Fore and Aft Motor mounts?

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Well-Known Member
Jan 20, 2009
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I'm still a newbie when it comes to building rockets, but from what I do know, we try to keep the CG's forward and the CP's aft, right? So what happens if we put a pair of smaller engines forward, angled outwards (like a Deuce's Wild) with a single larger engine aft (Traditional style)? The amount of drag from a forward-mounted angled motor wouldn't push the CP that much farther forward, and the forward motor weight would certainly help counter that.

Is there anything out there that says this idea WOULDN'T work? The recovery system would be center mounted, with the forward engines having no ejection charges.

I'm a bit under the weather at the moment, and codeine has induced a fog state - so if this is totally out of character for me - I'll apologize in advance.

In deference to forward mounted engines, your largest concern would be twofold:

First, the position of the CG as the motors loose their propellant weight would be critical. Thus, building a model with this type of forward type of motors would need threefold testing:

1) Swing test with no motor casings.

2) Swing test with empty motor casings.

3) Swing test with full motor casings.

This will help you see how stable/unstable the design is in each phase of flight.

Sure, it may be a small matter that the rocket is marginally stable once the forward engines burn out - and in further thinking this line of thought, I'd bet that IF you can keep the rocket stable through burn, and it becomes marginally stable near apogee/motor burnout - you should be safe.

Secondly, I'd certainly think a rocket of this design would indeed NOT be a candidate for multiple staging, and in addition, your also looking at a clustered upper section, which means you need a highly reliable launch system to ensure all motors light - or indeed using an air started type of system to further ensure proper ignition.

If not, even with a Duece style design compliment above the CG, you could have a stable rocket go unstable quickly. That said, though I've not seen a duece fly, I'm betting a single lit motor in a duece should still keep the rocket stable through flight.

Sorry if this post makes no sense. *blush*
I will add another test case to the list presented by Silverleaf:

4) swing test with empty cases forward and loaded (unfired) motor aft

This configuration should work fine if both forward motors fire at the same time. If not, things could become rather dicey. If you build it, you should probably conduct at least one test flight (in a REMOTE launch area) with all three motors installed buy only one of the two forward motors ignited, to see what flight characteristics you get in that case. To avoid a crash, you may have to design your test bird with ejection gases from the forward motors redirected to the rear to deploy your center fuselage recovery system.

A thought: you will have to be very careful about how you string your ignition leads to the forward motors so as not to foul or snag the main fins. You may have to use auxiliary towers to the sides to hold the wires out of the way.
Actually, I hadn't considered the separate launch towers for the top motor mounts - I was planning on running a conduit up the side of the rocket with a hard-mounted plug-and-socket configuration at the mid-body separation point. The power for the launch is a car battery, and that's worked very well with up to 4 simultaneous ignitions, and I've seen people ignite up to 7 at a time on the same launch system.

I appreciate the info here... any other thoughts?

I have a design I am optimizing using RockSim version 7 software (see attached file) that has just the two forward motors. Your design should work fine. I wouldn't be overly concerned about not having one (or both) of the forward motors ignite, the design should still fly stable since all the weight is up front! I would try to keep the motors canted to less than 20 degrees from vertical though. Think of the design as a large bottle rocket firework; besides being asymmetrical in design, many of the bottle rockets have canted motors or a vectored thrust, and fly stable.

Bruce S. Levison, NAR #69055
cool... i hadn't noticed this thread before... :)

One other concern that I would have...

...if the forward motors are *forward* of the separation point, you want to be sure that the delta-V isn't so great that it pulls the sections apart under boost.

Will the single rearward motor have a higher thrust curve than the combined cluster in the front? If not, then you have the chance of the cluster pulling the sections apart.

it *would* be interesting...

...but it wouldn't be *optimal*... :D
..if the forward motors are *forward* of the separation point, you want to be sure that the delta-V isn't so great that it pulls the sections apart under boost.
That is exactly what i thought of when i first read his post:eek:
i've been thinking of doing something similar myself, with two canted forward motors. of course i do allot of *thinking*. i wanted to have two forward and two aft motors tho, say two B6s up top and then 2 A8s 'down under'. the heavier B6s would keep the CG at a good position, and since the A8s (assuming they both ignite :rolleyes:) would burn out faster the B6s the rocket would get more stable during thrust of the B6s.
So far as recovery goes, the 'cute would be ejected by the upper motors frm the NC... the lower motors would either eject themselves of vet out some holes in the BT just above the lower motor tubes.

who knows when i'll get around to doing this, so if u wana try it have fun:)... its so cool how the duce has really led so many people to get creative, aint it??
I am planning to use a burn string (hook and line in the sharks mouth!) in my design that burns through upon ejection to release the nosecone to avoid nose cone sepration during boost.

Bruce S. Levison, NAR #69055