Krushnic Effect

[TRFfan]

Does this still occur in tailcones that have the motor recessed less than 1 tube diameter?

 

[Nytrunner]

It really shouldn't.
At that point I'd be more worried about the tailcone melting (if it was plastic)

 

[mpitfield]

Does this still occur in tailcones that have the motor recessed less than 1 tube diameter?

This is something I would like to experiment with at some point.

I only have experience with the GLR and Aero Pack tailcones, and they are both fine. I have experienced "performance anomalies" with the GLR bell retainer. Well to be factual, I have a Madcow Tembo with the GLR bell retainer (the one in my avatar) and the simulation in both Rocksim and OR was way off. I can't recall the numbers but I believe it was close to 40%, which is atypical for my sims and was a bit of a surprise.

In all fairness I jumped on the Krushnic effect immediately as the cause, however I really need to do some more flights. At some point I will build two test rockets for this purpose and use two 54mm retainers that I have in my build box (GLR Bell retainer, GLR slimline retainer), then do some AB flights, average out the numbers and do a comparison.

 

[rstaff3]

If the base of the rocket converges to the motor mount diameter, then the diamter of the motor tube is what the recess should be based on, not the main BT diameter. Also remember there is nothing magic about that rule of thumb. There is some loss of performance before the 1 caliber point.

 

[watheyak]

I guess I have empirical evidence suggesting it has an effect below 1 caliber. It way under-performed all the sims. It was about .5 calibers recessed. I blamed it on asymmetric thrust from a G-65 c-slot motor. Maybe I still do. Hmm...

 

[TRFfan]

If the base of the rocket converges to the motor mount diameter, then the diamter of the motor tube is what the recess should be based on, not the main BT diameter. Also remember there is nothing magic about that rule of thumb. There is some loss of performance before the 1 caliber point.

Yea but that was what I'm really asking about. Like if I'm building a MD and the nozzle is some 7 mm from the end of the tailcone (the motor is 24mm wide) would that result in a loss of performance?

 

[TRFfan]

I guess I have empirical evidence suggesting it has an effect below 1 caliber. It way under-performed all the sims. It was about .5 calibers recessed. I blamed it on asymmetric thrust from a G-65 c-slot motor. Maybe I still do. Hmm...

The G65 has a moonburner core which can offbalance rockets that are on the light or small side (like a MD).

 

[rstaff3]

Yea but that was what I'm really asking about. Like if I'm building a MD and the nozzle is some 7 mm from the end of the tailcone (the motor is 24mm wide) would that result in a loss of performance?

I guess I'm a bit slow. The recess rule of thumb should be based on the size of the hole in the base of the rocket. If the hole in the tail cone is 24mm, the recess should be no more than 24mm. And has been mentioned a couple of times, the degredation of performance begins before that. However, GHS must have thought that the loss wasn't that significant until the recess is one caliber. YMMV

 

[TRFfan]

I guess I'm a bit slow. The recess rule of thumb should be based on the size of the hole in the base of the rocket. If the hole in the tail cone is 24mm, the recess should be no more than 24mm. And has been mentioned a couple of times, the degredation of performance begins before that. However, GHS must have thought that the loss wasn't that significant until the recess is one caliber. YMMV

The hole is about 19 mm and the recess is 7mm. There shouldn't be too much of an altitude loss, should there?

 

[rstaff3]

I would assume not. That means I'd fly it that way. My advice comes with no warranty though :dark:

 

[tim cubbedge]

The hole is about 19 mm and the recess is 7mm. There shouldn't be too much of an altitude loss, should there?

Instead of all this speculation, why not build it, fly it and see what the performance loss is?

 

[TRFfan]

Instead of all this speculation, why not build it, fly it and see what the performance loss is?

Because I want to find out beforehand. I dont really want to spend too much money on reloads flying it again and again. And on top of that if the part isnt what Im looking for I'm going to have to make another one.

 

[tim cubbedge]

Because I want to find out beforehand. I dont really want to spend too much money on reloads flying it again and again. And on top of that if the part isnt what Im looking for I'm going to have to make another one.

What's wrong with flying it over again? Most people fly there rockets dozens of times. If you're after a record, more than likely you'll build it several times before you get it right.

 

[markkoelsch]

I think you likely would be better off having the tail one terminate at the motor mount diameter. I understand what you are after, but I think you will see a performance drop that might negate the gains of the smaller aft exit. Also, you might well melt the tail cone.

 

[rstaff3]

I think you likely would be better off having the tail one terminate at the motor mount diameter. I u dears tans what you are after, but I think you will see a performance drop that might negate the gains of the smaller aft exit. Also, you might well melt the tail cone.

+1 this would be best

 

[watheyak]

The G65 has a moonburner core which can offbalance rockets that are on the light or small side (like a MD).

It's not a moon burner. I think you learned about what you call "offbalance" from an old post of mine.

 

[watheyak]

Because I want to find out beforehand. I dont really want to spend too much money on reloads flying it again and again. And on top of that if the part isnt what Im looking for I'm going to have to make another one.

Part of going for max altitude is not being lazy. You're going to have to fly it over and over again. It's never right the first time. And when it is right, you'll have a boost that doesn't fly straight or a crappy ignition. Persistence is the key. There is no instant gratification in what you want to do!

I lit 5 G-65s before I decided they sucked.

 

[TRFfan]

Part of going for max altitude is not being lazy. You're going to have to fly it over and over again. It's never right the first time. And when it is right, you'll have a boost that doesn't fly straight or a crappy ignition. Persistence is the key. There is no instant gratification in what you want to do!

I lit 5 G-65s before I decided they sucked.

That is true. I am prepared to launch it more than once but i want to make sure that the design is proper before i build it.

 

[TRFfan]

I think you likely would be better off having the tail one terminate at the motor mount diameter. I u dears tans what you are after, but I think you will see a performance drop that might negate the gains of the smaller aft exit. Also, you might well melt the tail cone.

Yea probably that is what i am thinking of doing.

 

[watheyak]

I feel that reality may have been left behind in your planning. Krushnic is not the problem here. In your many other threads scattered about this forum, you have acknowledged that the custom aft closure that would be needed to accommodate this boattail would both invalidate the record you're trying for AND require a level 2 cert to fly, due to the EX nature.

Rather than adjust your approach to something that is actually possible, you start new threads where well meaning people offer good advice in vain. Any time a hard truth about your approach comes up, you either completely ignore that post or move on to an entirely new thread, duping more unsuspecting and well meaning people into your charade.

If you really want to set this record, you can. The CAR record low enough that you don't need carbon fiber biconvex airfoiled fins, boattails and custom closures. You can beat that record with a cardboard tube and balsa fins. No, it's not sexy, but it's real.

 

[TRFfan]

I feel that reality may have been left behind in your planning. Krushnic is not the problem here. In your many other threads scattered about this forum, you have acknowledged that the custom aft closure that would be needed to accommodate this boattail would both invalidate the record you're trying for AND require a level 2 cert to fly, due to the EX nature.

Rather than adjust your approach to something that is actually possible, you start new threads where well meaning people offer good advice in vain. Any time a hard truth about your approach comes up, you either completely ignore that post or move on to an entirely new thread, duping more unsuspecting and well meaning people into your charade.

If you really want to set this record, you can. The CAR record low enough that you don't need carbon fiber biconvex airfoiled fins, boattails and custom closures. You can beat that record with a cardboard tube and balsa fins. No, it's not sexy, but it's real.

Yea i am getting a normal rear closure for the rocket but i have a friend who is level 2..so if he preps the motor than maybe I can fly it that way even though i wouldnt get the official record it still would be nice to see a G motor hit ~11000 ft.

That being said I will try for the official record this year with the normal closure but maybe next year i can go for the unofficial one.

 

[markkoelsch]

Why not just get a level 2 cert, and do it yourself?

 

[Steve Shannon]

Why not just get a level 2 cert, and do it yourself?

According to the "Oldest Rocketeer" thread he's 14 years old.


Steve Shannon

 

[markkoelsch]

According to the "Oldest Rocketeer" thread he's 14 years old.


Steve Shannon

That explains the fragmented threads.