Vent holes to reduce ejection blast?

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car3107

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My little "Cyclops" horizon-cam D-engine launch vehicle (attached pic) cyclops.jpg flies fine, but after two launches I'm coming to the conclusion that a tube this short (20cm) and narrow (24mm) is just too small for a D-engine's ejection blast.

After chute and shock cord there's only about 2 inches for wadding; chutes are getting melted, shroud lines damaged. The nose pops out fine, but despite using plenty of masking tape for friction-mounting the engine, the engine shotgunned out both times.

Is a tube of this size just too small for D engines, or are there any special tricks for dealing with a D-sized ejection charge in a small tube? I am thinking of drilling two or three 1-cm holes in the tube, just above the engine to discharge some of the blast (covering them with sacrificial tape to keep things aerodynamic on the way up).

Anyone with any ideas/advice on this kind of thing?

Many thanks.
 
I believe vent holes are most effective just below the shoulder of the nosecone, at least that’s where I’ve always put them.

Theres a few other methods you could use. Apogee components mentions these two,
the first is made from music wire, and could easily be added to the built model
5CB86DAE-EC20-4C15-BC2C-49BB98909947.jpg
The second would take a bit more preparation to launch but may prove easier if you have no access to music wire
EECBB8C0-AE8B-489D-A8E8-17D9D9DCEDF5.jpeg
In your case you would move the windows up between the fins. Only two are necessary. Then you would insert the engine, and take cut pieces of masking tape the same size as the window and build them up from the engine case till it’s flush with the body tube.
 
With 1cm holes, you will see flames coming out of the side of your rocket :)
I would increase the length of the body tube, which would also make that rocket look even more cool!
 
My little "Cyclops" horizon-cam D-engine launch vehicle (attached pic) flies fine, but after two launches I'm coming to the conclusion that a tube this short (20cm) and narrow (24mm) is just too small for a D-engine's ejection blast.

After chute and shock cord there's only about 2 inches for wadding; chutes are getting melted, shroud lines damaged.

I could never get wadding to properly seal the cute from motor ejection charges either, so I moved onto Nomex blankets:
https://www.rocketarium.com/Heat-Shields

Properly sized for the tube (probably 4x4 for your 24mm tube), they do a much better job of creating a heat resistant membrane between the chute and the ejection charge.

Full disclosure - I saw experienced fliers relying on 2-4" of "dog barf", but never went that rout myself to avoid having more stuff to carry to launches and back.
https://www.rocketryforum.com/showthread.php?14619-Is-this-Dog-Barf


The nose pops out fine, but despite using plenty of masking tape for friction-mounting the engine, the engine shotgunned out both times.

I am not entirely sure what your main concern here is, or what "shotgunned out" actually means.
As long as the ejection charge is sufficient to separate the nose cone (confirmed), but not excessively so as to cause zippering when the shock cord and nose cone rebound (not reported), the only remaining concerns might be over damage to the tube (not reported).

The damage to the chute has everything to do with lack of proper insulation (and likely crappy plastic chutes), and not just the size/force of the ejection charge.


I am thinking of drilling two or three 1-cm holes in the tube, just above the engine to discharge some of the blast (covering them with sacrificial tape to keep things aerodynamic on the way up).

Anyone with any ideas/advice on this kind of thing?

I would not drill any holes in the airframe.
For one thing, you don't know how large of a hole to drill before the ejection charge becomes insufficient to separate the nose cone. If too much pressure is vented through those extraneous holes, you risk ballistic / unsafe descent.
For the other thing, you will be weakening the airframe by both making the holes, and creating more surface area for hot ejection gasses to singe and/or burn the tube. You could mitigate this by impregnating the effected areas with CA, but issues #1 above still persists.

a
 
Flames coming out the sides sounds VERY cool! Too bad it will happen at 300 metres up, so without a telescopic camera I'll never get to see it...
 
Thanks for your thoughts on this, afdeev. Trying a nomex blanket first sounds like a good idea.

I'm using chutes hot-cut from pertex (nylon). They are undoubtedly susceptible to melting but have been fine in my other launch vehicles.

By "shotgunning out" out I just meant that the engine ejected itself out the back. I found that surprising considering how tight I thought it was in the tube, and the fact that the nose and chute still popped out ok. Just goes to show how much pressure is in the body tube in the nanoseconds immediately following the blast, eh?

My concern with the engine ejecting (even though the nose and chute go out ok as well) is that without the 20g of the empty engine still in there I might get another 50m of horizontal drift on the way down - and in my confined inner-city launch field that could mean a lost rocket.

I agree that the wrong sized vent holes would cause more problems than they might alleviate. Perhaps best to try the nomex blanket and an engine hook to cure this model's ills first, eh?

Thanks again for the comments.
 
Another good reason to keep the spent motor in the rocket is the pain caused by a D casing falling from altitude. I was hit on the shoulder by one and I don't recommend it to anyone.
 
Reading through this thread I've got some data to share that's relevant to what you're asking even though I'm not sure if this is really what's needed.

A number of years ago I made a custom booster stage to lift an Estes Loadstar II upper stage. The booster uses a friction fit 24mm (D12-0) motor, and stages through a gap that's about 17cm between motors. Immediately above the 24mm motor the tube the internal structure steps down to an 18mm stuffer-tube that then opens out again to the full ~3cm body tube diameter at the transition section for the upper stage. Here's the full stack ready for launch at an undisclosed location north of Yuma, AZ.

YumaLaunchCropped.jpg

I'm thinking that the internal volume of my booster is at least in the same ballpark as what you're dealing with. (While I don't have any laundry inside my stuffer, your BT remains at the full 24mm dia from top to bottom.) Most relevant to your original question, I vented the top of the booster with three 3mm diameter vent holes. (One of which is visible to the left of the spring-pin in the photo below.)

DSCN3481.jpg

As noted above, my motor like yours is just friction-fit. I've lost track of exactly how many flights I've completed in this configuration (maybe 6 or so) but my success rate so far has been 100% with this setup both in terms of sustainer ignition and motor retention.

One question I would ask about your current setup is how tight is the fit of your nosecone? Same question about how tight your 'chute is packed into the tube? If the fit of either of these are really tight that might contribute to blowing the expended motor case out the bottom.
 
I have been conscious of the necessity of the nose not being too tight, but i may still have got it wrong. I wonder, though, whether the fact that in that short a tube, a 13-inch chute and 5mm shock cord (needed for the weighty nose & payload) mean that the wadding is pressed right up against the end of the engine so there's no air volume at all to dampen the shock of the ejection blast by even the smallest amount...

Sent from my XT1039 using Rocketry Forum mobile app
 
I have been conscious of the necessity of the nose nit being too tight, but i may still have got it wrong. I wonder, though, whether the fact that in that short a tube, a 13-inch chute and 5mm shock cord (needed for the weighty nose & payload) mean that the wadding is pressed right up against the end of the engine so there's no air volume at all to dampen the shock of the ejection blast by even the smallest amount...

Sent from my XT1039 using Rocketry Forum mobile app

No. That’s not a thing. Your chute, cord, and wadding don’t completely fill the volume. You’d need a hydraulic press to compress them that much.
You just need better motor retention.
Also it looks like your nosecone is solid wood. How heavy is it? That’s the mass that the force caused by the pressure must move out of the way. If the nosecone is heavy it accelerates more slowly. If your tape retention is toward the aft end of the motor casing it’s possible for the motor to be ejected. Show us how you wrap your tape around the motor case.


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Hi - the nosecone is hollowed-out balsa so not heavy in itself - but there's a 20g camera plus another 16g of ballast to get the caliper stability figure up to a healthy value - so I guess there is a fair bit of mass going on there...IMG_20180319_160639125.jpg

As far as taping the engine, I've been using widths of masking tape pretty much the full length of the engine.

Today I've gone ahead and rigged an external engine hook onto the rocket, and gonna take afadeev's advice and get some nomex to further protect the chute. Will also double check that the nosecone, chute, and wadding will all be able to eject without undue resistance.

Shame I'm not gonna have a chance to launch her again for another 6 weeks at least.

Thanks for all the help, guys.
 
By "shotgunning out" out I just meant that the engine ejected itself out the back.
[...]
I have been conscious of the necessity of the nose not being too tight, but i may still have got it wrong. I wonder, though, whether the fact that in that short a tube, a 13-inch chute and 5mm shock cord (needed for the weighty nose & payload) mean that the wadding is pressed right up against the end of the engine so there's no air volume at all to dampen the shock of the ejection blast by even the smallest amount...

No. That’s not a thing. Your chute, cord, and wadding don’t completely fill the volume. You’d need a hydraulic press to compress them that much.
You just need better motor retention.

+1.

Please consider positive motor retention by way of either Estes (plastic) or Aeropack / Rocketarium (aluminum) screw-on retainers.
Once properly epoxied (with high-temp JBWeld epoxy), both types are just about bullet-proof. Aluminum ones are less bulky and prettier:
https://www.siriusrocketry.biz/ishop/parts-tools-and-more-11/motor-retainers-117/

Those metal hooks will become too flimsy, and will bend out of the way and fail to secure the motor, if the ejection charge is sufficiently strong. BTDT.

Ejecting motors out of the motor mount is both unsafe (being hit by the motor is not fun, and ejecting the motor instead of deploying the chute is not good for neither the rocket, nor the spectators), and will get expensive once you start flying reusable motors.

a
 
Sounds like the motors friction fit was not enough and the nose cones friction fit was too much. The nose cone should only be tight enough for the ready to fly rocket to be held by the nose cone without the body sliding off. The motor on the other hand needs to be pretty snugly fitted ( almost too tight to remove without tools like pliers).
 
My little "Cyclops" horizon-cam D-engine launch vehicle (attached pic) View attachment 342696 flies fine, but after two launches I'm coming to the conclusion that a tube this short (20cm) and narrow (24mm) is just too small for a D-engine's ejection blast.

After chute and shock cord there's only about 2 inches for wadding; chutes are getting melted, shroud lines damaged. The nose pops out fine, but despite using plenty of masking tape for friction-mounting the engine, the engine shotgunned out both times.

Is a tube of this size just too small for D engines, or are there any special tricks for dealing with a D-sized ejection charge in a small tube? I am thinking of drilling two or three 1-cm holes in the tube, just above the engine to discharge some of the blast (covering them with sacrificial tape to keep things aerodynamic on the way up).

Anyone with any ideas/advice on this kind of thing?

Many thanks.

Cool rocket, nice engineering on the nose cone camera.

Regarding vent holes, I'd say absolutely NOT. You want all that hot gas and flames to go forward and blow out nose cone, chute, shock cord, and wadding.

Vents will not only reduce the force, but diverting the hot gas with holes anywhere near the engine will direct it out the sides through the holes, and will burn your tube around the holes. Don't ask me how I know this......

Your tube diameter is not a problem, D's do fine with minimum diameter tubes.

Length of your tube may be a problem, not regarding ejection, but in giving you enough room to LOOSELY place the wadding, chute, and shock cord between the engine and the nose cone (also think about the length of the shoulder of the nose cone, which takes up some tube space.) Estes Cosmic Cobra was a Helicopter nose cone rocket with a really short tube, I always had problems getting it packed with wadding, cord, chute AND these huge rotor blades.

A longer tube would make it much easier for you. You could add an extension to your tube, but looking at your design, looks like vast majority of your effort is on the nose cone, may be more worth your time and effort just to build another rocket body with a longer tube.

For minimum diameter rockets, if friction fit isn't working (usually does for me, I wrap a piece of masking tape around engine and fold it back and forth once so small area of triple tape thickness-- usually does the trick), then external tape works. Looking at your model, looks like fins go all the way to the edge, so you can't wrap around easily. Here's my solution.

First, Put small (say 3 cm long) pieces of clear cellophane tape at the tail on of the rocket between the fins (this will protect your paint job when you remove the USEFUL pieces of tape.)

when ready to launch, you can use either clear or masking tape to place over the butt of engine sticking out the bottom and overlappiing the clear plastic tape between the fins (three pieces for three fins, four for four.) Should not go as far forward as the forward edge of the clear tape you placed above. Press new tape down hard, should form fit to the butt of the engine. These should hold the engine in place. The underlying clear tape allows you to easily peel off the other tape without marring your finish when you're done.

Nothing is failure proof, but this has worked well for me.

Looking forward to seeing more of your designs and videos!
 
Cool rocket, nice engineering on the nose cone camera.

Regarding vent holes, I'd say absolutely NOT. You want all that hot gas and flames to go forward and blow out nose cone, chute, shock cord, and wadding.

Vents will not only reduce the force, but diverting the hot gas with holes anywhere near the engine will direct it out the sides through the holes, and will burn your tube around the holes. Don't ask me how I know this......

Your tube diameter is not a problem, D's do fine with minimum diameter tubes.

Length of your tube may be a problem, not regarding ejection, but in giving you enough room to LOOSELY place the wadding, chute, and shock cord between the engine and the nose cone (also think about the length of the shoulder of the nose cone, which takes up some tube space.) Estes Cosmic Cobra was a Helicopter nose cone rocket with a really short tube, I always had problems getting it packed with wadding, cord, chute AND these huge rotor blades.

A longer tube would make it much easier for you. You could add an extension to your tube, but looking at your design, looks like vast majority of your effort is on the nose cone, may be more worth your time and effort just to build another rocket body with a longer tube.

For minimum diameter rockets, if friction fit isn't working (usually does for me, I wrap a piece of masking tape around engine and fold it back and forth once so small area of triple tape thickness-- usually does the trick), then external tape works. Looking at your model, looks like fins go all the way to the edge, so you can't wrap around easily. Here's my solution.

First, Put small (say 3 cm long) pieces of clear cellophane tape at the tail on of the rocket between the fins (this will protect your paint job when you remove the USEFUL pieces of tape.)

when ready to launch, you can use either clear or masking tape to place over the butt of engine sticking out the bottom and overlappiing the clear plastic tape between the fins (three pieces for three fins, four for four.) Should not go as far forward as the forward edge of the clear tape you placed above. Press new tape down hard, should form fit to the butt of the engine. These should hold the engine in place. The underlying clear tape allows you to easily peel off the other tape without marring your finish when you're done.

Nothing is failure proof, but this has worked well for me.

Looking forward to seeing more of your designs and videos!
Thanks Babar - i've come to the same conclusion about length. The body tube is from a roll of aluminium foil (typically shorter, skinnier and thicker-walled here in the UK than US tin foil rolls I believe) and it was originally about 4 inches longer, which would've been perfect, but i cut it down cuz i was trying to approximate the proportions of an estes alpha III. Live and learn, eh?

Think i will indeed build a longer Mk II version to go with that camera nose. Gonna give the existing version one more go, though.

Gonna try protecting the chute with a bit of nomex in addition to cellulose wadding. For engine retention, i've already gone ahead and rigged up an external engine hook. Not ideal, but let's see how that goes.

Thanks again for the ideas and encouragement.

Sent from my XT1039 using Rocketry Forum mobile app
 
Similar concept to the Nomex is Quest style wadding (float paper), which is sheets about 6" you can wrap around whatever you need to protect. I also have a tendency towards rockets where the default ejection charge is excessive and dog barf is useless. When the motor is large compared to the airframe, the whole rocket is hot after flight.

P.S. the rocket that got hottest was also aluminum foil tube, U.S. version, just big enough to slip a piece of half-burnt Estes 24 mm tubing in and the old motor hook.
 
Just a thought: One thing that I didn't see mentioned... where in the flight path is the chute being deployed?

If the rockets speed is still significant there will be significant force on the NC, hindering release of the NC from the BT.
 
Not quite related to your original question, but the Estes Astron Scout K-1 used vent holes to allow ejection gases to vent after the engine pushes itself rearward to shift c.g. for tumble recovery.

https://www.spacemodeling.org/jimz/k-01.htm

k-011@.gif
 
Not quite related to your original question, but the Estes Astron Scout K-1 used vent holes to allow ejection gases to vent after the engine pushes itself rearward to shift c.g. for tumble recovery.

https://www.spacemodeling.org/jimz/k-01.htm

k-011@.gif
Wow, GlenP thanks for this. I remember seeing the astron scout in the catalogue as a kid but had never considered that Tumble Recovery would necessitate a CG shift and a way to vent ejection charge gasses... So at least there is some precedent to my idea of leaking out a proportion of a D's charge from a particularly small tube.

But I've now gone and built a mkII version with 10cm more tube length, added an engine hook and will be wrapping the chute in nomex, so won't be trying vent holes this time.

Still really like the idea of flames blasting out the side on ejection, though. Maybe it's time for me to put together one specifically to do just that, eh? (Though here we may well be moving away from model rocketry and into the world of pyrotechnics...)
 
There are some high temp rated BBQ paints you can use to help a vented duct last a little bit longer before it gets burned through. Another option is the metallic dryer duct tape, or other utility metallic tape to use as a liner, but that can get heavy if you use a lot of it. Or just plan for how you can insert a sacrificial paper liner or cooking parchment paper that you can replace. But if you close up the vented tube permanently, then you can’t easily replace a liner. Some things to consider in a vented design.
 
i dont know if this would help in your situation, but something i do with short, small dia rockets is pack the chute and shock cord, then stuff dog barf in through the MMT. then a bit of dog barf packed into the motor before installing.
 
The new, longer version. Hopefully the extra 10cm of space for wadding etc will sort out the probs.
MkII.jpg
 
You know, even without considering this has a side facing camera in it that thing looks pretty cool!
 
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