Hypertek Tips and Tricks

Over the past 200+ Hypertek fuel grains I have consumed, I have discovered many ways to do things wrong. One of the purposes of this page is to share that information, and make new hybrid users successful earlier. The tips can be broken down into the following areas:

General - N20 Filling - Oxyen Supply - High Voltage Supply and Wire
Solenoid Valves - Fill Stem - Extension Cords

When stripping ignitor wires, it is very easy to pull on the conductors, so that they pull inside the insulation on the other end. This almost always results in a miss fire, as the arc will not form. It is best to strip the wires first, then cut the other end off square. If you have already loaded the rocket, you must be very careful stripping the ignitor wires.

It is also important to tighten the tie wrap(s) firmly. If the tiewrap is too loose, when pressure builds as the N20 is loaded, the rocket will attempt to lift, pulling out the slack in the tie wrap, and possibly create a leak. An attempt to launch with a leak will most likely cause a CATO.

Large motors (L,M)

An ignition failure of a large motor can be expensive if you have to dump a load of nitrous. I recommend using dual HV ignitor modules. This should eliminate failures due to a bad ignitor wire, or a wire where the conductors got pulled down into the insulation when you were stripping the other end!

It is commonly recommended to tape the igniter wires to the inside of the grain for large motors to improve reliability. After trying this for a dozen M flights, I no longer bother, and just put dual ignitor wires on the stem. I have not had a single failure in the 20-30 subsequent L and M flights. Though taping them inside the grain does work, it is hard to get them to stick, and most of the time they have pulled out by the time your rocket is loaded on the pad!

General
Hybrid rocket motors do not have a pyrotechnic delay system, so most do not leave a significant smoke trail, and they require a separate electronic system for chute deployment. More details of the various commercial hybrid systems currently on the market can be found here. Because of some of the variables in hybrid motor total impulse, an altimeter or integrating accelerometer are preferred over a simple timer, but any can be used. The new magnetic apogee detectors may also work well, but I have not personally tried them.

N20 Filling
Most hybrid motors use liquid nitrous oxide (N2O) as an oxidizer. The most common failure of these systems comes from trying to operate without a full flight cylinder of liquid N20. This can occur for several reasons:

A. The supply tank is not full enough. Most N20 supply tanks are used with a siphon tube, or they must be operated inverted. The normal method to determine how much N20 is left is to weigh the cylinder, and subtract the empty cylinder weight from it. The resultant weight is composed of usable liquid N20, the liquid N20 below the siphon tube, and the weight of the compressed gas above the liquid. For a 20 lb cylinder, when 6 pounds remains, you should refill. It will be difficult to get a complete flight cylinder fill, but MIGHT be possible, if you are lucky, depending on environmental conditions. Don't risk it. Get a bigger supply cylinder anyway, the L's are here!

B. The supply tank was full enough, but when the last person used it, a speck of dust got stuck in the solenoid valve, and the N20 bled off without anyone noticing. It is best to turn off the supply tanks, both N20 and GOX between flights. In addition, the N20 tank can be sat directly onto a cheap bathroom scale, and the remaining weight verified before each flight.

C. The supply tank is too hot. In very hot conditions, the liquid N20 boils rapidly away as you try to fill the flight tank. Eventually it will be cooled from the expanding N20 and should fill, but the gas consumption and time required will be substantial. I have filled a 440cc tank (3/4 lb) for over 2 minutes, and used 4 lbs of N20, and it was still of such low density, that the flight was far from optimal.

D. You didn't completely fill the flight cylinder. Most flyers can verify when the plume of liquid N20 appears, signifying a fill. Here in the desert, I have had difficulty confirming this visually, due to our dry conditions, even from 25 feet away. However, the sound change as it transitions from gas to a liquid is very obvious, if you are close enough to hear it. We now use a Radio Shack 9V amplifier wired back to the LCO to confirm fill from a distance. Since the system was implemented, we have not have an incident caused by insufficient fill. Note: I do not know of an algorithm than can be used to calculate fill time from supply tank pressure, hose and orifice size, and flight tank temperature, or any other factors. However, Dave Griffith, of the RATT works demonstrated with his 29mm system, that fill time can vary from 20 seconds to over 4 minutes. You must have a system to confirm fill! Filling for 40 seconds and pressing fire is NOT a good system!!

Oxygen Supply
You should ensure you have GOX, and the pressure is set to 80-100 PSI. To maximize life of the regulator, it is recommended to turn it down to a low pressure setting for storage

High Voltage Supply and Wire
Hypertek motors are ignited by arcing a 7500V spark across the end of a 24ga speaker wire, in the presence of GOX. The high voltage supply is polarity sensitive, and contains a fuse. If you connect 12V backwards, it will instantly blow the fuse. When first setting up a system for the day, in addition to verifying gas solenoid operation, test the ignitor by connecting a short piece of speaker wire to it and applying power. If you want to build your own igniter box, you can get the "BOSS" module here. I am not affiliated with them in any way. Running 2 HV igniters for large motors is cheap insurance.

Solenoid Valves
The N20 fill solenoid was designed to supply N20 to race cars for a few seconds. It requires 11 amps to operate, and runs very hot. It is susceptable to burn out, but can be protected by a voltage reducing circuit that can be viewed here. You can purchase replacement coils and stem direct from Nitrous Oxide Systems for about $40. Also, you should check to make sure the solenoid body is electrically isolated from the electrical supply. (Check resistance between either side of the plug, and the valve body) If there is a connection, although it won't effect normal operation, it will make the entire tank and braided steel supply lines electrically hot. If it then touches the other side of your 12V supply, you will melt multiple holes in the inner teflon tubes of the braided supply lines. (Don't ask how I know this!)

Fill Stem
The old stem system works well with rockets up to 8 pounds. You can launch heavier rockets, but they will sit on the stem longer, possibly damaging it. For larger airframes, a mechanism to either drop the stem an inch or so, or a system to help lift the airframe off the stem and release the nitrous is much better. In the event of a mishap, (you use your stem as a cutting torch) you can purchase replacement 1/4 or 3/8 stainless steel tubing from McMaster-Carr or other sources. I use the more temperature and corrosion resistant type 316, but have found that welded tube works perfectly well, and costs significantly less than seamless tubing.

Drop Stem CTI has developed and put in production a new drop stem system. With the new, more powerful hybrid motors recently released, airframe weights have increased, and so the difficulty of getting them off the stem quickly has become apparent. Some homebrewed systems, like my own, drop the entire fill system out of the motor to release nitrous, or used a spring to help lift the airframe. CTI solved the issues by developing a system that only drops the fill tube itself. The fill tube mount is stationary, bolted to the rail/rod.

In general, this system works great, especially with big, heavy M rockets. However, it is extremely important that the stem be properly aligned with the motor. When many people are flying different size airframes off the same pad, the stem has to be adjusted in or out for each airframe diameter. It this is not done accurately, the stem can bind in the motor, causing the rocket to stay on the pad too long, while the oxygen works as a cutting torch on the inside of the motor. After a couple screwups, (one wasn't enough for me), I no longer install the tie wrap while the rocket is horizontal. I wait until it is vertical, and I can actually test the ease of the stem dropping from the motor. Even a dry Kline valve O-ring can make the stem "sticky". I am using lube on the stem tip and where it slides through the brass bushing. The easier the stem drops, the better. A sticky stem can cause half the grain to burn away before the N20 starts.

The stem should be set up to drop about 1 inch before the split collar stops on the bushing. If you are switching from J/K motors to L motors, the collar has to be adjusted. If you adjust it for a 1 inch gap when you THINK it's fully inserted into the motor (but it's not), when you try to fill, N20 will pour out of the motor, and you will look stupid. (ask my friends how I know this) The proper length in inches, measured from the tip of the stem to the upper surface of the split collar should be:

J-13 3/8

K-18 3/4

M-21

If you set to these dimensions, when you load a rocket, if 1 inch of stem is not exposed between the split collar and the bushing, the stem is not seated properly in the Kline valve.

There have been several cases where less than perfect alignment of the drop fill stem has caused binding of the rocket on the stem, which can cause damage to the stem, motor, and possibly even burn through of the grain and damage to the rocket. Early drop stem system came with a snug bushing to guide the stem. The J/L stem systems should have the bushing drilled out to 7/16 inch, or completely removed. For the M systems, remove the bushing.

Use dual ignitor modules when launching L and M motors. The price of redundancy is cheap compared to a couple of N20 dumps!

Extension Cords
The stock Hypertek control system comes with a 50 foot extension cord, and works very well. You can add another extension cord and still have reliable operation. When you add a 3rd 50 foot extension, you may find that while the solenoids will operate without gas pressure, when you turn the gases on, they will no longer operate. If you are going to run your system over long distances, we recommend a "battery at the pad" system be made. An old UPS works great for this, and already has the outlets needed, and usually batteries that will last for full days of launching. You just need to remove the electronics, add 30 amp 12V automotive type relays, and rewire the batteries for 12V operation.

HyperTEK now offers a 500 foot control cable and relay box to enable you to safely fill M motors without homebuilt systems. You supply the extra battery at the pad.