C6-0 make a C6-P

NO.

Inserting a smaller motor into the top of a larger motor would not be good. Reason: Nozzle on smaller motor is smaller than the nozzle on the larger motor, so you would not get a properly choked nozzle and you would get less thrust and less total impulse.

The point of tandem motors was to take two identically nozzled motors and epoxy them end to end with a reinforcement sleeve epoxied over the outside (like a C6-0 with a C6-7 on top). The sleeve was typically a CMR RB74 and the airframe then had to be RB77.

The firing of the booster motor produced normal results but when the upper stage ignited the thrust curve was 10% higher. Why (when the nozzles are identical)? Because the firing of the upper motor into the lower casing would scour away the inside of the lower casing nozzle. This additional mass being accelerated out the lower nozzle produced more thrust. F=m*a

Of course, it also resulted in lots of motor failures (remember Challenger?) where the lower casing burned through the sidewall.

-Fred Shecter NAR 20117

I did the opposite one time. Took an E9-P and made it an EP-0 to put into a Quest Zenith II. Had a ceramic plug and used a twist drill by hand to remove the plug by going down to the black powder. Launched and successfully recovered. Sim said 2700-2800 feet with a C6-7 sustainer. Note Well! I did this on my time at an isolated field, by myself and NOT at an organized launch. Was a cool flight but will not do it again as the risk of losing such a small model is so great.

I suspect you could achieve the same thing with some sort of heat resistant plug and a high temperature adhesive. Durham's water putty might make a satisfactory plug material follow by a glob of J & B weld. One has to remember the guy is going to use a C6-0 with no ejection charge. One only has to worry about burn through. My removal by hand of the plug in the E9-P was dirt easy and the plug actually didn't seem to be that strong.

Kurt Savegnago

Boy,

That is very novel Chad and if you've proved it, that would be the way to go to stay "legal".

Kurt Savegnago

Yes, I said this, but in reference to 1/4A BG, not to C BG.

But most (all but the PRK?) states incorporate NFPA 1122 into their state laws, so violating the NAR safety code may convert your legal model rocket into prohibited fireworks...

But now you can get A10-P motors that are already plugged. MANY years ago, Estes made a plugged B motor, for "static test" demos.

Booster motor burnout is pretty violent. That's what throws the particles forward to ignite the upper stage. It will eject a parachute or kick an empty casing. I would not think that some wadding in the end of the motor would be sufficient.

BTW, that extra propellant in a booster motor was also retained by the Tandem process, providing a bit extra thrust. I don't know if anyone ever determined how much of the extra thrust came from that propellant that wasn't lost int he staging process, and how much came from booster erosion by the upper stage.

I put tandem motors on the MIT test stand and had the curves.

There was a very short and not that violent blip when the booster motor burned through and then a slight dip as the booster propellant was completely consumed and the upper stage was just coming up to thrust.

The perfect example of the burn through spike that Bob is trying to describe is the AVI Gold Series motors. Let's look at two examples that used a looong 18mm casing and a looong 24 mm casing. They were basically booster motors and then they had a seperate smaller casing containing a delay train and ejection charge that was epoxied into the top. When first produced, the motors were extremely long and the 18mm was a full 20 Ns and the 24 mm was a full 40 Ns (unless I'm remembering wrong and they were just short of "full"). Anyway, when they burned, the casings got really thin from erosion. Not as bad as if they had been tandems, but still extremely thin. Some burned through the sidewall, but MANY blew through the sidewall as the propellant burned out. The forward face of the propellant would ignite as soon as the flame front blew through and since the aft face was already burning, it had a momentary spike of at least double the thrust. Since it was not open on top, the pressure equalized almost instantly on both sides of the rupture, so you did not get giant chunks of propellant breaking off and sending the peak thrust/pressure explosively high. However, the peak was enough to blow out the casing at it's thinnest.

Solution: they de-rated the motors, cutting about 5 Ns off each one and making the casings shorter. They also painted inhibitor on most of the forward face of the propellant before epoxying in the delay/ejection housing.

Estes had to do some casing magic to get the E9 motor to work with the long burn time, but they did not have to worry about the burnout spike, since they have integral delays.

-Fred Shecter NAR 20117

Hey Bob, Chad said that he did that with his boost glider and it never failed for him. If it works every time for him and he is not lying then I don't know how he could be refuted.

Kurt Savegnago

You are so right Bob. Wally Mart has them for just a few drachma per 4 pack so why bother to use anything else. I'm with you on this one. Didn't know about the static motors.

Tandem motors worked great. Many used them in competition even, that is of course what drove their design.

Some didn't like them. Once in a while, they might have gotten a burn thru, but for the most part worked great.

The NAR Model Racketeer published a nice report on their use and benefits in

1974. I'll post this article scanned sometime in the next 7 days for those who are interested, as I still have it.

The manufacture knew and looked the other way, it didn't bother them.

Someone(s) around 1977(?) who didn't like tandems finally asked estes if they had Estes' permission to make tadem motors. Estes of course has to say no for liability reasons, and hence they got banned from competition, which was what a few had wanted.

I would be very interested in that article. I'll be looking for it.

True, I've never had a failure. Now, I haven't flown 100 motors like this, but have done multiple. Will it eventually fail....yes, everything will. The key is packing the wadding as tight as possible AND several layers of tape.

I'm probably gonna jinx myself by posting this. I can see it now, some contest has C RG and I do this and completely toast my radio gear. :) Just because I opened my mouth and said it has worked. Always watch out for Murphy...

chad

WWHD?

bob:

do you seriously believe that anybody other than the approximate 8300 NAR/TRA members have any idea that a NAR Model Rockety Safety code even exists? The fact that the NAR has never had more than approximately 6000 members MAX even in its late 60's-early 70's "golden dayz" points out that the majority of people who do and have done model rocketry have no clue the NAR much less the NAR Safety code even exists. IF you don't know something exists, how can one "violate" it? . DO you think that the typical wal-Mart model rocket shooper is even aware that there is such a thing as NFPA 1122? I doubt it.

shockie B)

Ignorance of the law is not an excuse.

You can unknowingly violate a law.

-Kevin

Just as a point of order here, it would be very hard for anyone that buys an Estes kit to say they haven't seen the code. It's there every time you open the package and has been for a very long time.

Randy

True, but seeing the code and actually reading the code is whole other matter.

When I flying as a kid I NEVER read the code. Took too much time away from flying :)

Ted Novak TRA#5512 IEAS#75