Ducted Rocket Experiments

[iter]

I've been looking at pilots at my local R/C field who convert foamy EDFs to turbine power. They use stainless steel tailpipes to insulate their airframes from turbine exhaust. I thought I'd try the same idea.

I used a G40 in an EFlite Habu. The results are mixed so far. The duct protects the foam from heat, but has itself deformed from the heat.

Needs more work.

Ari.

 

[Worsaer]

Ari, I always enjoy watching your creativity.

 

[burkefj]

Are you getting reduced thrust due to krushnic effect? How much duct post nozzle do you have?

 

[iter]

Thank you for your encouragement, Bill!

Frank--the tube is open at the front and open to the original air intakes. There is plenty of airflow, and the performance looks right for the weight and drag of the airframe. I doubt we're seeing Krushnick effect. The tube is about 14" long. Perhaps relevant to your question, its front half if intact. The tail end is where all the damage is. It's hard to capture in photos without cutting it open. I showed photos to turbine enthusiasts, and their observation was that heat damage from a wet start looks similar to this.

Ari.

 

[fyrwrxz]

Ari! Good to see you post again! Land long and prosper, buddy!

 

[burkefj]

Ah, ok you still have the ducting, what's your boost weight in how high do you think you're getting on the g80?

Thank you for your encouragement, Bill!

Frank--the tube is open at the front and open to the original air intakes. There is plenty of airflow, and the performance looks right for the weight and drag of the airframe. I doubt we're seeing Krushnick effect. The tube is about 14" long. Perhaps relevant to your question, its front half if intact. The tail end is where all the damage is. It's hard to capture in photos without cutting it open. I showed photos to turbine enthusiasts, and their observation was that heat damage from a wet start looks similar to this.

Ari.

 

[iter]

Ah, ok you still have the ducting, what's your boost weight in how high do you think you're getting on the g80?

G40. I had the one flight. Hard to estimate altitude, too much excitement trying to trim it out. The weight is about 2kg. Don't ask. It's designed with an EDF in the back and a huge battery in the front, and adding the tailpipe means it needs even more nose weight. There's 13 oz of lead in the nose. I'm ashamed of myself.

The tailpipe is two layers (three, if you count the corrugated baffle between them). Photos in my first post sort of show that. The inner tube is what's distorted. The outer one is fine. You wouldn't know anything went wrong from looking at the outside of the tailpipe assembly.

 

[iter]

Ari! Good to see you post again! Land long and prosper, buddy!

You remembered! I miss you guys.

 

[aerostadt]

Interesting idea. Lift-off still picture looks exciting. The amount of nose weight sounds disappointing. Jet turbine exhaust might be very different than solid propellant exhaust. Jet turbine exhaust might be cooler, less corrosive, and definitely cleaner with regard to particulates.

 

[iter]

The amount of nose weight sounds disappointing.

Yes. Then again, this is a prototype. It's easier for me to adapt an existing airframe than design a new one. If I can make this idea work, I'd want an airframe with the motor way in the nose, MiG-21-style. That would improve CG distribution.

Jet turbine exhaust might be cooler, less corrosive, and definitely cleaner with regard to particulates.

Apparently, kerosene burns at about 600C. That's a lot cooler than our motors. I'm less concerned about corrosion and particulates. There is white residue clearly visible inside the pipe, but it cleans off. The heat deformation is the gating factor right now.

 

[tab28682]

Hey, Ari.

One thing that might be worth considering on a foam EDF model conversion like yours is the ceramic heat protection paint from BVM. You might be able to reduce the amount of steel tubing in the rear.

https://shopbvmjets.com/index.php?main_page=product_info&products_id=29
Some folks have used this material to coat a couple of layers of glass cloth to build up more heat protection.

Over the years I have picked up a couple of inexpensive Phase III Squall EDF models to do something similar with. I think I can hit 32-33 oz off the pad with this smaller model and a G40.

We need to get together with Dave Schaefer one of these days and have a RC RG day at the North Dallas RC field!

 

[Rob Campbell]

Nevermind. I looked more closely at the pics.

 

[aerostadt]

I am thinking that turbulent mixing of rocket exhaust (beyond the potential core) will start about 10 exit diameters beyond the nozzle exit. So, assuming the exit diameter of a G40 is around 0.5 inches, there might be about 5 inches of pipe just downstream of the G40 that doesn't look too bad. Probably, the G40 needs to be so far forward this does not help very much. It might be interesting to see if the bad heat affects start a few inches downstream of the motor.

The turbojet exhaust temperature is probably much lower than the rocket exhaust. There is so much nitrogen from the air that does not contribute to the combustion temperature, yet, is still being heated to act as a working fluid.

 

[iter]

Hey, Ari.

One thing that might be worth considering on a foam EDF model conversion like yours is the ceramic heat protection paint from BVM. You might be able to reduce the amount of steel tubing in the rear.

https://shopbvmjets.com/index.php?main_page=product_info&products_id=29
Some folks have used this material to coat a couple of layers of glass cloth to build up more heat protection.

I ordered a can of Rustoleum "2000°" paint (https://www.rustoleum.com/product-c...y-paints/high-heat?ls=248904&lc=Flat Aluminum) following a similar line of reasoning. It's hard for me to understand the relative performance of these coatings from the marketing literature. I'm curious if using an insulating material for the heat pipe might produce better results, e.g., cardboard or Blue Tube with one of these coatings on the inside. Cardboard tube can be ablative, replaceable after one or two flights.

With fiberglass on steel, do people coat the inside or the outside of the pipe?

We need to get together with Dave Schaefer one of these days and have a RC RG day at the North Dallas RC field!

Yes! Say when. I live a mile from the field and would always love to see you there.

 

[iter]

I am thinking that turbulent mixing of rocket exhaust (beyond the potential core) will start about 10 exist diameters beyond the nozzle exist. So, assuming the exit diameter of a G40 is around 0.5 inches, there might be about 5 inches of pipe just downstream of the G40 that don't look too bad. Probably, the G40 needs to be so far forward this does not help very much. It might be interesting to see if the bad heat affects start a few inches downstream of the motor.

The turbojet exhaust is probably much lower than the rocket exhaust. There is so much nitrogen from the air that does not contribute to the combustion temperature, yet, is still being heated to act as a working fluid.

G40 throat is about 1/8". The nozzle is 3/8" wide. My tailpipe is fine for the first half of its length (~7"). The bad distortion starts in the last third, about 10" from the nozzle.

 

[burkefj]

I'm curious how much you are saving, a G-40 is only 123 grams, and the nose is about as far from the CG as the rear end, so would cost you at most 123 grams of weight, versus the weight of the tail pipe and lining and any thrust reduction, you would have the CG shift, but the propellent weight is only 53 grams, in a large model like this I would think it would be trimmable and be very very much simpler, but I usually take the easy way out... It would also allow you to block off the intakes and get rid of some of the surface friction drag in glide that you'd have which in a high performance model like this might be worthwhile and be an interesting trade-off.maybe you just want to experiment which I understand.

 

[TheAviator]

Yes. Then again, this is a prototype. It's easier for me to adapt an existing airframe than design a new one. If I can make this idea work, I'd want an airframe with the motor way in the nose, MiG-21-style. That would improve CG distribution.

Apparently, kerosene burns at about 600C. That's a lot cooler than our motors. I'm less concerned about corrosion and particulates. There is white residue clearly visible inside the pipe, but it cleans off. The heat deformation is the gating factor right now.

A stoichiometric kerosene flame temperature is right around 2000 C. Clearly this would melt anything it came into contact with, so jet engines run quite lean to lower the turbine inlet temperature (TIT), typically to around 1200-1400 C for high performance engines. This number drops through each turbine stage, but I don't know what a typical turbine exhaust temperature (TET) is off the top of my head. I could see the TIT/TET for model jet engines being significantly lower for a variety of reasons, 600 C TET might be reasonable.

A standard aluminized composite propellant might have a flame temperature in the neighborhood of 2500-3000 C. The biggest difference between this and the jet engine though, is that the rocket exhaust isn't diluted as much by air and so will still be quite hot when it hits the duct. I would also wager that significant portion of your heat stress is generated by the aluminum oxide in the exhaust depositing on the duct walls and transferring a lot of heat that way. That's part of the white crud you can see on the duct walls. You'd probably have better luck with a low-smoke propellant like Classic or Blue Thunder.

Also, bear in mind that APCP propellants have a lot of free chlorine and hydrogen chloride in the exhaust, so that's going to corrode most duct materials pretty quickly.

 

[iter]

I'm curious how much you are saving, a G-40 is only 123 grams, and the nose is about as far from the CG as the rear end, so would cost you at most 123 grams of weight, versus the weight of the tail pipe and lining and any thrust reduction, you would have the CG shift, but the propellent weight is only 53 grams, in a large model like this I would think it would be trimmable and be very very much simpler, but I usually take the easy way out... It would also allow you to block off the intakes and get rid of some of the surface friction drag in glide that you'd have which in a high performance model like this might be worthwhile and be an interesting trade-off.maybe you just want to experiment which I understand.

You're absolutely right. In fact, if I can't make the duct work, I'm thinking of putting an MMT in the back of this airplane like you describe. My purpose in running this experiment is to find if ducting works. G40 is just a nice little motor to figure out what works. On a model this size, imagine a J33 way in the nose. (The Gary flew at Hamster Dance). Or a K67. It's the wrong airframe for this setup, but this is what I'm aiming at if I can make the ducting work.

 

[GlenP]

Is there any way to let some cooling air into the tube upstream of the engine? That probably won’t really help much, but another approach is to make part of the tube perforated with small holes. That will help cool the tube itself, but will also let more heat escape.

 

[iter]

would also wager that significant portion of your heat stress is generated by the aluminum oxide in the exhaust depositing on the duct walls and transferring a lot of heat that way. That's part of the white crud you can see on the duct walls. You'd probably have better luck with a low-smoke propellant like Classic or Blue Thunder.

This is a fascinating idea. Empirically, the mechanical distortion coincides with the white deposits. The idea that the heat transfer occurs from residue as much as (or more than) from gases that flow past has interesting implications. What if I cover the inside with something non-stick like kapton? Or conversely, with something heat-dissipating like grease?

 

[iter]

Is there any way to let some cooling air into the tube upstream of the engine? That probably won’t really help much, but another approach is to make part of the tube perforated with small holes. That will help cool the tube itself, but will also let more heat escape.

There is airflow though the original air intake past the motor.

 

[TheAviator]

This is a fascinating idea. Empirically, the mechanical distortion coincides with the white deposits. The idea that the heat transfer occurs from residue as much as (or more than) from gases that flow past has interesting implications. What if I cover the inside with something non-stick like kapton? Or conversely, with something heat-dissipating like grease?

Funnily enough, I think your best bet would be to line the metal tube with a piece of paper or phenolic body tube. The char that forms carries the heat away as it ablates and it would prevent the deposits from forming on the metal. I would try paper first and that isn't enough, either go with thicker paper or try phenolic.

 

[rocket_troy]

The char that forms carries the heat away as it ablates and it would prevent the deposits from forming on the metal. I would try paper first and that isn't enough, either go with thicker paper or try phenolic

Another option could be either a paper-glass composite (eg. cardboard but with much more waterglass than typical) or an aramid-glass composite.

TP

 

[GlenP]

Maybe try some of that Aluminum dryer duct tape in the hot spots?

 

[TheAviator]

Another option could be either a paper-glass composite (eg. cardboard but with much more waterglass than typical) or an aramid-glass composite.
TP

Composite materials would probably work, but the goal of the liner is to be an insulator and ablative, which means it would need to be replaced every so often. I'd make a consumable as cheap as possible if I could.

Maybe try some of that Aluminum dryer duct tape in the hot spots?

Aluminum tape works well if the heat transfer is primarily radiative, as the aluminum reflects the infrared heat back away from the surface. However, as demonstrated by the slag build-up, the transfer here is primarily conductive and convective, in which case, the thin aluminum tape will just conduct the heat into the pipe anyways. A paper or phenolic tube would keep the heat down in three ways:

1. Conduction. Paper and phenolic are thermal insulators, they don't easily pass heat from one surface to the other. This is a similar principle to wrapping your copper hot water lines with foam to keep the heat in.
2. Charring. It takes a fair amount of heat to turn the liner into carbon char. This conversion absorbs some of the heat and lowers the surface temperature of the liner tube thereby lowering conduction through the liner.
3. Ablation. The fast exhaust stream can strip some of the hot charred surface away from the liner and exhaust it out the back, removing heat from the inside of the liner. Again, this lowers the temperature of the inside surface of the liner, lowering the conduction of heat through the liner. You need to be careful with this, though, because you can ablate all of the liner and end up back at the original problem.

 

[burkefj]

curious how you are doing ignition, long wires up the spout or onboard?

 

[iter]

Funnily enough, I think your best bet would be to line the metal tube with a piece of paper or phenolic body tube. The char that forms carries the heat away as it ablates and it would prevent the deposits from forming on the metal. I would try paper first and that isn't enough, either go with thicker paper or try phenolic.

Looking for a tube that would fit inside the stainless tailpipe, it looks like Aerotech 54mm liners are just about the right OD. They have phenolic in different lengths but the paper liner only comes in one-grain length from RCS. I ordered the 54/1706 phenolic liner which is about the right length. It says 6.4 grams/inch, so around 86 grams.

 

[iter]

I'd make a consumable as cheap as possible if I could.

Yes, what you said.

as demonstrated by the slag build-up, the transfer here is primarily conductive and convective

I noticed something interesting. I tried to unbend the pipe by hand and noticed that the slag readily flakes off the stainless steel, exposing clean surface. It did not corrode the pipe or pit it or react with it.

 

[iter]

curious how you are doing ignition, long wires up the spout or onboard?

Long wires. I was going to use the stock speed controller, it's a nice unit with battery voltage telemetry. (you can use brushless controllers to drive resistive loads, just use any two outputs). The problem I ran into is that the ECS beeps as part of its startup sequence and when you arm it. It accomplishes the beeping by driving current through motor windings, and it's enough to set off an igniter. So I decided to use the simpler solution for now and use my hot box. It has a receiver bound to the same transmitter I use for flying, and I have a locking switch on the TX to control arming and ignition.

 

[burkefj]

Cool, Curious about the brushless esc, I thought that if they didn't detect windings it would not arm the esc, interesting about the current being enough to blast the igniter...

Long wires. I was going to use the stock speed controller, it's a nice unit with battery voltage telemetry. (you can use brushless controllers to drive resistive loads, just use any two outputs). The problem I ran into is that the ECS beeps as part of its startup sequence and when you arm it. It accomplishes the beeping by driving current through motor windings, and it's enough to set off an igniter. So I decided to use the simpler solution for now and use my hot box. It has a receiver bound to the same transmitter I use for flying, and I have a locking switch on the TX to control arming and ignition.