Rocketry Planet

The AeroTech RMS/Hybrid motor system (Figures 1 & 2) has been available since November of 1995, when it was first certified by Tripoli Motor Testing. My first interest in the hybrid began over a year before I finally made the decision to try one out.

While considering a purchase, I found little information about the hybrid system — and I only witnessed one hybrid launch during that time. This review will provide you with the information you need to decide if the AeroTech RMS/Hybrid is in your future.

Pros:

• Five AeroTech RMS/Hybrid reloads are available, all “J” class J145, J170, J210, J260, J390
• The cost per regular hybrid “J” launch is about $35.00 (list price, including nitrous oxide - NOX)
• The cost per EFX “J” launch is about $45.00 (list price, including NOX)
• The cost per Turbo “J” launch is about $55.00 (list price, including NOX)
• Long burn times — 3.5 to 5.5 seconds
• Majestic launches
• Uses a standard launch pad, nothing extra required

Cons:

• Rocket must be designed to be compatible with a hybrid
• Altimeter, timer, or other electronic recovery system is required
• The NOX tank requires filling
• The reload kits have more parts to assemble
• The standard reloads lack smoke and flame
• Higher hardware weight to total newton-seconds ratio, about 3.5 pounds launch weight
• Higher initial hardware cost

There are several considerations for any rocket using the RMS/Hybrid motor. I wouldn’t choose a short stubby rocket — a long sleek design (Figure 3) works well — and make sure you will be able to set up the recovery system for fully electronic operation.

The RMS/Hybrid is a long 54mm motor, measuring approximately 28” in length. Many existing rockets and current kits already have a motor mount tube which will accept the RMS/Hybrid’s length.

You might think it not necessary to worry about motor retention because there is no ejection charge associated with a hybrid to give it the reverse kick. However, unless the parachute deployment occurs exactly at apogee, when the rocket is barely moving, there will be a jolt when the parachute deploys.

The RMS/Hybrid assembly weighs about 2 pounds empty, which may be enough mass to push it through wimpy motor retention.

The recovery setup requires the use of electronics for deployment. I have used both single stage and dual stage recovery on my hybrid launches with backups for each parachute.

Because of the high cost of the RMS/Hybrid motor hardware (list price about $300.00), I put a lot of time and effort into the planning and setup of the recovery system. Because each rocket is different, it is not possible to go into the recovery system here, however it is a major issue you will need to address if you decide to use a hybrid.

The hybrid’s nitrous oxide (NOX) flight tank (Figure 2) requires filling before use. AeroTech sells a filling adapter/hose (Figure 5) which connects to the flight tank and to a NOX supply tank (Figure 4) using an adapter.

An accurate scale is also required for the filling process. The cost of the adapter/hose, scale, supply tank, and tank adapter will be close to $400.00.

I purchased a 10 pound supply bottle which I had filled at a hot rod shop at a cost of $40 plus tax. You might be able to take your flight tank to a hot rod shop for filling, but you will still need the filling adapter hose, and it’s unlikely they would have the equipment needed to accurately weigh the filled bottle to confirm 299 grams of NOX.

If someone at your local launches are able to fill flight tanks, you can forget about all those extra accessories, and have them fill it for you. At the Tripoli Wisconsin launches at Bong, I offer that service.

The AeroTech RMS/Hybrid manual explains the process in great detail, so I will only go over the experiences I have had at this time — you can obtain the manual via the their web site. The NOX flows from the supply tank to the flight tank using the filling adapter/hose.

There needs to be a substantial temperature difference between the two tanks for the entire 299 grams of NOX to transfer into the flight tank. When I’m home, I put the flight tank in the freezer overnight, and the supply tank in a warm location.

On-site fillings are more challenging. At a recent Danville launch, I planned to launch two or three hybrids which meant on-site fillings. I was guessing the temperature at Danville in late October to be 40 to 60 degrees — too cold for the supply tank.

My plan was to warm up the supply tank to about 90 - 100 degrees and roll it up in two sleeping bags just before departure to the launch. At the launch, the flight tank was put on ice for 45 minutes before the fill was attempted. And it worked. I was able to fully fill the flight tank for my second launch of the day.

I use my new “Sunbeam Precision 70 oz. Electronic Digital Scale” (Figure 5), available at Office Depot for under $70, to weigh the flight tank. I had been using a triple beam balance type scale for my first fills, so the Sunbeam was a huge improvement. Always wear eye protection and leather gloves for filling operations.

Once filled, the flight tank can be stored for weeks or months before used, however I weigh my tank after it has been stored for a while to confirm it is still full. I usually will consider my tank full if I have over 270 grams of NOX.

The RMS/Hybrid motor assembly will take you about twice as long to complete as a standard RMS™ motor reload, not including the filling process; however, everything goes together easily. A special grease, Krytox™, must be used when assembling the RMS/Hybrid motor — this is a non petroleum grease.

After the forward closure is assembled (Figure 6), a leak test is performed using a filled flight tank. If you made an error, or the Pyrovalve™ (a black ceramic-like disk which ruptures at ignition to release the NOX) is defective, you need to know that before final motor assembly.

Assembling the lower half for the RMS/Hybrid motor, which is a standard 54mm 1252 RMS casing and aft closure, is a simple task. The igniter is made from an electric match (included in the RMS/Hybrid reload kit), masking tape, and black powder. I found the best way to pour the black powder into the tiny tube you create is to cut a straw at an angle which forms a mini scoop.

It’s very important that the igniter is in contact with the Pyrovalve. For the standard RMS/Hybrid reloads, you install the igniter assembly while building the motor, the red end cap should keep the igniter from slipping back. Screw on the flight tank and it’s ready to launch.

Between my daughter, Amy, and myself, we have had five flawless RMS/Hybrid launches using J145, J260EFX, J390Turbo, and two J170 reloads. The standard J145 reload (#1) in a Rocketman Preying Mantis produces the least amount of smoke and flame. In (#2), the J260EFX produces more power, smoke and flame. The J390Turbo (#3) is the most impressive of the RMS/Hybrid.

Unlike other AP motors that need to build up pressure at ignition, the RMS/Hybrid leave the pad in a hurry. The sound and roar of the AeroTech RMS/Hybrid is about the same as their other AP motors. Post flight cleanup is similar to any AeroTech RMS motor, except that I use a Q-tip and vinegar to clean the area of the Pyrovalve.

The AeroTech RMS/Hybrid is not for everyone, but if all of the above sounds interesting and challenging to you, then you might consider giving it a try. For me it has been a very enjoyable experience.