NAR: Tandem Motors?

I seem to recall, back in the day, that people use to be able to glue C's into the backend of D's or something. Then it was banned. Is it permissible again?

I'm going to take a guess that the idea here is for the upper motor to be ignited when the lower motor burns out, then the upper motor's exhaust exits through the lower motor's nozzle. Further, I'm going to guess that gluing the motors together is modifying the motors and thus not allowed. Taping motors together, on the other hand, is certainly allowed because many multi-stage model rockets depend on it.

This is effectively a two-stage rocket whose booster is not going to separate. Therefore one workaround might be simply to not glue the motors together. Either have a two-stage rocket with some way of locking the booster so it doesn't separate, or have a double-length motor mount which can accommodate both motors and which has a retention system on the back to prevent the lower motor from being kicked out when the upper motor ignites.

It was banned and still is. I believe at the time the manufacturers were asked what they thought of it and the reply was that the casings weren't designed to see the long burn times that the lower motor would experience. A burn through would be possible.

A burn through was not only "possible" it was common. Very common.

The motors had to be of an identical nozzle size since putting a larger nozzle motor under a smaller nozzle motor would result in 'bad results'. Putting a smaller nozzle motor (like a C6) under a larger nozzle motor (like a B14 or C5) wouold result is "extremely bad results".

The idea behind tandem motors was that the upper stage motor would thrust through the lower stage motor and the hig speed exhaust would erode casing material and expel it through the booster nozzle along with the exhaust gasses from the upperr stage motor. Those gasses have to go somewhere, a the nozzles are the same, the thrust would be the same assuming minimal thermodynamic losses, BUT the added mass of the eroded casing would actually add 10% to the thrust. We tested them at MIT on the old test stand and the thrust time curves were interesting.

The booster motor casings would indeed burn through and one solution was to coat the booster motor and lower end of the upper motor with epoxy - LOTS of epoxy - and then slide a body tube over them and allow to cure. Yes, this solved the burn through problem "most" of the time, but it added thickness and weight and this was clearly motor modification and use contrary to the motor manufacturer's recommendation and intent.

Hence the prohibition since this is clearly motor modification and modification is already not allowed.

So, instead of wasting massive amounts of time and money epoxying C6 motors together and building special rockets with longer and larger diameter motor mounts to accommodate these Franken-motors, we simply buy an Aerotech D10 or D21 motor and use it.

Duh.

shreadvector said:

We tested them at MIT on the old test stand and the thrust time curves were interesting.

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I was hoping this thread would prompt the current East Coast S&T group to test the effect.
But if it's been done before, I'm sure I could prod good ol' Mr. Spad to tell us all about it at the next dinner after launch or the next club meeting he hijacked from the usual July meeting venue.

Shreadvectors post is dead on!
Tandem Motors were a Super BAD idea back in the day and Still are. Use a higher impulse motor or stage the model.

KennB said:

I was hoping this thread would prompt the current East Coast S&T group to test the effect.
But if it's been done before, I'm sure I could prod good ol' Mr. Spad to tell us all about it at the next dinner after launch or the next club meeting he hijacked from the usual July meeting venue.

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Remind me in enough time before the meeting for the epoxy to dry and we can put a motor on my stand and try it. Or you could just read the copy of Trip's report from many years ago that I have on my desk. More information but not as much fun.

billspad said:

It was banned and still is. I believe at the time the manufacturers were asked what they thought of it and the reply was that the casings weren't designed to see the long burn times that the lower motor would experience. A burn through would be possible.

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Correct. Tandems are motors being used in a manner not recommended by the manufacturer.

Do they stay connected or is this the same thing as CHAD staging?

Brent said:

Do they stay connected or is this the same thing as CHAD staging?

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They stay connected. As far as I know, CHAD staging is okay.

billspad said:

They stay connected. As far as I know, CHAD staging is okay.

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Well, as I understood it, "CHAD" (cheap and dirty) staging was just taping a booster engine to an over stable rocket so the booster motor didn't need fins and would just fall away after it lit the upper stage.