Apparently, in 3 bar oxy/hydrogen, DDT runup distance is about 12". Following the conclusion that DDT runup distance doesn't really decrease that dramatically in higher pressure mixes, you should be fine at any reasonable pre-ignition pressure you can generate, so long as no area of unburnt fuel more than 12" long is present during the combustion process. Think multiple, closely space spark gaps.

5.Using oxy/hydrogen is a good way to start. Acetylene has even shorter runup distances, but its instability at over 15psig makes it unsuitable for this purpose. Assuming you are using oxy/hydrogen at a few bar pre-ignition, 2 feet of runup distance should almost guarantee DDT. Schelkin spirals and other obstructions in the chamber, and creating turbulence before and during ignition can also help your cause.

Just to clarify, DDT of a large amount of unburnt fuel in a steel or copper chamber can and will cause extreme fatigue, and likely failure if it is repeated. You probably don't need to be told that PVC is obviously out. Chambers for hypothetical detonation guns have ranged in material choice from polycarbonate and ABS, to brass, aluminum, or lead.
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Unread postAuthor: Fnord » Fri Jan 25, 2008 1:09 pm

Edit: beaten. Oh well DYI probably answered it better than me.


1.Mapp burns hotter than propane, which affects a number of things. It is generally accepted that it increases power.

2.You can, but you will need a very accurate gauge and a ball valve to protect the gauge from ignition pressures.

3. What is acceptable is however long you're willing to have it.
A CB of 1:10 is probably less than maximum efficiency on a 4x hybrid.

4.No one has really used hydrogen in a hybrid yet, so I can't *pretend* to tell you what's safe or not. You'll have to figure that out for yourself.
Hydrogen is much more prone to detonate than mapp.


5. One spark gap in the back of the chamber and a schelkin spiral will probably cause it in a fairly short distance (in hydrogen/oxygen).
You'll need a chamber made from/lined with the most impact-resistant material you can find if you don't want it to wear out/asplode eventually.
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Last edited by Fnord on Fri Jan 25, 2008 1:13 pm, edited 1 time in total.
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Unread postAuthor: DYI » Fri Jan 25, 2008 1:12 pm

No one has really used hydrogen in a hybrid yet

Maybe they have, but just haven't got around to posting it yet :wink:
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Unread postAuthor: Jeeperforlife » Fri Jan 25, 2008 11:39 pm

Thanks that gives me some to think about. I should state my resources are vast. My dad runs a manufarturing plant for air craft refuleers and has all kinds of steel I can get for cheap with machining capabilities. Including stainless 6" OD with 1/4" wall seamless :twisted: . I have been welding since I was about 10 and have some pretty good fab skills from my other hobbies.

I have a design in my head but am not yet ready to share because it will change. I will problly build a small one first in my spare time at work for testing pupesess. No even I can't test at work even though my employess want me to, just build.

one more question I meant to post but was in a hurry to get to work. Has anybody used nituress as a oxidizer?

I am thinking Mapp gass and N2O2 at 200psi :shock: .
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Unread postAuthor: Ragnarok » Sat Jan 26, 2008 12:29 am

Jeeperforlife wrote:I am thinking Mapp gass and N2O2 at 200psi :shock: .

I'm not sure you mean N<sub>2</sub>O<sub>2</sub> - I think you mean N<sub>2</sub>O, commonly known as Nitrous Oxide, Nitrous, or Laughing gas - because N<sub>2</sub>O<sub>2</sub> is an exceptionally rare compound.

Also, at those pressures and with an oxidiser as dangerous as N<sub>2</sub>O (Per molar quantity, Nitrous oxide is a better oxidiser than oxygen itself), I hope you're either doing it in very tough pipe, or aren't standing in the same county as the hybrid.
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Unread postAuthor: DYI » Sat Jan 26, 2008 12:22 pm

Per molar quantity, Nitrous oxide is a better oxidiser than oxygen itself


You learn something new every day... I always had the assumption that pure O2 was a better oxidiser than N2O. Even an oxy/MAPP mix at 200 psi pre-ignition would be roughly equivalent to an air/MAPP mix at 400 or so (what I'm going to try :roll: ). As long as detonation didn't occur (which is likely a real danger with an oxidiser that potent), SCH 80, or preferably SCH 160 seamless steel pipe should be alright.
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Unread postAuthor: SpudBlaster15 » Sat Jan 26, 2008 2:58 pm

Ragnarok wrote:Per molar quantity, Nitrous oxide is a better oxidiser than oxygen itself


My chem is a bit rusty right now, but that just doesn't sound right.

Taking propane as an example, the balanced reactions are;

10N<sub>2</sub>O + C<sub>3</sub>H<sub>8</sub> --> 3CO<sub>2</sub> + 4H<sub>2</sub>O + 10N<sub>2</sub>

5O<sub>2</sub> + C<sub>3</sub>H<sub>8</sub> --> 3CO<sub>2</sub> + 4H<sub>2</sub>O

For the molar enthlapy change of formation for each compound, we get;
C<sub>3</sub>H<sub>8</sub> = -104.7kJ/mol
10N<sub>2</sub>O = 82.0kJ/mol
3CO<sub>2</sub> = -395.3kJ/mol
4H<sub>2</sub>O = -142.9kj/mol
(Negative value means the reaction is exothermic, it liberates energy)
Oxygen and Nitrogen are pure substances, so they are considered to have no change in heat energy during the reaction.

Assume we have a chamber containing one mole (22.4 liters) of each mixture.
For the Nitrous Oxide/Propane reaction, we have 1/11th of a mole of Propane, and 10/11ths of a mole of Nitrous Oxide to begin with. The reaction ends with 4/11ths of Water, and 3/11ths of Carbon Dioxide.
For the Oxygen/Propane reaction, we begin with 1/6th of a mole of Propane, and 5/6ths of a mole of Oxygen. The reactions yields 3/6ths of Carbon Dioxide, and 4/6ths of Water.

For Nitrous Oxide/Propane, this gives us (104.7*(1/11)) = 9.5kJ for Propane, (82.0*(10/11)) = 74.5kJ for Nitrous Oxide, (-395.3*(3/11)) = -107.8kJ for Carbon Dioxide, and (-142.9*(4/11)) = -52.0kJ for Water.

For Oxygen/Propane, we get (104.7*(1/6)) = 17.5kJ for Propane, (-395.3*(3/6)) = -197.6kJ for Carbon Dioxide, and (-142.9*(4/6)) = -95.3kJ for Water.

(Wondering where the negative signs on the propane values went in the last calculation? Well, keep in mind that I used enthalpy of formation as the base value, and the reaction in question requires that the bonds within the molecules are broken, which requires the opposite energy value)

Now we can add these all together to obtain the net energy for each reaction.

For Nitrous/Propane, we get;

E<sub>net</sub> = (-107.8-52.0)-(9.5+74.5)
E<sub>net</sub> = -243.8kJ from 1 mole of gas

For Oxygen/Propane, we get;

E<sub>net</sub> = (-197.6-95.3)-17.5
E<sub>net</sub> = -310.4kJ from 1 mole of gas

So the Oxygen/Propane reaction should produce about 20% more energy per volume of reactants.
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Unread postAuthor: BrianMigs » Sat Jan 26, 2008 3:46 pm

SpudBlaster, you arent actually Mr. Nixon, my chemistry teacher are you?

I hate stoichiometry, haha
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Unread postAuthor: Jeeperforlife » Sun Jan 27, 2008 11:56 am

Thanks, to everybody.
I think I will stay away from nitrous oxcide. I designed a hydrogen - oxygen water generator yesterday. Now I will need to complete my 24volt generator, it will be capable of over 200 amps output. The hydrogen/oxygen generator will be a 6 cell design and liquid cooled. I will plan it to pull 180-220 amps depending on how the 24 volt generator works. I even have a torch tip designed for it.

I probally won't post up for a few days as I will be very busy at work but I will try and post up if any of you have questions or suggestions.

Marcus
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Unread postAuthor: Jeeperforlife » Mon Feb 18, 2008 11:56 pm

A couple of thoughts about a piston hybrid.
I finished a all copper pneumatic burst disk gun. It works great but I hate changing burst disks every time. This leaves me to build a simple but reliable piston style gun. I have looked at Fnord's design and am wondering if instead of a metal spring what about using compressed air. as the piston moves back from the pressure of the explosion a tube will expose a hole emptying all of the pressure from behind the valve leaving it open to release all gasses from the combustion chamber. this could be set up to auto refill to make a rapid fire gun. I will build one but it will be some time before I have the time to do so. I hope I described it well enough to make sense. what do you guys think?
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Unread postAuthor: psycix » Tue Feb 19, 2008 7:53 am

DYI wrote:
No one has really used hydrogen in a hybrid yet

Maybe they have, but just haven't got around to posting it yet :wink:


Or those members died from the failing chamber.....
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Unread postAuthor: Fnord » Tue Feb 19, 2008 12:01 pm

@Jeeper:

If you're looking at my original design for ideas, you'll have better luck with a barrel sealing piston valve like version 2 (which is basically what you described, just not what you were thinking).
Even though the spring piston idea did work for me, It required tight tolerances because it had to seal the walls of a pipe with orings. Also, it tends to damage to piston, as opposed to the barrel-sealing method. I only got a couple of shots before it had to be repaired.
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Unread postAuthor: Jeeperforlife » Tue Feb 19, 2008 10:05 pm

_Fnord wrote:@Jeeper:

If you're looking at my original design for ideas, you'll have better luck with a barrel sealing piston valve like version 2 (which is basically what you described, just not what you were thinking).
Even though the spring piston idea did work for me, It required tight tolerances because it had to seal the walls of a pipe with orings. Also, it tends to damage to piston, as opposed to the barrel-sealing method. I only got a couple of shots before it had to be repaired.


Thanks for posting.
I will look into that.
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