Ordered parts!

I have ordered all parts for the hoverbot except for the IMU and micro controller. I am still uncertain of the requirements for the micro controller, so we haven't bought that yet. Rob has a ardwino with a atmel328 which i plan on using to do basic development. From my reading i have heard different things about the processing requirements, the diy uav people seen to have developed a 2 axis pid controlled helicopter using only a reprogrammed 2.4ghz receiver! but it required alot of optimization. I am not the best programmer, but i imagine the atmel328 should be powerful enough even with inefficient code.

I have decided that although i could probably cobble together a vehicle relatively quickly i would like to make it as professional as possible. I had started a 3d drawing of the vehicle bu decided it to be unnecessary. It would help in determining the weights of the structure but i think the simplest thing to do is to just build it.

I am planing on building a prototype first, and then a proper vehicle. This may not be nessicary but i do not think i will get everything (weight distribution in particular) right the first time. When i tried to make the first compressed air airframe i had a hard time making all the frame square, this could also be an issue. I may have to make some sort of jig to hold the pieces in place while i drill and rivet them together.

As for the shape of the airframe i am currently thinking about a simple rectangular prism. A cylinder would be nice aesthetically and also more rocket like but with the shape but the way i plan on building the gimbaling mechanism its not really possible. Layers of perspex of aluminum plate joined together with welded rod are temptingly simple, but i from my first attempt of building the frame a rivited frame would be more rigid.

The gimbal will be just like one of those gyroscope toys:


Accept on one end of each joint will be a servo and there will be a ducted fan where the spinning mass is. Also instead of being circular it will be square.

One issue i just considered is that there will be extra force from resisting the spinning mass of motor shaft and impeller, hopefully the servos should be powerful enough.

I am not fully aware of the isues with response time etc, but just from thinking about it i can imagine that a brush less engine and servos will have a better response time than moving/changing the thrust of a bi-propellant rocket engine, although for the engine it does not matter that it is producing more thrust than it needs to hover, as we will need to speed up the engine when we thrust vector just to stay level. I ordered digital servos as they were not much more and from my reading they seem to be more accurate. I am considering the possibility of gearing down the servo at present i believe they can traven from -180 to +180 digrees. This amount of movement is not required and if i could gear them down i would have more accuracy.

Charging batteries could be a minor annoyance as we are using three battery packs and the charger i purchased only charges one at a time. I imagine it would take about 30-45 minutes to charge one pack. This may become a constraint during later development of software but initially i imagine we will only do small runs.

I am feeling good about the project at the moment but i haven't started building anything! I have a tendency of half finishing projects, so i am a but anxious to prove to myself that i can see this one through. The goal is a completely autonomous hovering robot that is as rocket like as posible, capable of moving between two points in space reliably. Dont forget that! I think if i work steadily i should be fine. The temptation to quit will be when i build the first prototype that semi works (perhaps in one axis), but i will be disappointed with myself if this happens.

Also sorry for anyone who is reading this! i can only imagine what you are going through..... I will try in future posts to make it more readable and understandable (with pictures). I have trouble expressing my thoughts into words.

Getting closer to a final design

I think i have all the major mechanical components of the hover bot now sorted.

Ducted fan:
Weomtec midfan pro + ARC3675-1 motor

Apparently this motor will output 3.6Kg of thurst @ 2480 watts and 4.5kg of thrust @ 3000+ watts. Rc people seem to love knowing the absolute maximum performance of all their parts. My concern is that the engine will not be designed to operate continuously with thrusts this high.

Batteries:
2 x Race 2600MAh 4-cell batteries 40c. In series the system will operate at 28v. I should be able to draw just over 3KW from this. The website says that it does not recommend operating the batteries continuously at over 35c (2.7Kw). I am hoping that with good ventilation i can push this a bit farther (if i need to). Initally we will only be operating the hoverbot for seconds at a time so least if we do blow the betteries it will mean we have a functional hovering system! The batteries are definitely the limiting factor. Capacity is not the issue its how fast the batteries can deliver their power.

Speed controller:
Platinum pro 120A
This should get the job done. We will draw about 105 amps hovering stationary, this is rated to 120A continuous (or so they say) so it should be fine

All this comes to about 1.5Kg. I hope to keep the whole vehicle under 3kg. Granted that i havent accounted for the structure, micrococontroller servos and other components but i think 3kg is quite achievable.

Now that i have all my major components (and weights) finalized i need to get on designing the structure and gambling mechanism. I would like to draw up the vehicle so i know exactly how much the airframe will weigh as it could easily blow out the weight.

The other thing i have to do is calculate the range of moments that will be applied to the vehicle from the main engine so i can design the reaction wheel.

I hope to get these two things done in the next week.

Thoughts on propulsion system for hoverbot

Well i have been doing ALOT of thinking about the propulsion system for the hoverbot and have three main candidates with a few variations.

Hover time is not as important as one might think. Because this project in intended to work out the control system only (something i have no experience with). Two seconds of stable flight is about as difficult as an enternity of flight. That said it would be really cool to demonstrate the hover bot an something flying for two seconds would need to be tethered as it would not have enough time to take off and land properly.

Something with a actual nozzle takes preference over a fan because it would at least introduce me to the problem of actuating a nozzle (a very difficult problem).....


1:
Compressed air.
This one would result in the lowest running time, calculated to be about 2-5 seconds using a 3000 PSI wrapped fiber paintball tank. This has the advantage that it would be the simplest of the candidates with an actual nozzle.

A large part of the cost with compressed air is filling the tank. With 3000 PSI you cant use just any compressor. The simplest thing to do would be to do what the paintball people do and use a scuba tank and a adapter to fill the paintball tank. The scuba tank will be a large part of the cost with this option. Also there is an isue of getting the tank filled. In australia to get a scuba tank filled you need either a dive license or a statuary declaration saying what it will be used for. I have never been good with bureaucracy and have no idea where to start getting this, although i cant imagine it would be too difficult.

A very cool variant of compressed air came from someone on the ausrocket rocketry forums. If you add some fuel to the compressed air (in a combustion chamber) you would add to the impulse and the hover time. It also gets significantly more complex and it would probally be an upgrade to option one once a working concept had been flown.

2:
Steam.
This one is sort of the same as compressed air but with steam. There would be a boiler in the vehicle which would be heated with an external heat source prior to flight. Once a set amount of water was at the desired temperature/ pressure the heat would be removed and a actuated valve (same as 1) would control the flow of steam to the nozzle.

This is probally my favourite of the three as it would yield a moderate hover time with not much more complexity than 1.

Unfortunately it seems unfeasible with a gimbald nozzle as piping the steam through a flexible tube would be quite difficult and result in pressure loss. The other option would be vanes in the exhaust but i have never really been a fan of vanes, they would be fine for steam but seem a bit tacky.

3:
The third option, which at the moment seems the most likely is a ducted fan from a remote control plane such as this one. This would be by far the simplest option if its feasible.

The problem is that although the fan can output a maximum 45N of thrust i doubt it is designed to operate like this continuously. At max thrust it is drawing 3KW! that's a lot of power for a small engine. A 120A speed controller operating at 28V (8 cell lipo) would easily be able to put that out, but it seems like alot and i am not sure the motor could handle it.

If i can keep the weight of the vehicle around 2kg then the motor will only have to operate at about %50 capacity (simplification) and it should be so able. The weights of the major components are:

Fan + motor: 700g
Batteries: 500-700g (depending on configuration)
ESC: 80g
IMU: 50g
Microcontroller: 100g
Structure + wireing etc: 500-700g
Inertia wheel (counter main fan) + Controller + Motor: 200g
Servos: 100g

Hopefully the whole thing will come in under 2.5kg.

One isue isue (raised by Ariel) is that the time taken for the fan to spin up/down would be too great to enable adequate control. I do not think this is an issue as surely the time taken for a impeller to spin up/break is less than the time taken to actuate a valve and for propellant to flow into a combustion chamber, but i will try to calculate the response time.

Another issue (also raised by Ariel) and neglected by me is that the spinning mass that is the impellor will have significant inertia which will apply a moment to the vehicle when its speed is changed. The mass of the impellor would be about 20-50g (no weights avilable). One way of countering this is to have another motor spinning a inertia wheel to counter it. This will add some mass to the vehicle but needs to be there.

This option is definitely the simplest. And will likely be the option chosen.

Why am i making this blog?

I like to have some way of documenting my progress on my various projects. I was a bit hesitant to put this up publicly, but i really enjoy reading other peoples engineering projects blog so i thought i would but this up so that somebody might enjoy it.

I am not a verry wordy person, and i often struggle to turn my thoughts into understandable words so be warned! This blog probably wont be particular understandable at times, and as you may have already realized my spelling is atrocious.

That said something is better than nothing and i hope people can take away some useful ideas.

Thoughts on hovering rocket.

I have been doing alot of thinking on the propulsion system for the hovering rocket.

My first idea was a a rocket tethered to a compressor. After talking to a nasa guy at aerosapce futures this turned out to be impractical because the relatively rigid tether would mess up the vehicle dynamics to the point where its no longer an exercise in hovering but fighting the tether.

My second idea was to use steam. There would be a boiler which would be filled with a small amount of water. Heat would be applied until the steam was superheated at about 500*c. The heat would be taken way and the rocket could be turned on.

There are two main issues with this idea. First and more importantly any motor gimballing would involve some sort of flexible pipe to conduct the steam. This is not practical. I could possibly make some sort of universal fluid joint, but that would be quite complicated.

Vanes in the exhaust are definitely possible

I am really thinking that i will just use compressed air with an internal tank. I may only got 5 seconds of hover time but as a proof of concept its definably the easiest.