Solid fuel questions

[jthompso]

I remember reading about a way to increase the burn rate of a solid propellant. As far as I remember, if you embed metal rods or staples into a grain of solid fuel it will serve to increase the rate of combustion. I'm envisioning some kind of adjustable spike here, not sure of the feasibility. I can't remember the details but I could try to find the reference if you want. Might be something to look into.

[heada]

If some of the metal becomes loose and is significantly large enough, it could clog the nozzle, over pressurize the motor and "very bad" things happen. You could add a burn catalyst to the formula, but getting it to a very specific location in the motor would be difficult at best. Having layered motors is an idea as they would burn at different rates and pressures. Either outer layers in a BATES grain or upper layers in an end-burning grain.

The Krushnic effect happens when a motor is placed further up a tube than what is "normal" If you have a body tube that is 3 inches wide and place your motor so that it is 3 inches or less from the base of the body tube, you'll be OK. If it is more than 3 inches up the tube, then the tube will act like a secondary nozzle and the exhaust pressure would drop too much and you'll get no thrust. If the motors have a 29mm diameter and are 3 inches long and stacked 3 deep then by the time the second motor ignites, it's nozzle is 3 inches into the "tube" holding the motors which is significantly more than 29mm and you'll experience the Krushnic effect. If on the other hand, you have 5 motors side-by-side in the same tube, all at the base of the tube, you'll be able to fire any combination of 1-5 motors at the same time and not have the Krushnic effect.

Graphite for an aerospike would work as it is cheap, easily obtained and machines very well. Controlling the position would be critical but I think it could be done. A major drawback would be that it is brittle and can crack very easily. (I should have thought of the graphite before as thats what a lot of my nozzles are made from)

-Aaron

[jthompso]

In a well designed system there shouldn't be a chance of the metal rods coming loose--at least not a big enough chance to completely demerit the idea yet. I found the reference too, it's in rocket propulsion elements. If there's any interest I'll post the text.

[Viv]

In a well designed system there shouldn't be a chance of the metal rods coming loose--at least not a big enough chance to completely demerit the idea yet. I found the reference too, it's in rocket propulsion elements. If there's any interest I'll post the text.

It would be interesting to see how this works, since you mentioned it I have been trying to work out how it works and to use it.

Viv

[Viv]

If the motors have a 29mm diameter and are 3 inches long and stacked 3 deep then by the time the second motor ignites, it's nozzle is 3 inches into the "tube" holding the motors which is significantly more than 29mm and you'll experience the Krushnic effect.
-Aaron

I sense a bit of a misunderstanding over this effect, I would only have one nozzle on the end of the tube with a number of grains stacked inside but separated to control firing.

Does that clear it up?

Viv

[Viv]

Heres an airospike project I was looking for around for.

http://www.csulb.edu/colleges/coe/mae/v ... 2003.shtml

THis is the one with the graphite plug I mentioned, there should be another video of a failed run on the first engine on this site too

Viv

[heada]

Viv,

It does make more sense. You'd have a single motor with a single nozzle and just feed it grains. I don't know how that could be made to work, but yes, it wouldn't have the problems of the Krushnic effect. You'd have to have a seriously thick wall to the motor in order to run it without a liner or some way to insert the grains with a liner. The liner keeps the walls of the motor cooler so that thinner metal can be used for the case. This also will keep the flame front that happens between grains (as well as on top and bottom of the grain) from hitting the motor case and producing a burn-through.

When reading the link you provided, it looks like they fixed the problem with the aerospike for you.

"The static fire test came a little more than a year after the static fire test of their first aerospike engine during which the plug failed after 200 ms. The new engine design was based on the previous one with modifications to address issues raised with the first one, such as the addition of a second ignition port. More importantly, the modified design incorporated a titanium rod running through the center of the graphite plug and isolated with RTV."

A guide rod so that it can't get out of alignment. The aerospike rides up and down a rod that is fixed from the forward closure and going all the way down the motor. As long as the core size of the grains were adjusted and the nozzle throat was widened and there was a limit placed on the aerospike so that it couldn't move too far forward...(a lot of ifs, but it could work) Another option would be to have the aerospike actually form the nozzle. By moving it up and down, you wouldn't be plugging and unplugging the nozzle but more dynamically changing the nozzle size and configuration.

-Aaron

[Viv]

Aaron,

The classic shape plug nozzle used in the first test has the spike larger than the opening formed by the shroud, hence plug nozzle I suppose:-) the problem is a tendency for lateral forces to develop in an oscillatory fashion, if the plug is off center a little and a little flexible the flow pulls the plug to one side then the other a bit like a dog shaking its head:-)

In the first test the engine lasted a very short time before the graphite shaft supporting the plug snapped, next was the plug closing the gap with the shroud;-( as you say the reinforcing rod sorted it out.

I have seen none blocking geometry's for plug nozzles that would look nice for type of motor we are talking about here, I will try and look it up again.

An issue with the liner design and material is to equalize pressure along the length of the tube as different grains burn, compressive effects of a 1000 PSI at one end could cause problems or even move the charges around.

JTompson.

I am looking forward to reading the reference to adding metal to the grain mix as a spiked liner maybe possible, this would answer a few problems.

Viv

[jthompso]

http://www.rimworld.com/pgh/drake/index.html

This is similar--essentially they built a large "magazine" style system.

Heada-As far as I understand it the Krushnik effect does not apply until the exhaust gasses are expanding to atmospheric--as in exiting the nozzle. In this "MetalStorm" type motor it shouldn't have any effect.

Viv-When I get home I'll type up the section, I think it's a neat idea, it's really outside of the box.

[Viv]

J,

Well that was a really good find! I found that really interesting for many reasons not just the fact some one has also been trying for a significant time to work up another way of doing things.

I think the real treasure is as much the conclusions they draw from experience as the reasons and perceived benefits of their magazine approach to staging.

Watching them go the complete motor magazine route brings me back to the single motor magazine feed concept as an alternative, very interesting I will carry on reading.

Viv

[heada]

The metal storm I know of is a tube filled with alternating projectile and propellant. The propellant is ignited electrically (if I remember correctly), forcing the projectile out, making room for the next projectile. The stack does not move forward but is stationary thus after each projectile leaves the tube, the distance to the end of the tube gets larger. What I thought the goal here was something similar. To place multiple complete motors into a tube and fire and then eject them in succession. In that setup, the Krushnik could happen as the complete motor has a nozzle on the end and would be sending the motor's exhaust into the tube. The tube aft of the current motor would be at atmospheric pressure. Now that I know different, I agree that the Krushnik effect would be highly unlikely to occur.

I found the site about the drake rocket very interesting and I totally agree that it could be made to work. Looking at the drawings, it appears that they got some of their ideas from an aeropack motor adapter (54mm to 38mm) The amount of metal required to do something along the same lines would restrict this to high power motors or someone with very good metal working skills.

-Aaron

[jthompso]

"In addition to the use of chemical additives to increase the burning rate, certain physical means have been effective, such as placing of wires or other shapes of good metal heat conductors in the propellant grain. Another method uses slits or slots in the grain to promote erosive burning in the slits. One extensively used technique has been the inclusion of several silver wires arranged longitudinally in an end-burning grain. Depending on wire size and the number of wires per square inch of grain cross section, the burning rate of a grain can easily be doubled. Aluminum wires have about half the effectiveness of silver wires. Other forms of heat conductors have been wire staples (short bent wires) mixed with propellant prior to the casting operation." --Rocket Propulsion Elements

Picture an aluminum mesh cast into the fuel grain attached to a peltier heat pump--digital control of the thrust level up to 2x the baseline thrust. That would be something to see.

[Viv]

The metal storm I know of is a tube filled with alternating projectile and propellant. The propellant is ignited electrically (if I remember correctly), forcing the projectile out, making room for the next projectile. The stack does not move forward but is stationary thus after each projectile leaves the tube, the distance to the end of the tube gets larger. What I thought the goal here was something similar. To place multiple complete motors into a tube and fire and then eject them in succession. In that setup, the Krushnik could happen as the complete motor has a nozzle on the end and would be sending the motor's exhaust into the tube. The tube aft of the current motor would be at atmospheric pressure. Now that I know different, I agree that the Krushnik effect would be highly unlikely to occur.

I found the site about the drake rocket very interesting and I totally agree that it could be made to work. Looking at the drawings, it appears that they got some of their ideas from an aeropack motor adapter (54mm to 38mm) The amount of metal required to do something along the same lines would restrict this to high power motors or someone with very good metal working skills.

-Aaron

A good description of the metal storm and of the basic idea we are trying to explore, in summery a metal with the bullets and a nozzle on the end of the barrel:-)

I found the Drake project interesting for a different take on an old problem, I never take it as read that a given solution is the best solution so am always interested to see some one else's thoughts on a problem.

Drake was a refreshing change and a lot of the thought that has gone in to it is applicable to what we are looking at here, the problems are as you say complexity and mass in the magazine loader.

Viv

[Viv]

"In addition to the use of chemical additives to increase the burning rate, certain physical means have been effective, such as placing of wires or other shapes of good metal heat conductors in the propellant grain. Another method uses slits or slots in the grain to promote erosive burning in the slits. One extensively used technique has been the inclusion of several silver wires arranged longitudinally in an end-burning grain. Depending on wire size and the number of wires per square inch of grain cross section, the burning rate of a grain can easily be doubled. Aluminum wires have about half the effectiveness of silver wires. Other forms of heat conductors have been wire staples (short bent wires) mixed with propellant prior to the casting operation." --Rocket Propulsion Elements

Picture an aluminum mesh cast into the fuel grain attached to a Peltier heat pump--digital control of the thrust level up to 2x the baseline thrust. That would be something to see.

I figured it would have some thing to do with preheating the grain, I do however wonder how the metal alters the basic chemical reaction? is the metal oxidizing and adding to the reaction?

My first thought was a liner with spikes to carry heat in to the fuel and regenerative cooling from the nozzle region for the heat supply.

Second idea was a simple heating coil cast in to the grain, the turns of the coil would be very close together at the nozzle end of the grain and then spread further apart further from the nozzle, stopping about half way along the motor.

This would give the ability to preheat the grain before launch so one end was hot and one still cold, this would give a x2 boost in the early phase falling off to x1 in the sustainer phase, electrical power would be supplied from the ground before launch so no addition of mass apart from the coil.

Viv

[heada]

Does grain temp pre-ignition really matter that much? Once the motor ignites, the CC is going to be above 1500 degrees C in a matter of less than 1 second (I'd hazard to guess that it would be up to temp in much less time, something along the lines of once the motor comes up to pressures)

Even if pre-heating the grain helps, you'd be walking a very fine line up to the self ignition temp of the grain.

-Aaron