RCS - too complex for a hobby rocket?

roid · Post by **roid** » Wed May 04, 2005 11:30 am

has anyone heard of an RCS (Reaction Control System) being used on a model rocket/aircraft? like a hobby rocket?

basically an RCS is what they actually use on spacecraft to steer them. it's little rockets in strategic points in a craft that fire short bursts to control yaw/pitch/roll.

as you can see from the picture - a similar method is used to steer the harrier jet while it's hovering, since it's control surfaces (rudders/flaps etc) don't have enough air flowing past them to do anything while it's hovering. so it shoots pressurised air outof nozzles in the wings/tail etc.

to quote:

In addition to the vectoring engine nozzles, the Harrier also requires a method of controlling its attitude during jet-borne flight, when the normal aerodynamic surfaces are ineffective. To this end, a system of reaction control nozzles in the nose (blowing down), wingtips (blowing up and down) and tail (down and lateral blowing) are fitted to the aircraft (Fig. 4). These nozzles are supplied with high pressure air bled from the engine and are operated by the normal flying controls. Pilot command operate valves in each nozzle that allow powerful jets of compressed air to provide the desired movement in pitch, roll or yaw. The system is energized once the engine nozzles have been partially vectored, with the amount of engine bleed air increasing with reducing airspeed, allowing for the seamless passing of authority from aerodynamic to reaction controls. This frees the pilot from any excessive workload during the transition to and from conventional flight, increasing aircraft safety.

so has anyone (maybe one of you model rocket or model aeroplane builders) ever heard of a complex system like this being used in a hobby plane or rocket?

i'm trying to figure out if it can be done

Stryker · Post by **Stryker** » Wed May 04, 2005 11:33 am

Why? So you can make a Lander ship?

roid · Post by **roid** » Wed May 04, 2005 11:50 am

maybe

MD-2389 · Post by **MD-2389** » Wed May 04, 2005 1:44 pm

I personally haven't seen it being done, mainly because of the added weight involved for such a system.

AceCombat · Post by **AceCombat** » Wed May 04, 2005 2:10 pm

ill have to dig for it, but i remember seeing something of this nature made for models. but it was very complex and required massive amounts of hours to get it fully operating properly.

MD is right also, it would increase the vehicles weight immensly.

snoopy · Post by **snoopy** » Wed May 04, 2005 2:53 pm

I don't know... but here's my thoughts on it. First, if you are planning on using one rocket per point, you probably won't get even thrusting, and you'll end up just spinning the thing. If you use a single rocket, it is probably feasable, but but ducting will be both heavy and inefficient- you would probably have a very hard time getting it efficient enough to actually lift off the ground. (I'm not sure how much those engines develop, but I don't think it's really too much.) Finally, the lifetime on this rocket is somewhere around 10 seconds... which means you're going to have your throttling valves open and ready to go as soon as you light it, and you still don't get a very impressive flight if you are trying to have any sort of control over the plane (You won't have enough time to take it up and land, you will probably just have to dead stick it.) I'd suggest looking into a gas powered or electric powered fan. In either case, if you build light, you can produce more thrust than the the weight of your plane, then I'd just steal a little of the flow and duct it out towards the extremities... the fact that the air will be ambient temperature will enable you to use a lot lighter materials for the ducting, while still keeping the vast majority of your thrust right at your cg. (For example, an O.S. .61 FX engine can produce about 7 lbs of thrust with a 13x4 prop at a stand still, and the engine weights just under two, so if you get a really light frame around the engine, you're good to go. (You can probably build a 5 lb aircraft around it if you go really skimpy) Look at ducted fans... I'm not sure if they produce better thrust at 0 airspeed than normal props.... Also, you can get gobs of thrust/weight with an electric system- just make sure you gear it down to give it tons of torque. With either a gas of electric system, you're probably looking at 10-15 minutes of flight time, rather than a matter of seconds.

It looks like a C6-4 rocket engine produces a peak of about 3.4 lbs, over a duration of 2 seconds.... I wouldn't even try the rocket engine- go with a fan system if you want to have any luck. link

Dedman · Post by **Dedman** » Wed May 04, 2005 2:58 pm

I have seen something similar done using small electrically driven props.

http://www.reallycooltoys.com/toys/i3info.html

However, I don't think that is what you are looking for. I don't see why you couldn't emulate the real deal by using bleed air from the turbine if you were talking about a model turbine powered aircraft.

Today's model turbines are only single stage so tapping the compressor section would introduce some inefficiencies into the system, but it should work although you may have to go with a higher rated engine then would normally be the case.

This may or may not have been done before. I don't know. I will ask around though.

snoopy · Post by **snoopy** » Wed May 04, 2005 3:14 pm

Dedman wrote: Today's model turbines are only single stage so tapping the compressor section would introduce some inefficiencies into the system, but it should work although you may have to go with a higher rated engine then would normally be the case.

There are two problems with turbine engines:

1) Cost- they are worth a ton... you lose a lot of money if you wreck one.
2) They produce better thrust than prop systems at high speeds, but don't really hit their sweet spot until you get going at a decent speed- I'd still reccomend good old prop for hover or near hover applications.

Dedman · Post by **Dedman** » Wed May 04, 2005 8:35 pm

snoopy wrote:There are two problems with turbine engines:

1) Cost- they are worth a ton... you lose a lot of money if you wreck one.

I guess it depends on what you consider a lot of money. You can buy one for around $2,000. That is a lot of money for a model engine yes, but the complexity he is talking about with an RCS is going to increase the cost no matter what propulsion system he uses.

snoopy wrote:2) They produce better thrust than prop systems at high speeds, but don't really hit their sweet spot until you get going at a decent speed- I'd still reccomend good old prop for hover or near hover applications.

I don't see this to be an issue. Excluding the thrust needed to stay aloft in hover mode (thrust=weight) which he did not ask about, all the RCS would need is some good old fashion bleed air. Now, there may be an minimum engine RPM setting required to produce enough bleed air pressure to be useful in an RCS, but that should be fairly easy to figure out on the bench.

roid · Post by **roid** » Wed May 04, 2005 11:34 pm

because of a possibly BINARY (on/off) nature of the main thrust of this thing - bleeding it's thrust into an RCS system may not be possible. because idealy i'd like the RCS to still be in control of the craft even when it's main engine is not thrusting. so i'm keeping the whole design modular for now - tackling one seperate system at a time.

what do you think about a centralised CO2 canister for RCS? (image) i'm thinking it could be hooked up into an inteligent valving system which is then hooked up to pipes which bring the CO2 to the nozzles.

CO2canister ---> valve(s) ---> nozzle(s).

snoopy · Post by **snoopy** » Thu May 05, 2005 9:21 am

I still don't think you will get much thrust out of it.