Sugar Rocket Motor Mods
 

Hi,

A question for any of you rocketry/chemistry experts out there. I am a radio-control enthusiast who has recently been experimenting with rocket motors. What I am trying to do is create a "rocket plane", but unlike typical rocket planes, I am putting much time and care into making a craft that is both light and aerodynamic to achieve high flights.

Here is my question: I want to use sugar motors (KNO3 and sucrose) to power this thing. I have made a few already, but the problem is that they burn way too fast with too much thrust. What I want to do is modify the motor to slow the burn. Here is my idea:

Add baking soda to the KNO3/sucrose mixture since baking soda is non-flammable. I know this is done to create a delay charge, but I don't want it to burn that slow. Also, I would probably decrease the nozzle diameter to accommodate the pressure decrease.

What are your thoughts on this? Does it seem doable? Are there any formulas/methods for determining how much baking soda to add? If I could extend the burn from ~2s to ~10s then I would be happy.

Thanks for your help!

This probably isn't the best forum for motor-building questions. I'm sure some here have dabbled in it, but you won't find the concentrated expertise of . . . well, I'm not sure what to suggest!

It is an interesting project, though. I'd love a super-low-thrust motor for airplane uses; a modern Jetex without the clunkyness.

Try here... open discussion of EX motors, without the stupid "kid-proof" forum garbage on TRF...

http://www.rocketryspot.com/index.php

YORF doesn't discuss EX motors either... for whatever reason. Guess there's not a huge interest.

Best of luck! OL J R :)

Thanks guys. I already looked at TRF, but apparently it is against their rules to post anything about rocket motor manufacture (which is ridiculous of course.) I'll try your suggestion, "The Rocketry Spot." If you happen to think of anything, though, please don't be afraid to share your thoughts!

It might not be discussed, but I don't ban it like TRF does. It really doesn't pertain to the theme of this forum which is why people don't talk about it here.

Hi wjwj,
Before trying a burn rate modifier, have you considered different grain geometries?
C-slot, moonburner, end-burner?

You can make smoke bombs with the same ingredients, so the mix ratio, surface area (grain geometry), and nozzle will make a huge difference in thrust and burn time.

Yes, didn't mean to imply otherwise...

I'm glad that you take the stance that such discussions are "allowable"... (I put that in quotes because I don't accept the premise that topics of discussion should be allowed to be controlled by a third party, or that "permission" is required to discuss such things).

I also can understand that YORF is primarily about "classic rockets" and therefore not, by definition, likely to include much about high power or experimental motors. I'm glad that such topics CAN be discussed, but also that YORF focuses more on the MODEL rocketry part of the hobby rather than high power and experimental motors, which seem to dominate the discussions in other boards like the Terribly Run Forum (TRF), the Rocketry Center, and Rocketry Spot...

While those discussions are interesting, the model rocketry content is small enough to be "drowned out" by the greater volume of such discussions...

Later! OL J R :)

Here is a page that I found which seems to be exactly what I was looking for.

http://www.jamesyawn.net/endburner/

Could someone read this and help me decipher this guy's methods? My main questions are:

- What fuel (specifically) is used? It says KN/sucrose/Fe2O3, but how much of each?
- Can 3/4" PVC pipe be used for the case?

Also, I am trying to figure out how simply using an end-burner motor increased the burn time from 1-2 sec to nearly 7 sec. Does that make sense to you guys?

End-burning KNO3/Sucrose or KNO3/Dextrose results in a Smoke-Bomb with zilch for thrust.

Try experimenting with room temperature mixed Epoxy Glue (instead of Sugar) for the fuel with KNO3.

Really, epoxy? Isn't epoxy flame-resistant?

Your post got me thinking, so I did some theoretical physics calculations. Here is what I know:

The rocket plane is expected to weigh no more than 300g. According to the aforementioned site, the 7sec sucrose rocket produces an average thrush of 2lbf, or 9N.

If the plane weighs 300g, its weight is 3N. Therefore, the net force of the rocket is 6N upwards.

The acceleration at that weight and thrust is 20m/s^2, or 2G. Plenty for a rocket plane, since a rocket plane, by definition, is controllable!

This calculation reassures me that if I can mimic the end-burner sugar rocket, my rocket plane has a shot at working.

(P.S. - if the rocket plane accelerates for 7sec, it would reach speeds of >300mph without air resistance. Of course, air resistance will greatly limit my speed, but still cool!)

That is why I suggested an endburner.
I have zero experience with rc planes boosted with rockets, but had remembered that Aerotech have 24mm RC hardware available with end-burning reloads so I figured it might work for you.
7sec with 9N average is similarish to an aerotech E6(7.1s, 5.3N)
http://www.thrustcurve.org/motorsearch.jsp?id=38

You were right! That's why I asked. Thanks for the suggestion.

I was actually going to use either E6's or F10's originally. However, I want to do many launches, and each motor costs over $20 USD. That's as much as it costs me to fill up my car! :(

You can end--burn APCP (Ammonium Perchlorate Composite Propellant) and get low-thrust rocket motors.
You can only core-burn KNO3/Sugar or KNO3/Epoxy motors and expect any sort of aprreciable thrust. The amount of thrust and burn time can be adjusted by core geometry and nozzle size.

You can do whatever you want. So do it. Ignore the nay sayer. (I do, as do others around here)
Go with James (Recrystallized Rocketry) semi-positive experience with endburning and build on it.
Make some motors and do the tests, make more motors do more tests and if your results match or exceed those of James' tests, go fly.
Keep in mind that the E6 has an initial kick of ~12N (Red trace), which I suspect is done with a short core before transitioning to end burning.
An example of this short core is shown at the nozzle end in the second pic of a BlackPowder motor (eg, Estes, Quest).
The length of the core will affect the initial thrust and also the length of burn.
You will need to test this, along with nozzle diameter, to tweak your thrust curve.

You may also want to have a look at simulation programs like BurnSim. Will save you lots of time.
I've never actually tried to sim an endburner in BurnSim, but I'm sure there is a way to sim a short core transitioning to end burn. You might have to cheat, with a short Bates grain and a solid grain. Have a play :)

You've got me intrigued now, I'm going to fire up BurnSim :D
Edit: If you want the vital statistix for KNSU in Burnsim(they're not included) send me a PM.
More Edit: More reading: http://www.thefintels.com/aer/exper...ropellants2.htm
MORE edit... BurnSim has a Cored End-Burner option, make life simpler ;)

The quest for cheap motors isn't cheap. In the end, you might develop the capability to make inexpensive motors, but the process of getting there carries some expense.

I'll just try both! :) That's the beauty of experimental motors. I'll first try the sugar rockets as James' tests looked promising. If that doesn't work, I'll try dabbling with something else. As I mentioned before, though, I am more of an RC enthusiast and not so much a rocket enthusiast, so I probably won't do too much experimenting.

Also, do you think it would help if I increased the diameter of the motors? This would cause more propellant to burn and might increase the thrust a bit.

The other idea is to drill into the propellant a little, creating an initial core-burner for a powerful takeoff, then a long end-burner would follow to sustain the flight and add altitude.

"The other idea is to drill into the propellant a little, creating an initial core-burner for a powerful takeoff, then a long end-burner would follow to sustain the flight and add altitude."

In a home made motor you can do this, but limit the depth and the width. In commercially made motors, I wouldn't do it. At all.

Interesting. Can I ask why? I never planned on modifying commercial motors.

By drilling out part of an end burner, you make it into a 'mini port burner', which burns faster. This will give you the kick to get off the ground. To deep, too wide, causes the grain to burn quite fast, sometimes too fast, with negative results. Altering a ready made motor increases the likely hood of having that motor blowing out the nozzle, the top cap, or both. At worst, splitting the sides open. Not cool.

From and old micro-grain enthusiast.

Not meaning to discourage you from dabbling, but if you are not much into experimenting, I recommend shelling out the bucks for those long burn rocket glider motors and have fun with the gliders.

Long ago I once thought, "oh what's the big deal, just mix it up, pack some grains, and start flying..." I've been to local EX workshops and those guys develop some impressive motors, but not after investing a lot of time dedicated to experimentation and study, and of course spending a lot of money to learn the process. I learned first hand that it's a non-trivial process. I don't recommend cook-booking up some sugar motors just to save some money. See if you have some EX guys in your area and contact them.

Ok, I'm going to get started experimenting this week.

Here's what I'll do: I'll build the rocket plane (design is almost done), then I'll test it out with some sugar rockets using the end-burner technique. If that doesn't work, I might try ammonium perchlorate fuel - there are some "formulas" online, and it looks like you can buy the components relatively easily.

The beauty of the internet (and Youtube) is you don't always have to experiment much - there is usually already someone who has. ;)

KNSU doesn't have the same level of financial investment as experimental AP but it does still have the highest cost of all. Time.
As astronwolf has said, If you're only interested in saving money but you're not so interested in the experimenting, even with the negligible cost of KNSU, then yes commercial motors are for you.

I spent a few hours on BurnSim last night and got some very encouraging results.
You will get results from this if you are determined.

So for a point of comparison, S8E/P (RC rocket gliders with 1.1 meter minimum wingspan) models flown in FAI spacemodeling competitions are usually at or below 300g in mass.

We routinely fly on Aerotech E6 reload motors. These motors burn for 7 seconds, have a peak thrust of just under 12N and an average of 5.27N. I've flown them to 800-1200 ft.

They are slotted, end burning APCP motors, and because they are reloads and not single use motors, cost about $6 per flight on average.

kj

Drop us a couple of photos.

kj

It also has the potential to get a little hot when it's being cooked. KNSU propellant has started its share of house fires. A kitchen fire is very expensive, but not as expensive as the whole house and contents. If you do this, do it outside with a hotplate on a calm day, preferably on concrete. At the very least, have a hose handy. Use PPE (personal protective equipment) too.

Thanks for all your thoughts. To be honest, I'll probably stick with powdered KNSU for now since the thought of cooking it scares me. :(

I'll also probably try some commercial motors. The problem with these is not just the price point - they don't come in the sizes I need (I'm ultimately looking for G-size motors.) Aerotech makes an F-size, but it is $25 and not reloadable (F10). The other problem is they all have ejection charges.

This is why I am experimenting. I don't mean to say that I "won't experiment." I am just saying that a homemade motor with decent performance will do for me. I don't want to spend hours upon end on the workbench experimenting, and I don't think I'll need to if the kind folks from these forums, along with Youtube, will help me. ;) There are plenty of formulas online that I can follow, and if in the end they don't work, I can always try commercial motors. You should never discourage someone with experimenting (as I would say to any RC newbies.)

Aerotech make a G12 in their RC range, but in the 32mm hardware.
http://www.aerotech-rocketry.com/cu...eloads/G12.html

Also available for the same hardware:
F13, F16 and F23.

To get a 'G' size with KNSU you will have a considerable weight gain. The E6 'equivalent' I modelled in BurnSim needed 36g of propellant(max that will fit in a 24/60 case) for ~35N total impulse, with optimum KNSU(ie, cooked)
Compare that to the E6 which has 22g propellant in a 24/40 case for 37.5N total impulse. A 63% increase in propellant weight and the 24/60 hardware weighs more.
The AT G12 has 51g propellant, so KNSU 'equivalent' would need ~83g.
If you're into competition flying you're at a disadvantage. Sport flying it might not bother you...

Simon, thanks for your help. Your posts have been particularly useful.

I was not aware of these motors, but they change things a bit. I did not realize that the KNSU motors needed to be heavier. It looks like Aerotech G-12's are available in 2-packs for just $21 - not bad (although I'd have to pay that annoying HAZMAT charge.)

Perhaps I will use commercial motors. They are pretty cheap, have no ejection charge, and clearly have superior characteristics compared to the KNSU rockets.

I'll look into these more. Thanks for your help!

Simon, here is a link to the NAR's test of the Aerotech G-12:

http://www.nar.org/SandT/pdf/Aerotech/G12RC.pdf

It gives the before and after weight. Do you know if this includes the weight of the case?

No problem at all.
KNSU doesn't have to be heavier, but there are trade-offs.
KNSU has higher density and lower ISP than APCP, so you can shoot for equivalent performance with a size/weight trade-off, or equivalent motor size or weight with a performance trade-off.
Either way there is a compromise. APCP is the dominant propellant for a reason ;)

Are there any retailers within driving distance that regularly stock AeroTech?
If you can sweet-talk them into getting you RC reloads with their next order you might avoid the HAZMAT.

Yes that includes the hardware.
NAR states the initial mass is 131g.
AT state loaded motor weight as 126g.
No idea why there's a 5g discrepancy.
Maybe NAR included the igniter and nozzle cap while AT didn't? Who knows!

I found this video on Sugar rocket motor manufacture this morning on Transterrestrial Musings...

http://www.transterrestrial.com/?p=67547

I found this very interesting, but the nozzles seemed crumbly when he drilled them. I wonder how often these motors blow their nozzles out...........


Joe

I was wondering that myself.