Will my engine "nozzle" get torched?

I didn't really know where to put this, so I put it here because this is a scratch build, sorry if it should be somewhere else!

Anyway, I am in the final stages of building a low-power rocket modeled after the game "Kerbal Space Program". Her it is balancing on its nose:


My one question is about the black engine "nozzle". I am worried that it will be torched by the engine exhaust. Here is a closeup of the engine mount inside the nozzle:


Is this too close? I planned on using C engines for it, if that matters.

It will get torched. Coat it with epoxy, may make cleanup easier.

jimzcatz said:

It will get torched. Coat it with epoxy, may make cleanup easier.

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Ok thanks. I was worried because I don't want to create a fire hazard with flaming paper falling from the sky

Wait, what did you mean about "cleanup"?

Thanks!

Not to be disrespectful, but are you sure that the exhaust would expand enough to catch the nozzle on fire? Here is another shot of it with the engine loaded if that helps.



And sorry for three posts in a row

The exhaust won't hurt the nozzle. That doesn't look much different than the back end of a Big Bertha with an 18mm motor mount in a BT-60 body tube.

It may get some soot on it at lift-off, but you can help prevent that by providing a bit of a stand-off so that it is not right on the blast deflector at ignition.

-- Roger

I find even with flat bottoms of rockets, with the engine protruding anywhere from 1/2" to an inch from the bottom, that the bottom of the rocket gets torched. Figure that the air flowing past this part of the rocket may trap heat and smoke in that area.

Thanks guys! I will take your advice and make sure the rocket isn't right on the blast deflector. And Jeff, I have never had those results myself, I only notice the bottom of my rocket getting torched if it is literally sitting on the blast deflector

+1 coating the nozzle inside with epoxy, depending on the depth of the motor inside the nozzle the "Krushnic Effect" may come into play which if it does will scorch or destroy the nozzle. There are several threads about the Krushnic effect, and how it can also reduce thrust.This link is to the NAR's Tech Report about the Krushnic Effect its a PDF https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCIQFjAAahUKEwjnwr3T8PLGAhXB0IAKHSEXCLg&url=http%3A%2F%2Fwww.nar.org%2Fpdf%2FTCR1.pdf&ei=TLixVefsJsGhgwShrqDACw&usg=AFQjCNGvWDpAQwZKkFGQIzRAe_aVHPJxTw&bvm=bv.98476267,d.eXY

Krushnic effect is a concern but also research "Bernoulli Lock"-a phenomenon that essentially 'sucks' the rocket to the launch pad thru fluid dynamics that creates a low pressure zone so it 'locks' the bird on the pad-sometimes long enough to drain the alititude to mere feet.

Another option is to just make it removable and replace it when needed. It will get "torched" to a certain extent (and possibly get damaged upon landing) regardless.

Coating the nozzle with epoxy may help to keep it clean but I doubt that you need to be concerned about the nozzle catching fire. The exhaust exits in a cone that is much narrower than the nozzle that you have and I believe that it is unlikely to cause a problem.

Based on the width to length, I also don't think that you have to worry about Krushnic effect. The exhaust cone should be enough smaller than the aft end of the nozzle for this to be an issue.

I agree with the idea to coat with epoxy. It will probably help and not hurt and it is readily available. Years ago, I sometimes used spray barbecue paint on The Launch Pad models, which had recessed motors. It seemed to work, but maybe nothing at all work, too. I don't know if spray barbecue paint is readily available. Recently, I have used sodium silicate solution (waterglass). This appears to work, too, but it is expensive and it takes time to order. I bought a bottle a few years ago from a website for $7.50 and it cost about that much to ship it. I consider it a lifetime supply.

It may be possible with that geometry for the flow to transition to behaving like it has a seriously overexpanded nozzle. If this happens, thrust will have a significant reduction as exhaust velocity will experience a large drop.

Gerald

I believe it will get a little crispy in there, so some kind of coating is advisable (looks like a D12 loaded in there ?). I think rocketgeek101 is correct also; there is a better than 0% chance of a ding on landing. That said I really like the design.

Thinking outside the box, one other option.
Place the engine block a little bit more tailward, so that when the engine is in place, the motor nozzle is at the level of the bottom of your cosmetic nozzle. This also will shift your CG back a bit, so make sure your rocket has enough stability (fin area) to handle this or you get skywriting. This should solve both any burn problems to the nozzle and prevent Krushnic effect.

Also use a standoff to prevent blow back from your blast plate as mentioned.

samb said:

I believe it will get a little crispy in there, so some kind of coating is advisable (looks like a D12 loaded in there ?). I think rocketgeek101 is correct also; there is a better than 0% chance of a ding on landing. That said I really like the design.

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That is a C motor, and this is a rear-eject rocket so it shouldn't get damaged in landing.

Thank you all for you advice. I think I will see how it goes and then take it from there. The Kerbal way!

FishInferno said:

That is a C motor, and this is a rear-eject rocket so it shouldn't get damaged in landing.

Thank you all for you advice. I think I will see how it goes and then take it from there. The Kerbal way!

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That's the way to do it! There is a lot of good advice here but the final decision is yours. It doesn't count if we take all of the fun out of it for you

FishInferno said:

That is a C motor, and this is a rear-eject rocket so it shouldn't get damaged in landing.

Thank you all for you advice. I think I will see how it goes and then take it from there. The Kerbal way!

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Nice, more pictures Please !

one could I suppose use a paper punch and put in some holes (4 - 6) about even with the aft end of the motor. that would help reduce the 'krushnic' effect(and drag).
Rex

I don't think it will get torched. Maybe a little been after multiple launches

You can always use some HVAC Aluminum Tape on the inside.

I use it a lot, and sometimes on the TLP kits that have the motor mount recessed into the body tube further than you would think is healthy.









It's only exposed to the heat for a short time, and it holds up great.
Just make sure you get decent stuff like the 3M brand.