Re: Engineering Problem

[John Scott]

Hey Again. 

In terms of choice of propellant I have given some thought to other propellant combinations, and even built a nitrous oxide/ethanol thruster (although I never got it going) but eventually settled on peroxide for a number of reasons. Really the only liquid oxidiser options for the ammeter are liquid oxygen, peroxide and nitrous oxide. Liquid oxygen is often used for its high performance but that comes at the cost of system complexity and cost. Liquid oxygen would definitely be my choice if we were building a large vehicle that we wanted to perform, but it is not really suited for a small vehicle like we want to develop. Nitrous oxide does have advantages in that it is self pressurising but that comes at the expense of being quite sensitive to temperature. For example if stored at above 37 degrees it becomes supercritical, and its density decreases drastically. Still it is widely used in ammeter hybrid rockets, and is quite well suited for it. Despite having its issues, peroxide is in my opinion the easiest/safest to handle. Something I learnt in rocketry very quickly is that there are very few accepted "correct" answers. While America went for high tech/performance engines the USSR went mainly for lower performance storable propellants. Its really a age old debate what the best propellant choice is and often there is no accepted answer. As for the vehicle we want to build what we want is simplicity, and also low cost. By using peroxide as a monopropellant you can do away with half the tanks, plumbing and complexity and thats the main reason I like it. 

My main motivation for the project was to design a vehicle that can be used to develop the technologies needed for a larger actively controlled rocket. 

As for the vehicle itself there area a few posts on my blog about it:

http://chokedflow.blogspot.com.au/2012/01/lander-concept.html

http://chokedflow.blogspot.com.au/2012/01/catlyist.html

There may be one more but i cant seem to find it

The main design constraint in the vehicle is that we are stuck with off the shelf components. As I talk about in that second link everything is designed around the tank, because there are very few commercial products which suit our needs. The tank in that concept is a nitrous oxide tank designed for auto racing. I have given some though to making my own spherical tanks by welding spun hemispheres together but that is a whole exercise in itself and for now I just wanted to focus on getting something flying.  After the tank actuated valves (one for each motor) are probably the next constraint in the design, as there are no affordable (sub $5000) valve that is suited to our purpose. I have previously tried to make a actuated ball valve myself by coupling a motor to a valve but this was not particularly successful. One technique i came across to overcome this problem is to pulse a solenoid (fast) to achieve precise flow control. I haven't actually tried it yet, but apparently it works quite well. This is what armidillo aerospace did with their first hovering vehicle, which is very similar to ours but uses 4 thrusters (http://www.youtube.com/watch?v=-BstN0peLjI). I would love to hear any thoughts you have on the airframe and vehicle design, as we are so busy getting the engine running that we haven't given it nearly enough planing

In rocketry, programming and microcontrollers are a useful skills to have as they are used quite a bit. To control the peroxide test stand (command the engine and take sensor readings) I originally used a arduino (atmega328) which interfaced with a python program, at first via a xbee then via a usb server connected via wifi link. This had a few issues (probably more down to my programming) so I eventually started using labview. I do quite enjoy programming and messing around with microcontrollers but I found I was just spending too much time programming and not enough time on the actual project. When we eventually start flight testing and developing the control system we will probably go back to a custom solution and there will be lots of opportunity to write code if thats what interests you.

Anyway, I hope I have given you at-least a basic understanding of the project, and I hope I haven't rambled too much. There are too manny details to cover in one email, so please let me know if you have any questions, as I would be happy to answer them. If you have any  ideas on the airframe I would love to hear them, feel free to propose something completely different. 

.

On 14/03/2012, at 7:51 PM, Jared Crutchfield wrote:

Hi John. Thanks for the message.

Your hovering platform project sounds really cool.

I did a few space subjects at RMIT, including space propulsion, but unfortunately I'm no rocket scientist.

Hydrogen peroxide seems at first glace to be a bugger of a propellant to work with, but its great that you're having success. I was wondering why you have chosen not to use a bi-propellant system for your platform, using maybe liquid oxygen and kerosene. I guess its more complicated, and also from the size of your motor on your blog scale is probably also a factor; from reading LN so far I've been impressed at how small things can be made, including the little 433 MHz acceleration telemetry payload.

I would be interested to see your platform design so far. I've been thinking about it this afternoon, and without knowing specifics of platform characteristics or the intended mission, a simple truss framework would seem fairly efficient structure-wise. Many aircraft use welded trusses to support engines. If you have any engine thrust data and some dimensions I can try coming up with a specific design using materials that I've had experience with in aviation, such as 4130 chromoly steel (you can get fairly small tube, but depends on the scale of the project) or 6061 aluminium alloy. Welding some of these is a bit trickier than mild steel welding, but I should be able to find info on it in my aviation refs. It also depends on whether you want to source mil-spec or commercial grades though. I think I have a copy of AR-MMPDS-01 (formerly MIL-HBK-5) somewhere that I can source mil-spec material data. Depending on the complexity of the platform structure required, I can analyze a truss framework by hand or using a FEA package (I use Strand7 at work for bread and butter structural comps).

Is there any info about the hovering platform project on the LN website?

I also have an atmega128 development board and AVR Studio. I've done some basic A/D demos, and interfaced with Delphi programs through RS232/USB, including very basic low res spectrum analysis using FFT. One of my "gunna do" (but probably never will) projects is a music spectrum analyzer with an LED matrix, but I probably need at least 16 bit A/D on a separate chip from the micro performing the FFT. I might be more motivated to work on a rocket project though. I have Delphi, web langauges, VB/VBA and some C/C++experience, but I'm keen-ish to learn assembly for the AVR. If I really get stuck I also have bascom; I would love to someday figure out how to get the avr tools (such as avrdude and avr-gcc) in linux to work with either my parallel programmer or my USB JTAG (I think this problem has been a bit of a rut for me). All my stuff is from ETT via Futurlec, but I've also been drooling over an STM32 board for ages that I'm pretty sure has better linux support. I've worked with an RS232 3-axis accelerometer from Sparkfun when I was helping to develop a certification flight test data logging rack at my old work, and I know Sparkfun has some awesome stuff. From my reading of LN, it seems like a few people here have been infected by Sparkfun-itis.

Cheers,


Jared

> From: johns...@me.com
> Subject: Engineering Problem
> Date: Tue, 13 Mar 2012 17:09:39 +1000
> To: jared.cr...@hotmail.com
> 
> Hi Jared. I received your message through the lunar-numbat mailing list. My name is john, I am 21 and currently in my last year of a degree in mechanical and aerospace engineering at the university of Queensland. I have been interested in designing and building rocket motors since leaving high school. I started out making a few simple solid motors, but after consideration of the legal aspects of solid motors I became interested in liquid motors. My current project is building a rocket powered hovering platform using 3 monopropellant hydrogen peroxide thrusters, analogous to the lunar lander vehicle (I was inspired by the lunar lander x-proze). I keep a blog at: http://chokedflow.blogspot.com.au/ where I document some of work (please excise my spelling, I don't spend enough time proofreading). It is quite an ambitious project and I have no delusions about how long it will take (a long time). To date we have have achieved manny test firings of a boiler plate engine to test consumable catalyst types and configurations, and are making good progress on that front. If you are interested in getting involved in the project we could really use more people, and we have lots of interesting problems and challenges if thats what interests you. One aspect we are working on now, which you might be interested in is optimising the vehicle for low weight. We have a basic design but it is by no means optimised.

[Jared Crutchfield]