Size of ventholes for my hybrid rocket

Hello

As you may have read earlier i am, as part of my mechanical engineering education, building a hybrid rocket. It is not an official project that is required by the curriculum for the students to partake in, but rather an interesting side project for the purpose of learning and having fun.

The rocket i am building is based on the design for the skyripper 56mm hybrid engine, alltough all the parts are manufactured by me and my team (based on pictures and prints of the engine). One of the issues i've been having with building hybrid rockets is the seemingly illogical requirement to put in ventholes in the engine oxidizer tank. I've already started a thread on that subject and got it partly explained.

Now as i understand it, the oxidizer (which in our case is n2o) has to be filled immidiently prior to launch, and as soon as you see the vaporized oxidizer burst out of the holes you ignite your fuel. First of all, if you get your hole diameters to large you would throw away valuable thrust. The instruction manual for the skyripper engine states that you should fill your oxidizer tank with a suppy pressure of around 700 psi. However, taking into account the ventholes, you really have no idea of what the maximum internal pressure will rise to. N2o vill pressure condensate at 746 psi (at 20 degrees celsius), and i can only assume that is the reason you would like to put the supply pressure at that level. However, with the part absence of a pressurized environment, the condensated n2o will very rapidly boil of. Perhaps the idea is that the time required for it to do that still is enough to give it time to provide the fuel with oxidizer. What if you just skipped building these holes, and instead relied on a pressure sensor in order to not overpressurize your oxidizer tank (and knowing when to ignite the fuel). You would still get a small percentage of air in the oxidizer, but even accounting for that, you should get a much more effective propulsion. What is the secret here?

Lets get down to the engineering part of this question. If you do decide to make the holes, what diameter should you use? Also, i am unsure if you in the skyripper model are supposed to squeeze in a hose or a tube in the ventholes that are supposed to lead out the oxidizer outside of the outer fuselage. In that case, the size of the holes of the tank doesn't really matter, only the internal holes on the hose or tube (see pic). Maybe but some silicone sealant around the holes too to prevent leakage. If that is done, then my question is, how large holes do you drill in the plastic tube?

The oxidizer tank (only the tank) in question is cylindrical 56mm diameter and ca 900 mm long. I am going to do a pressure test of it using water, so an idea of how high a pressure to use would be helpful.

If there are any good books (or webpages) that answers these more engineering related questions of building rockets, that you know of, it would be most appreciated if you shared them here.

Finally, yes i am very much aware of the dangers associated with this kind of project. Thank you for your concern.

Pic:
https://oi41.tinypic.com/13zco7r.jpg

The key words, are vent hole, singular. You only need one vent hole, and it needs to be fairly small. As long as the solenoid remians open till you fire the ignition system, the pressure will be nearly at max levels, and only minimal nitrous is lost... assuming you do not use a 10-count.

How small a hole? Not sure, but will look it up for myself now.

I was doing some research on hybrids and if I recall correctly, the Rattworks mid-power hybrids have 1/8 inch vent holes.

CarVac said:

I was doing some research on hybrids and if I recall correctly, the Rattworks mid-power hybrids have 1/8 inch vent holes.

Click to expand...

the 1/8" hole is for inserting a rubber tube that diverts the vent to the outside of the airframe. The actual motor vent hole is around 1/32" (maybe smaller).

Ah. Makes more sense. And when I said mid-power, I actually meant Level 1.

-_- my standards have changed...

Rock

From the questions you are asking it is clear that you are not sufficiently knowledgeable with hybrid rocket motors or rocket motors in general to be making your own motors. A mistake in your design can have bad consequences. Additionally TRF does not advocate that inexperienced amateurs make their own rocket motors, nor will we disseminate the information to do so.

In your post, you state that this is not part of your official university program, and your IP address indicates you are located in Stockholm, Sweden. I would suggest you find a faculty adviser who is familiar with propulsion and go to your university library and get a few books on rocket propulsion in general and hybrids in particular before you proceed. Sutton's text on Rocket Propulsion Elements is a good place to start.

I don't know the legal requirements to conduct rockety in Sweden, however hybrids are universally considered high power rockets and as such require a waiver from your national air traffic control system before you launch them.

Bob Krech, TRF Moderator

First I would like to say bobkrech is completely correct.

As someone who has made over 30 different hybrid motors myself I found that you can replace a vent line with a valve. Fill till the vent stream appears more solid then lock the valve. Also a good idea when doing this is have a back feed on the fill line which releases once the tank is a certain preassure.**Edit** Make sure that you do this remotely /edit

Again if what I am saying is giberish do what I did. Learn with someone elses project and then take it on yourself when you are confident. Last thing you want is a CATO or wose. You learn with rocketry imagine how many things can go wrong and then times it by 3.

Thanks

Or you could use the Aerotech method with a prefilled tank of Nitrous, a pin valve, a burst disk that is opened when a black powder pellet is burned at motor ignition to release the nitrous and a preheater grain to start the decomp.
The only problem is the tank should be DOT rated to transport Nitrous Oxide.

hybridhighflyer said:

First I would like to say bobkrech is completely correct.

As someone who has made over 30 different hybrid motors myself I found that you can replace a vent line with a valve. Fill till the vent stream appears more solid then lock the valve. Also a good idea when doing this is have a back feed on the fill line which releases once the tank is a certain preassure.**Edit** Make sure that you do this remotely /edit

Again if what I am saying is giberish do what I did. Learn with someone elses project and then take it on yourself when you are confident. Last thing you want is a CATO or wose. You learn with rocketry imagine how many things can go wrong and then times it by 3.

Thanks

Click to expand...

There are 2 types of hybrids: vented and non-vented hybrids.

A non-vented hybrid has a prefilled tank of nitroous attached to a thrust chamber with a pyrovalve. The pyrovalve contains BP or Pyrodex(R) that when iginted releases liquid nitrous from the supply tank to the fuel grain. It can sit on the pad for extented time periods without using nitrous, and can be used as an upper stage without difficulty.

A vented hybrid is filled at the pad with liquid nitrous through a nylon fill line terminating in a compression fitting on th e flight cylinder that acts as the nitrous flow control orifice once an ignition pyrogen burns the line an ignites the motor. Liquid nitrous will not fill the flight tank unless it is vented. The vent is small so little nitrous is lost. There is no advantage to use a valve to close the vent on a vented hybrid.

Bob

Old thread but not properly answered. The vent hole is quite small like a pin hole orifice. I am checking on the exact size but I know that it is small enough to allow pressure to build in the tank while venting off gas.