Hybrid Rockets

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Experimental Rocketry

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Payloads

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Avionics

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Telemetry

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Onboard Power

Rocket Onboard Avonics and Payload Power Systems

With the proliferation of altimeters, accelerometers, GPS's, onboard video and other devices onboard amateur rockets, one area that has largely not advanced has been power, or namely batteries to power these systems. To a large extent, the majority of people use standard PP3 9-volt batteries, which whilst sufficient, provide little margin for error in terms of power capacity - if your battery has been used a few times, it may be better to use a fresh battery, and if the rocket has to remain with the altimeter powered up and armed on the launch pad for any length of time, the drain on the battery can be sufficient to prevent proper operation of the altimeter when it is time to launch.

The problem with using fresh batteries is cost. Regular replacement of batteries for avionics systems adds up. It is at this point, it is useful to consider taking a leaf out of the radio control aircraft fraternity's book, and using rechargeable battery packs. Once you take this step with rocketry avionics, and start using rechargeable battery packs, you'll find it hard to go back!

First of all, the myth of rechargeable battery packs being expensive needs to be exposed. A replacement, rechargeable, 9.6 volt battery pack which provides much more power capability for an R-DAS or G-Wiz altimeter for example, costs around £12. In other words, within half a dozen launches using one of these rechargeable battery packs instead of standard alkaline batteries, the flier will be saving money.

Take as an example, an R-DAS flight computer or a G-Wiz altimeter/accelerometer. Whilst most of the time, it is fine to use both these devices with a single PP3 9 volt battery, the manufacturers manuals point out that for firing some igniters, it is advisable to use 2 batteries to ensure there is sufficient current supplied both to the device, and to the igniters. With regular launches, the costs will add up.

Now, instead of fitting 2 x 9 volt batteries, it is just as easy to fit one of these 9.6 volt rechargeable battery packs, as used in in radio control applications. In this case, the battery packs used for investigation were 2 different configurations of 9.6 volt Nickel Metal Hydride (NiMH) battery packs, each composed of 8 AAA 1.2 volt cells soldered together into a heat shrunk pack, with a capacity of 700 miliAmp hours (mAh). The advantages of this are considerable; To start with, these battery packs are rechargeable, so the need to buy regular supplies of batteries vanishes. Also, the battery packs have a considerably higher capacity at 700 mAh than 2 PP3 batteries at 150 mAh, so not only is the problem of having sufficient power for firing and operating the device resolved at a stroke, but also, with that capacity, the rocket can sit on the pad with the altimeter powered on and armed for considerably longer, with the flier being safe in the knowledge that there will be sufficient power for the altimeter to function properly. Lastly, the battery pack weights at 98 grams, are almost identical to that of 2 PP3 9 volt batteries, so there is no increase in mass when this option is chosen.

Onboard Video Power

A similar approach can be adopted for 12 volt power systems for onboard video. Lightweight 12 volt NiMH rechargeable battery packs can be bought for £20-£25 which have capacities of between 650mAh and 1100mAh (larger capacities than this start to increase the volume required, and get heavier). Current draws for Video systems are often of the order of 250 mA for both a CCD camera and a transmitter, so one of these rechargeable battery packs can provide between 2 1/2 hours to almost 5 hours of operation when the device is powered up on the launch pad.

Connectors

As with onboard video systems, connectors can make or break a rocket avionics system, since they represent a point of failure. Rechargeable battery packs are frequently supplied with either BEC connectors or Tamiya connectors. Both offer good connections, although for ultimate piece of mind, connectors such as the HRS/Hirose HR10 miniature multipole latching connectors sold by Farnelll and RS, are hard to beat, albeit pretty expensive at around 10 pounds per connector.

Charger

For a battery charger, I decided to use a rechargeable battery charger for Ni-Cd and NiMH rechargeable batteries manufactured by Norwegian company, Mascot. The Mascot chargers are available through Farnell and RS. The particular Mascot charger I use, is the Model 8714 Constant Current Charger, which plugs into the mains, and also has a charging output socket fitted. This charger is designed to charge from 1 to 10 x 1.2 volt rechargeable batteries at 50mA to 400mA. I have changed the output connector on my charger and fitted one of the 4-pin HRS/Hirose HR10 miniature multipole latching plugs, since that is the connector type I use on my onboard avionics power systems, however, any suitable connector can be fitted if required.

References

The battery packs used in this article are available from www.modelpower.co.uk, and are as follows:

R-DAS Power: LG 9.6 Volt 700mA AAA Ni-Mh rechargeable battery pack (98 grams) - £15.94 inc VAT each

Video Systems Power: 2 x MMC 6.0v 650mA 1/2AA Trapese Receiver Pack Ni-MH (104 grams each) - £9.51 inc VAT each ()

Video Systems Power: 2 x MMC 6.0v 1100mA 2/3 AF Trapese Receiver Pack Ni-MH (104 grams each) - £12.95 inc VAT each ()

Video Systems Power: 2 x MMC 6.0v 1050mA 2/3 A Receiver Pack Ni-MH (104 grams each) - £12.95 inc VAT each ()