Ramjet Design Discussion

[Blackhawk]

This is a more suitable place for a continuation of the ramjet discussion that was taking place in the rocket thread in the pyrotechnics section, go there if you need the background.

[Bert]

O.K.- You want to push a 15 cm wide object perhaps a meter long past the speed of sound on a sugar rocket. You think this will work because the pressure your motor tubeing is spec'd for and your motor design program seem to indicate it could work. Is your design in the CENTER of the envelope, or right out at the edge of several parameters?

Why isn't anyone I know flying sugar rockets that break the speed of sound, let alone do it pushing and object much WIDER than the rocket's body on top of it? As you said, the fuel is nice and cheap. But for some reason, everyone I know who's cracking mach 1 is using AP composites, nitrous oxidized hybrids or, in a few rare cases, LOX and liquid fuels.


Supersonic speeds with shapes more complex than a rifle bullet or the simplest rocket shapes are apt to experience destructive turbulence. Got a supersonic wind tunnel? It would come in handy for testing the stability of combustion in your engine too-


How big is your budget? If you have to put one of these on a rocket and fire it to test, you will lose one jet for every trial. (Please don't tell me you're budgeting a recovery system for this toy!) And I don't think your first "by guess and by golly" try is going to just roar right over the horizon on a stable sustained burn. But if it does, I will be the first to congratulate you!

[Blackhawk]

I'm not expecting much from this, just for the rocket to decelarate slower than the rocket would if it had no engine at all. The rocket itself is as I said only 64mm wide, and while the ramjet is 15cm it is a tube and will offer less resistance than a solid transition. There is no reason you can't break mach with sugar motors as far as I'm concerned, but you would need a larger motor than if you were using higher performance fuels, but that said there is no reason why I couldn't use a hybrid motor design or even AP fuels for my booster, because this is in the DESIGN stage, I have in no way chosen any specific path yet.

As for recovery, of course there will be a recovery system, I already have the altimiter system, loosing a jet per test flight would just be stupid, and a waste of what money I have, as you wouldn't be able to get any hard facts back from the way it ran (The person whom I am designing it with also makes and sells custom electronic deployment altimiters that arm once the pressure begins rapidly changing, once the pressure stops canging the rocket deploys a drogue chute, once the atimiter reads the pressure to be ~700ft it deploys the main chute. The pressure effects caused by mach braking are surpassed by adding a timer to the system that disregards all pressure readings before a certain time)

"Why isn't anyone I know flying sugar rockets that break the speed of sound, let alone do it pushing and object much WIDER than the rocket's body on top of it?"
-Because there is no point in breaking mach except for bragging rights and adding a larger rocket on top is just making it harder.

"Supersonic speeds with shapes more complex than a rifle bullet or the simplest rocket shapes are apt to experience destructive turbulence. Got a supersonic wind tunnel? It would come in handy for testing the stability of combustion in your engine too-"
-Yes I have seen rockets break up when passing the sound barrier and I wish I had a wind tunnel, but the design will be such that there 'shouldn't' be any unexpected problems with turbulance, I'm not going for a crazy asymetric design with rectangular inlets or something, KISS, even though simple is not a word to describe this sort of thing. And even if I do have problems, I LEARN from them, I find out what is wrong and I'm willing to loose rockets for it, you know why? Because afterwards I will know more than i did at the start and I will be closer to being succesfull, isn't that what life is? I sure as hell don't have a life wind tunnel to jump in if you catch my drift but I still live.

By guess and by golly is simply not the approach I am taking here, it may be the way nasa oparates and look how they are going ;) I don't intend on completing the design phase before I am ready whenever that is but I will put as much effort into designing it (and as much thought) as is put into much larger commercial engines. I never make grandiose claims, like my first flight will be perfect, or that I will do everything right and the motor will be perfect, but that dosn't stop me from trying now does it.

"You think this will work because the pressure your motor tubeing is spec'd for and your motor design program seem to indicate it could work. Is your design in the CENTER of the envelope, or right out at the edge of several parameters?"
-I think this will work becauase these motors have been repeatedly static fired on load cell thrust guages, I was just talking about the motor pressure because you seemed to think it was too high. As for the design software, it is usually wrong about max altitude by a few hundred feet but I know that it is not the hight of accuracy, and I know that just becuase it says exactly Mach 1 the rocket will get to exactly mach 1, you seem to take me for a fool, which I can assure you I'm not.

Your input is highly welcome in designing the motor though, and WHEN it works (and it will, with a lot of time, hard work, money and testing) you will be the first to hear about it ;) ) But don't decide that I am a bumbeling idiot that will try to slap some carboard tubes together with a cylinder of gas and try to light it, beacuse that simply isn't the type of person that I am. You seem to be knowledgable in the field howerver, may I ask how you have been involved in amature rocketry in the past?

Also have a look here, more experimenters with the same ideas http://www.rocketresearch.org/

[Bert]

I don't think you're an idiot, if I did I'd have just called your post to the attention of a moderator rather than take the time to point out a few potential weak points.

I DO think you haven't gone through an R&D project of this complexity, and that you will soon know my good friend Mr. Murphy much better than before. But, that can be a good thing as long as you live through it.

When you actualy finalize a design and put tools to metal, this thread will be worthy of inclusion in the main site, I would judge. I certainly hope you go further with this than some of the other projects I've seen flash across my screen.

And BTW, 30 odd years ago before AP composites were commonly made by amateurs, I had this little plan... I was going to use MAPP gas for the fuel in my little ramjet, it would self ignite from the compression, it was gonna be BEAUTIFUL... But I didn't have the engineering support or the fabrication facilities, nor the huge base of practical amateur experience now available on the internet. You do. Break a leg, kid.

[Blackhawk]

Thanks, yes this is my first complex sort of project and I'll do my best at it, I'll post updates here, probably some of the maths I am using and pictures of the design progression. I have no doubt I'll meet a sea of problems but thats the fun part, if everything just went together and it flew than where would the accomplishment be (I enjoy fixing my RC car a lot more than driving it for some reason).

"But I didn't have the engineering support or the fabrication facilities, nor the huge base of practical amateur experience now available on the internet."
-Ahhh, but you do now, why don't you give it a go again ;)

[Marvin]

I'm worried you are presurising the fuel with an oxidiser, I'm furthur worried that at this pressure the oxidiser will dissolve in the fuel to a large extent....

N2 at room temp is well above its critical point, so no pressure however extreme will liquify it.

"....so the only way you can increase the thrust is to make what is flying away heavier, or make it fly faster..."

This is rather flawwed thinking.

If you increase the mass of the fuel without changing its energy, then that rocket will perform worse than a rocket in which the increased fuel mass is simply due to using more fuel.

If you increase the mass of the fuel and its energy content, thats equivalent to saying 'make a bigger rocket' or 'burn fuel more rapidly' and this doesnt work out to producing a faster rocket, unless its staged.

If you make the fuel lighter while keeping its energy though you end up with a faster rocket. So the key is making lighter fuel (and hense exaust), not heavier fuel.

"Seeing as the only particles used in most rockets are very light gas particles the only way you can get better performance is to make the gas fly away faster"

I think the concept you are searching for here is that the exhaust is limited to being the same mass as the fuel. While this seems obvios, its the key reason increasing mass to get a bigger impulse from an amount of energy doesnt work.

I look forward to hearing more about the project and I'm sure if you spend long enough on it you'll succeed. I do think it would be helpful if you started dealing with the design more formally. (Read the math that would have explained all te above).

The last thing I'll add is while I'm sure you think you can avoid unexpected problems anything you can forsee in order to avoid is automatically not an unexpected problem is it :)

Everything I have ever done has turned out to be more complicated than I thought it would.

[Jacks Complete]

Does anyone else think we need a new area for stuff like this?
Putting it in the watercooler seems wrong!

For what it is worth, I wish you both good luck. As someone who knows the complexity of these systems, (and the things like cruise missiles, on another water cooler thread) I know it won't be easy. However, the journey is often better than the destination!

I also believe that too few people these days ever try to really stretch themselves, or try something new, that actually has a reasonable chance of success. I think that building a homemade ramjet is one of the things that is definately do-able, and was something I tried some years back. I only worked it out on paper, however.

Of course, on paper, it looks simple...

On a practical note, you can build a wind-tunnel at home, with just a pair of room fans and some board, and a tube of silicon. It won't be very fast, but it will let you try out designs for stability at the low speeds that are encountered at launch. A simple ping-pong ball can be used for a windspeed meter. http://home.zonnet.nl/sicco55/anemometer.htm is one example. I used a scanner window for visibility.

[Blackhawk]

Yes I have heard of using fans for home made wind tunnels, also you can make a simple 'fog wand' by sticking a short section of tube onto the tip of a soldering iron (so that the tip is in the centre of the tube) and filling the tube with generic fogger liquid, you can use the fog to check for any eddies and disturbances in the air flow. I think that I might begin with a subsonic ramjet as they are much simpler in design and construction, to first verify that the systems are working properly, the rocket will most probably loose velocity while the motor is running but with the extra thrust a higher altitude would be attained than without the motor running at all, which could be used to verify it's operation. If everything is working from there I will progress to a higher performance supersonic version which I should be able to get to increase in speed rather than just slowing down faster ;)