EX Designs

So can someone tell me why no EX users seem to do motor ejection. I realize these are usually higher power and as such need dual deploy. Is there any other reason?

Al

That's one reason. From what I've seen/heard, getting the delay elements calibrated for difference propellants is a fair amount of work. Work that could be spent woring on the motors themselves.

Now lets see if the EX folks substantiate this.

That's just more complexity to add to a motor which you are designing and making on your own. See what I mean?

Your point is correct, too...usually the vehicles that EX motors are built for are well in the dual deploy "size range".

Even the largest 54mm aero-tech [k-700]is used in a plugged foward closure configuration.....along with all it's 75-98mm bigger brothers...mainly because when you are using these larger motors, you are attaining altitude levels that require dual-deploy anyhow.....some ex'rs I flywith do use motor ejection on smaller 54-38 and 29mm stuff...plus most ex motor's come with solid forward closures.....then there is the matter of formulating a consistantly accurate de-lay column,if you wanted to use motor ejection.....I would not want to risk losing my investment with motor ejection, when I can use altimeter insurance

I forgot to mention that you can buy commercially available delays for smaller motors that can be used when your propellant is a clone of commercial motors propellant....even then,at best, it's trial and error...

The pressure in the motor will vary between the different propellant formulations. The higher the pressure or greater the Kn= the faster it will burn,which includes the delay. I have used a delay for tracking only. Some have gone through testing of ejection charges for their propellants , but it requires more testing rather than flying. Just ask Jeff Taylor from Loki Reserach he has a better take on how much testing is required to get an accurate delay timing.



JD

Originally posted by Hospital_Rocket
So can someone tell me why no EX users seem to do motor ejection. I realize these are usually higher power and as such need dual deploy. Is there any other reason?

Al

Click to expand...

The big reason is that the TRA EX safety code forbids using motor ejection with EX motors. Rockets catoing on the pads don't scare me (much). Rockets dropping from the skys sans parachute scare me.

Why do it anyway? If you are making motors there's little reason not to make one big enough to lift an altimeter. Besides, motor guys are always wanting to get data on how the motor performed and flight data, even just an altitude, can tell you a lot.

Most EX'ers simply can't be bothered. With most EX being 38mm and up (and UP ), most rockets you'll fly them in will have electronic ejection.

It's simply too much work to bother with. If you happen to be a fan of say EX 29mm SU's, then fine, that's justified I guess. Most EX'er are more interested in making the motors go, rather than fiddling around with little things like that .
And go they do .

Another thing to consider is that delays burn slower at high altitude and can't be reliably predicted. This often happened in rockets using our 98mm motors and was the main reason for discontinuing delays for the larger motors. That, and the cost of testing and certifying all those delays.

I could be wwrong, but I think yet another reason is that with the larger motors, the coast stage to apogee would require a longer delay and that just takes up more room that could be filed with propellant that could help in lifting the altimeter.
Reed

I was just wondering (and killing time on a really monotonous conference call).

I think Jeff's is by far the best reason to avoid this practice.

I have a propellant that has characteristics similar to White Lightning and when using Aerotech hardware, the corresponding RDK works well. I do have a delay formulation but I haven't calibrated it and use it just for smoke. Also, with a delay, a bubble can take several seconds off the expected burn time. Careful vacuum processing and casting is a must if using for ejection.

Originally posted by Loki
Rockets catoing on the pads don't scare me (much). Rockets dropping from the skys sans parachute scare me.

Click to expand...

Jeff

No truer words were ever spoken. Ever.

Chuck

delays burn slower at high altitude and can't be reliably predicted

Click to expand...

Gary, any ideas why that might be true? Is it just the lower atmospheric pressure changing the burn rate?

just remember that EX is a TRA "construct"... Outside TRA, EX is more commonly known as AR: Amateur Rocketry....

NAR has no "construct" for AREX

The point is this: Except for CA, AR is essentially totally unregulated.

I have had good success with a forward closure I made for my 29-100 casing. I use AT RDK kits, and so far have a successful static with 2 successful flights.

Here's a picture of it firing:

Delay smoke...

I swear I attached the pic...

and the ejection charge firing

This motor turned out to be a 29-100 G56-7 Blue. Not too shabby if you ask me.

Originally posted by cls
Gary, any ideas why that might be true? Is it just the lower atmospheric pressure changing the burn rate?

Click to expand...

Yes. I believe Chuck Mund proved that green fuse did the same thing.

Originally posted by Loki
Rockets catoing on the pads don't scare me (much). Rockets dropping from the skys sans parachute scare me.

Click to expand...

I thought that way too till I went up to use a rail at a launch and the stop on the rail was a casing that had completely wrapped around the rail, effectively having turned itself inside out and they just decided to leave it there for use as a stop (and as a reminder).

If you observe the proper safe distances, I am still much more afraid of a rocket coming in ballistic. I recently had the opportunity to see what a largish, slow moving modroc (maybe BT-60 and D12 powered?) can do to a person's leg. Much less somthing barrelling in from 1000+ feet. I'll take my chances with CATOs any day.

At the first BALLS launch in 1991, an N motor blew. The concussion could be felt at the tracking stations 2 miles away. The flight line at 600 feet got a good jolt. None of the debris went beyond 300 feet. The tower was turned into a pretzel and no pieces of the rocket were larger than 4".

Originally posted by Rocketjunkie
At the first BALLS launch in 1991, an N motor blew. The concussion could be felt at the tracking stations 2 miles away. The flight line at 600 feet got a good jolt. None of the debris went beyond 300 feet. The tower was turned into a pretzel and no pieces of the rocket were larger than 4".

Click to expand...

Concussion wave-2 miles
Parts-300 feet

Give me the CATO any day.

Sound doesn't make pain, parts can only fly so far depending on the overpressurization, but no one is safe from a mile high auger, or a 1,000 foot auger, or even a 200 foot high auger.

Chuck

The purple line is a camera effect from the light, as this paraffin hybrid was like White Lightning...only zero smoke. 8 second burn too. Roughly 1.5 pounds of paraffin consumed with 5.5 pounds of nitrous oxide.

The venting at the top of the set-up, is the pressure gauge at the tank failing, as the diaphram burst.

Originally posted by cls
Gary, any ideas why that might be true? Is it just the lower atmospheric pressure changing the burn rate?

Click to expand...

It's kinetics, and here's an oversimplified explanation.

The hot gas radiates energy back into the propellant causing it to react making more hot gas, etc.

In a rocket motor operating under normal conditions, the high mass flow chokes the nozzle and effectively controls the the pressure and energy release in the thrust chamber. A delay column doesn't produce a very high mass flow (compared ithe the propellant grain) to the nozzle doesn't choke the flow or control the pressure in the thrust chamber.

To a first approximation, the burn rate is proportional to some power of the chamber pressure and is pretty constant if the nozzle chokes the flow, but when there is little mass flow, the pressure outside the rocket controls the chamber pressure and the burn rate will be much slower.

Bob Krech

For propellant to keep burning you need to put a certain amount of energy back into the propellant to vaporize the components, and then burn them. Close to the earths surface the burning material is kept closer to the unburnt material, so the energy transfer is faster and more easily completed. Higher, the burnt hot material spreads out faster and energy isnt transfered as quickly or easily. The reason they burn slower is the exact same reason burnrate of propellant increases with an increase in KN (pressure)

As to overpressurizing vs ballistic recovery. I would MUCH rather take overpressurization. Kermie, imagine if that peice of Al had traveled towards the crowd. It would have been flat (no rail to rap around) and started at high speed but VERY quickly it would have slowed down. Try to throw a dinner plate (not like a frisbee) and see how far it will go, it will stop dead.
When something overpressurizes you can see where the rocket is, and probably get behind something you know which direction stuff is moving (which 95% of the time is straight up and down because the case will fail at the nozzle or bulkhead) When something comes in ballistic often no one knows where it is, and someetimes not even that something is coming in.

A magnetic apogee detector would be good, $30 kit, $60 built, aerocon systems makes them, timer would work but what if the motor doesnt perform as expected? Then the timers ejection would be off, a MAD is always accurate.

Most EX motors I make are large enough to warrant avionics. Usually mid K's to mid L range. To date.

I am waiting on some nozzles for 76 mm motors. Then I will be able to say I built an M.

Let me add this. Jeff Taylor has been a great deal of help. Along with a guy named Ed Jacoby. I'd still be tearing casings up if it weren't for them.

i'd prefer the auger any day, while the kinetic energy is obviously more focused (ie bullet vs shrapnel grenade) I augered in a 4 inch 8 foot vehicle from about 7000 ft this past w/e. Beautiful ascent on a J295 then electronics failed. The small group on hand had all kinds of time to get out of the way, in fact i heard some short prayers behind me as the apogee failed and she turned over, then, many more exhortations to the gods as she went ballistic down, and more still until about 600 feet when the consensus went to shes a goner....point is ample time. Now if she had landed on our heads no doubt might have caused serious damage to a car. A cato happens instantly. no chance to move or duck and at ex launches I have seen j, k impulse motors ignited at 20 feet.
J
PS but what a rush!