Rear Engine/Parachute Ejection

Is there any reason why there are no mid/high powered rocket kits using a rear engine/parachute deployment system?

I’m thinking of up-scaling a model I built that uses this method to 24mm mid-power, up to Gs, and I’m asking if there are any good reasons why this might be a BAD IDEA.

Sounds like a good idea to me.

I saw an upscale Gemini DC some where, maybe on youtube?

boomtube said:

Is there any reason why there are no mid/high powered rocket kits using a rear engine/parachute deployment system?

I’m thinking of up-scaling a model I built that uses this method to 24mm mid-power, up to Gs, and I’m asking if there are any good reasons why this might be a BAD IDEA.

Click to expand...

Funny you should ask this. I've been thinking about that for an HPR rocket to keep the airframe all in one piece for an ultra high-speed launch. The idea I had floating in my brain is to kick the entire motor with recovery harness attached and the drogue 'chute just above the motor. I thought about this because I saw it done once unintentionally and thought it might be cool to try on purpose. It really separates the heavy pieces.

The things that concern me are: 1) if the motor is loose enough to eject out the back, it could be drawn out early by pressure change inside the rocket or drag; 2) kicking a long motor might be iffy and probably not worth the risk; 3) having the motor flailing about in the breeze seems like it could come around and whack the airframe, possibly doing serious damage to both; 4) the sudden stop at the end wouldn't do a large motor any good, either. If I were to try it, I think I would use two charges separated by a small amount of time so in case charge #1 only kicks the motor part way, charge #2 would finish the job.

I would do a lot of ground testing to make sure you can get the motor/mmt out the back. It seems like you would need a long mmt, too, because you might want most of the recovery harness and the drogue 'chute inside the mmt so it wouldn't get hung up on other stuff inside the rocket on the way out.

I probably won't ever do it, but it is an idea. A mid-power size rocket where the motor isn't long or heavy seems like a good place to start to mitigate the challenges.

--Lance.

I haven't done this with anything other than low power, but there are a couple of other issues.

If you rear eject, your recovery device is hanging out the bottom of your rocket proper (all of the rocket except the motor ejection pod.) Unless you attach your shock cord to the OUTSIDE of the rocket, your rocket will descend and land nose first. Meaning nose will impact the grass, rocket, pavement, or whatever. This may or may not matter, depending on how tough the nose is. You CAN attach the cord on the OUTSIDE of the rocket, but it is a little tricky. If you do this,your rocket can hang whatever way you want, depending on the attachment point relative to the CG.

It is actually possible to have a motor pod ejected and have the rocket proper and the motor pod recover separate, each with it's own chute. This was done, and worked (at least once) for the Uranus Explorer (see RocketReivews.) For low power rockets, this can be kind of nice, as the motor pod is going to be pretty tough and has to come down safely but not that gently, in fact a streamer is often enough, and by separating the motor pod, the rocket proper is pretty light, so a relatively small chute will work for the rocket. But..... for rockets that don't go up too high, having two targets to track coming down may not be a big deal. If it's really going way up there, may be more difficult and you may have hard time recovering both parts.

Again, assuming your motor pod unit is tougher than the rocket, if you DO have the two coming down together, setting up the attachments so the motor pod touches down first may reduce the impact stress on the rocket proper.

Hope this helps.

Tom

So I am edging closer to a 29mm design that uses rear/motor chute deployment.
I know that in my original post I said 24mm but after looking at how I want to do this; the weight/size aspects changed my mind as to what size/power motors I want/need to use.

Big Gs to “Pony Hs” is my new standard.

But this got me to thinking, always a bad idea, as to delay times. It seems to me that a rocket that deploys its chute out the back should actually be well into its descending arc before it goes POP!!

My reasoning, such as it is and what there is of it, is this; if the rocket is still ascending when the chute pops out the back the drag could stop the rocket dead-in-the-air and it could conceivably fall straight back right into the deploying chute/shroud lines.
If the rocket is over the top of the arc then there is little or no chance of this happening.

I figure I could just take the recommended delay time for a rocket of comparable weight and add an additional second or so.

Any thoughts?

Recommendations?

Dire warnings?

The Gemini DC that flew at LDRS...27, I think, used rear ejection.

The challenge is to keep the recovery system in place during thrust and coast phases, but allow it to come out when it needs to.

On the Gemini, they rigged something up that had to go a bit sideways in order to let things come out -- Roy hangs out around here, and he was on the team, so he'll know the particulars.

For Lance's idea, the easy answer is a piston at the top and a couple shear pins that hold the motor mount assembly in place during thrust/coast.

-Kevin

This reads like a bit of overkill for something flying on G76s or H128s and probably won’t weigh over two and a half pounds max.

I can’t think of any reason why the motor mount assembly would fall-out while the rocket is under boost nor even when coasting. It’s not like an air-drag separation of a two stage rocket.

My chief concern is what happens right after deployment. I don’t want the rocket to foul its chute; thus my delay time question?

I went through a rear-eject phase with both LP and MP rockets. I built the motor mount as usual, but did not glue it into the rocket. A bit of coupler glued into the rocket kept the motor mount from moving forward. The chute was wrapped around the motor tube and inserted into the rear of the rocket. A kevlar shock cord attached the motor mount to the nose cone, which was glued or taped to the rocket. I even used a baffle at the top of the motor tube to protect the shock cord.

This worked fine for LP rockets, though it was a bit more complicated to build than the usual arrangement. For MP rockets, I had to add a layer of insulation around the motor tube. Otherwise, it would get hot enough to melt the chute.

I think about using the same ideas for my HP rockets, but haven't done so yet. The major down side to this technique is that there is less space around the motor tube for a chute than there is in the usual configuration. In practice, you need a large diameter airframe compared to the motor tube. For my HP stuff, this has not been the case so far.

boomtube said:

Is there any reason why there are no mid/high powered rocket kits using a rear engine/parachute deployment system?

I’m thinking of up-scaling a model I built that uses this method to 24mm mid-power, up to Gs, and I’m asking if there are any good reasons why this might be a BAD IDEA.

Click to expand...

Apparently you haven't seen this Brand spanking new Thresher:
https://www.rocketryforum.com/showthread.php?t=30165

It flew last weekend at our club....great flier and good looking kit. definitely different with the rear eject.https://www.baddazzrocketry.com/BAR/ZZtore.html


There were no problems when it ejected the chute, nothing tangled, just have long enough shock cord. I think this weighs about 1.5 empty.

boomtube said:

But this got me to thinking, always a bad idea, as to delay times. It seems to me that a rocket that deploys its chute out the back should actually be well into its descending arc before it goes POP!!

My reasoning, such as it is and what there is of it, is this; if the rocket is still ascending when the chute pops out the back the drag could stop the rocket dead-in-the-air and it could conceivably fall straight back right into the deploying chute/shroud lines.
If the rocket is over the top of the arc then there is little or no chance of this happening.

Click to expand...

You want the recovery system to deploy at apogee when the rocket is moving slowest in relationship to the air. If the 'chute deploys early or late, the rocket may be moving fast enough to sever the shock cord.

A big problem with rear-ejection (especially with large rockets) is that the rocket is still stable after deployment. If the shock cord breaks or the 'chute fails to open, the rocket is going to continue its ballistic course all the way to the ground.

-- Roger

KevinDunn said:

For MP rockets, I had to add a layer of insulation around the motor tube. Otherwise, it would get hot enough to melt the chute.

Click to expand...

what did you use for insulation? (I know I'm resurrecting an old thread here... sorry)