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.
Tuesday, July 27, 2010
Monday, July 19, 2010
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.
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.
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.
Thursday, July 15, 2010
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.
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.
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