Multi-engine modular booster concept diagram | Booster | Forum

Here is a very simple conceptual diagram of my proposal for our modular booster unit, as discussed in this thread.  Sorry it's so simple, I don't have Rocket's impressive SketchUp skills yet.  Of course this diagram is not at all to scale.

Basically this diagrams shows that the body of the modular booster contains a number of thin hybrid engines (the exact number has been discussed briefly in the thread linked to earlier).  This approach lets us keep the length:diameter ratio of the fuel grains high (which is good) and lets us use differential thrusting to control the booster in flight, rather than more mechanically complicated gimballing of a single nozzle.

At the top is an avionics module which contains, amongst other things, an inertial measurement unit which enables a small onboard computer to be aware of the module's altitude, velocity and orientation.  This module is able to send control signals to the oxidiser valves and ignition units mid way down the body, so it can implement differential thrusting, attempt to restart stopped engines, or shut down all engines in an emergency situation (e.g. if the rocket is tilted say more than 10 or 20 degrees toward the ground than it is supposed to be at its current altitude, or if the rocket is tipping over very rapidly).

Optionally, there may be a parachute on top of the avionics unit so that we can recover the booster after burn out.  The actual body of the booster would probably be damaged too much by the landing to reuse safely, but the avionics module or at least parts of it may survive.  The oxidiser tanks would probably be reusable.  The combustion chambers may or may not be refurbishable, we would have to see.

In clusterings of these modular boosters, like Selene 2 or 3, the avionics modules of the individual modular boosters would need to be able to communicate with one another, since differential thrusting would be implemented on entire modular boosters.

If everybody likes this idea, maybe we should update the Wiki and when Rocket gets time he could do a SketchUp rendering?

Rocket-To-The-Moon said:

I assume that each modular booster would contain 7 individual hybrid motors (circle packing).

This came up in the thread where I originally posted the idea: I think we should wait on deciding the number on motors until we have a better idea of how much thrust we can expect to get out of one.  There is no point in maximising the volumetric efficiency with 7 of them if it turns out 4 will provide the amount of thrust that we want for an individual booster.

Also worth considering, if we have an even number of individual motors, we can compensate for one faiiling by throttling down the one that is radially opposite it – this will keep the overall thrust balanced.  With 7 or any other odd number ofmotors, we could not do this.

As for thrust issues on Selene 3, well, this is the big problem with deciding how many modular boosters will go in each stage before we have thrust numbers.  If we need less thrust in the final stage, we could consider either using a reducing conical adaptor between the second and third stages, so that the third stage is a cluster of, say, four modular boosters.  Alternatively, if each modular booster is designed to hold an even number of engines, we could produce lower thrust versions by simply leaving every second engine slot empty (or mounting inert but massive "balast" rods in their places if we need more mass for stability).  That would be another advantage of an even number of engines.

Ah, true enough.  Scratch that as a compelling argument, then.

I still think the thrust output of individual engines will be the most important factor in determining the total number to cluster.

I'm not too sure what other arrangements could work.  Having our modular booster be one big single hybrid engine would probably result in a lower length:diameter ratio than is ideal.  There's also the complexity of gimballing.  Admittedly, the differential thrusting for my proposal could get complex too with trying to keep weight evenly distributed and correct for malfunction motors – but that is algorithmic complexity, which is much more amenable to being subdued by distributed open source efforts than the mechanical complexity of gimballing.

My main opposition to fins (aside from wondering how well they will work at higher altitudes and whether or not we will need them there) is one of the problems I have with gimballing – the knowledge is not reusable.  Nozzle gimballing and fin control would both be acceptable solutions for single stage / low control rockets, but by the time we scale up to Selene 3 there would be more sensible options available (differential thrusting).  I think we should avoid "wasting" effort on technology that we won't use a lot.  The nice thing about this proposal is that differential thrusting is all we ever need – we can focus on learning to do one thing and do it well.

The electromagnetic steering option sounds very cool, but at first consideration I am skeptical that it could be made less complex than the many-engines-per-module proposal.  Do you know of any commercial rockets that do this?  Particularly chemical rockets?  This would obviously be a great approach for ion engines, but with chemical rockets there are heat issues – what would we wind our coils from that is both a good electrical conductor, light weight and won't melt when in close proximity to a chemical rocket nozzle?

Probably best to put this proposal on the "definitely worth considering later down the road" list and wait until we have some more motor experience from OHKLA or some good theoretical modelling abilities to see if it will work better than other proposals.

Just a quick idea to think about when/if we do revisit this concept later when we have some experience from OHKLA: Rather than having a cluster of individual oxidiser tanks above the cluster of fuel grains, with one for each engine, perhaps we could just have one large common tank with several valves on it.

Pros: More efficient use of space; Less mass contributed by oxidiser tank walls; Easier to fill the rocket with oxidiser before launch – just one tank instead of n.

Cons: If a fault is found with one of the valves, the entire common tank may need to be swapped out for another one, as opposed to just one of the individual tanks.