Engine Mount Questions

Quick synopsis of the project...

I am kit bashing a Estes Star Orbiter into a sport scale rocket which is primarily going to consist of shorting the body and changing the fins. I would also like to fly it with Estes "D" engines instead of the "E" and "F" as originally designed and that is where my questions come in.

First, I have seen some engine mounts where the tube the engine slide in to is much longer than the engine, say 8 inches or so. However, the tube is not that long at all on the BT-60 to 24mm engine mounts I am finding on the web. Does the tube length on the rocket really matter, and if so, why? Also, would the length factor in at all if I wanted to install a baffle?

Second, would it be best to change the engine mount over to 24mm or would it be more advisable to keep the 29mm tube and use a plastic spacer assembly to fit the "D" engine? Certainly less work going the spacer route but which would be better in the long run?

Thanks,
-Bob

The longer motor tube reduces the internal volume that the ejection charge needs to pressurize to pop the nosecone and get the parachute out. On a BT-60 based model, I don't think it's going to be a problem for D motor, so you shouldn't need the extended tube.

The only argument for keeping the 29mm motor mount is to have the flexibility to fly the model on the larger engines in the future. If you are only going to use Estes D motors, then there's no need to keep the mount and make an adapter.

kj

In 29mm, the length of the motor mount typically doesn't matter too much. Most 29mm motors include a thrust ring at the back end, so the motor mount just needs to hold the motor straight.

In 24mm, it varies. Black powder motors do not have thrust rings while composites typically do, so if you plan to fly BP you should make the mount around 4" long. That way you have space for 24/60 and Estes E motors if you want- both are 3.75".

If it were me, I'd build with the stock 29mm mount and either build or buy an adapter. That way if you decide to really go insane you have the option to. Just make sure you're still stable with the modifications. My profile pic is my 3" Saturn 1B- my biggest regret with that build is putting a 24mm mount in it instead of 29mm.

Just curious- what are you kitbashing it to?

One possible issue that may influence the length of the motor mount tube, just slightly, is if you are planning to have through-the-wall (TTW) fins and the chord of the fin tab that extends through the wall requires a certain engine mount length tube. The slotted portion of the fin does not need to be the full length of the external fin root, but still you might need a certain length to mount to the engine mount for the inner portion that goes through the external tube slot. you probably don't need to worry about that for D power, surface mounted fins would work fine. (just realized that body tube is already slotted, so as long your internal engine mount lets you match up with those existing slots, you should be fine.)

Its a good idea as well to ditch the motor block ring as well, CTI makes some fun 24mm reloadable motors and the longest wont work with a motor block. If using BP motors that dont have thrust rings just wrap tape around the nozzle end and build up your own external thrust ring, works like a charm.

29mm engine casings can vary greatly in their lengths (We are talking about composite propellant motor casings). That is why 29mm motor tubes can be longer than the 29mm motors you are using. The 24mm motor casings do not vary in length as much. That's why the 24mm motor mounts you are talking about do not have tubes as long. If the long motor tube has a centering ring at the forward end then it might be a "stuffer" tube that kjohnson was alluding to. It reduces the volume of space that the ejection charge has to pressurize to pop the recovery gear out, and increases the reliability of deployment. The length of the motor tube may be a factor if it doesn't give you enough room to install the baffle and still leave enough room ahead for the recovery gear. Also baffles should be installed as far ahead in the tube as possible while still leaving comfortable room to pack the recovery gear. This is for stability reasons: you do not want to move the CG too far back and make the rocket unstable.
If you are certain that you will only fly this on a black powder D engine, then you can glue in a 24mm motor mount and thrust ring for that engine. You will not be able to use the longer E engine. (Well, you can use it but it would stick out past your motor retention system). If you want to leave open the possibility of flying the longer 24mm engines and 29mm engines in the future, then you can glue in a 29mm motor mount and leave out the thrust ring. Composite motors have a flange at the rear of the motor that acts as a rear thrust ring. The Estes 24 to 29mm red adapter also has this flange at the rear. Estes black powder 29mm motors will need to be taped with a narrow strip at the back to act as a rear thrust ring. Finally, if you don't have a motor retainer system, a wrap of tape on the nozzle end over the end of the motor tube and rear of the engine will keep your motor from kicking out at ejection. Many modelers use this system to fly the full range of 24 and 29mm motors.

Thank you all for the information. I'll share some info about me to let you know where I am coming from.

This is my first rocket in about 40 years so I don't want to get too "advanced" with it. I want to keep it fairly simple and not too expensive until I decide if it's something I like and want to pursue further or not. I was into r/c aircraft for quite awhile and I always did love building more than the flying but I just don't have the time, space and eyes to do that anymore.

LithoSphereRocketry: I am bashing it into a IQSY Tomahawk

GlenP: You are correct in that the tube is slotted, but it's only slotted for three fins and I need four. I need to lose some length anyway, so It's no big deal to cut those slots off and make some new ones.

Kuriran: This rocket came with some sort of plastic threaded part that screws on the end of the motor tube. There is no internal motor block. The instructions call out putting a ring of tape around the bottom of the motor like you wrote, then screw one the plastic thing to hold the engine in.

It seems a no-brainer to just keep the 29mm mount and toss an adapter to fly a "D" motor. The only worry really is if the extra weight of the adapter would make the rocket unstable. You know though, in thinking about it, I wonder what the difference in weight is between an Estes E engine versus a Estes D engine with the adapter. The "E" motors are physically much larger. I'll have to see if I can find the weights... It may be that there will be no difference...

Heck, it may not fly anyway with the changes I need to make. Those are: (1) rocket will be 36.5 tall instead of 45.2 and (2) it will have 4 fins instead of 3. The new fin is about the same area, but I am also changing from 1/8th inch balsa to 3/32nd basswood. I'll need to do the work up to make sure it's good to go once built.

Thanks again for all the info,
-Bob

So if I am reading the Estes catalog right, here are the motor weights:

D12-5 1.61 oz Size motor Hobby Lobby carries
E16-6* 2.92 oz Recommended
F15-8 3.69 oz Recommended

The only place I could find a weight for the Estes 9753 adapter was Amazon and it listed the package weight as 1.6 ounces. They come two to a package so less than .8 oz considering the package they come in. That would put the combined weight of the D12-5 plus the adapter under the weight of the smallest recommended motor (E16-6).

Sometimes I just need to talk things out to come to the logical conclusion Seems to me that sticking with the 29mm mount is the way to go.

Thanks again for all the help,
-Bob

A bit of removable nose weight can help stabilize for rear heavy motor selections.

A very useful tool is a free program called OpenRocket where you can quickly and virtually throw together rocket components, see the CP, get an estimate of the weight, and, most usefully, simulate a flight with a ton of different motors to see the CG shift and altitude predictions.