Determining length of MMT tube

[lalligood]

I'm currently working on a scratchbuild HPR project that I plan to fly on 38mm motors in the 240ns to 720ns range (like an AT H123/Pro38 H153 to AT J350/Pro38 J285 and everything in between). The total rocket length is 63" with 36" allocated to the main booster airframe. Flying this on hybrids is very unlikely.

How do you best calculate how much MMT tubing to use? What would be the shortest recommended with the maximum length motor? (Did I just ask the same question twice two different ways?!? )

Thanks for the help

 

[gerbs4me]

I'd cut it to 18" long, that way you have lenth for a J350, which is around 13" I think. If you want you could cut a little shorter I'm not sure how long the Pro38 casing is.

 

[rstaff3]

The norm is to have enough tube to hold the longest motor you plan to use. In reality, it doesn't matter that much. Your motor tube and centering rings only have to hold together and distribute the forces along the airframe. I have flown rockets with MMTs shorter than the motor but don't have stats or even rules of thumb. Other reasons to design with longer mounts exist (ie to reduce pressurized volume), but 12" should be enough for you IMHO. More won't hurt until you eat into the space for your chutes, etc. There are other considerations for not having a lot of extra tube. Probably doesn't apply here but if the motor tube is long, you rely on a forward ring to hold it together, AND the tube burns through due to excessive use , THEN you may have problems.

 

[BHP]

I've not flown hybrid motors nor do I know anything about them so this relates to my experience with solid fuel motors.

I've often wondered why kits come with such long motor tubes. It seems to me that anything longer than the span of the fins plus some extra for a fore CR is unnecessary. I've scratch built a 65" rocket with a 20" motor tube and a 10' rocket with a 28" motor tube (which happened to be what I had left of a 48" fiberglass tube). In my view the motor tube simply centers the motor and provides a means for the tranfer of the energy of the motor to the airframe. I've had many motors extend through the length of the MMT. It gives you more room to pack a 'chute, too.

The two mentioned rockets have a 4" MMT and have only been flown with 75 and 54mm motors. The reducers I make are 12" long and I've had no trouble since they simply center the motor.

IMO - just make your MMT as long as minimally necessary for a structurally sound airframe regardless of motor length. It reduces weight and frees up interior space.

BHP

 

[DynaSoar]

Originally posted by lalligood
I'm currently working on a scratchbuild HPR project that I plan to fly on 38mm motors in the 240ns to 720ns range (like an AT H123/Pro38 H153 to AT J350/Pro38 J285 and everything in between). The total rocket length is 63" with 36" allocated to the main booster airframe. Flying this on hybrids is very unlikely.

How do you best calculate how much MMT tubing to use? What would be the shortest recommended with the maximum length motor? (Did I just ask the same question twice two different ways?!? )

Thanks for the help

Build for the largest available 38mm casing in case you want to use. For everything smaller, get some 38mm tube coupler stock, and cut spacers (presuming you'll be using a motor block rather than risking it going through the roof).

The longest 38mm motor I see is an Ellis 38/910 (J330), which is 17 inches long.

If there's going to be a lot of volume above the engine (large diameter/long body) you might consider using a longer MMT tube and put the forward centering ring higher up. This minimizes the amount of volume that the ejection charge has to compress and make it less likely you'll have an incomplete recovery ejection. The only weight it adds is the extra length of tube beyond the minimum length (maybe 1 ounce per foot), and where it's added (and where the forward ring ends up) moves the CG forward. That's a good thing.

If the motor tube were long enough, you could even stuff in one of those stainless steel very coarse scrubbing pads, like AT uses in their recovery baffle system. I haven't put one of these in a scratch built yet, but when I do I'm going to make sure to build in an easy way to pull that thing out for cleaning. And I'll probably use copper instead.

 

[DumasBro2]

I have to side with BHP on this, the length isn’t really important as much as how it fits into the design of your rocket. I usually use longer motor tube to get the forward c/r further up the tube so I can reach it with my stubby little arms to attach the recovery harness to the hardware on the forward ring. Having the motor stick past the front of the motor tube isn’t a problem and I have done this with a monotube hybrid, as they are quite long as well as with some solids. It’s really what you prefer for your design.

Steve