Task - Propulsion Control Avionics

Variable throttle control avionics using pre-determined commands on a CAN bus is the goal of this endeavour.

ASRI hardware evaluation

In cooperation with the ASRI A2.5 mission the LN build team will investigate building the avionics to take instructions on a CAN bus and translate these to motion of a ball valve which is ultimately used to control propellant flow into the rocket engine.

The engineering of the throttle system has three main components:

i. Electronics

The electronic hardware interfaces to the CAN bus and to the motor control interface.

At this stage, it seems that the general plan (for now) is to implement this via an AVR microcontroller such as the Atmel ATmega328, combined with a CAN-bus interface chip such as the Microchip MCP2515. The use of an AVR microcontroller which is compatible with the Arduino system is advantageous, as it allows leverage of the ease-of-use of Arduino, existing experience with Arduino and existing Arduino firmware technologies such as the Aiko framework.

The servo drive board being used at this stage is the Rutex R2020. One of these units, donated to ASRI by Rutex, has been made available to the LN build team by ASRI.

The R2000 setup and tuning documentation from Rutex is valuable, as is the other documentation available on these devices from Rutex.

The main power supply that the Rutex servo drive board board takes to run itself (on the 16-pin IDC connector) is 24 V, but it requires little current, well under 1 A. If the electronics assembly is supplied with 24 V (assuming that AUSROC's power supply bus supplies 24 V) this 24 V rail can also be supplied to a voltage regulator on the microcontroller board to provide the 5 V required for the microcontroller.

(To document further: What kind of power supply bus does the A2.5 launch vehicle have on board? What voltage(s)? What current capability?)

The Rutex board has four large, high current terminals - two connect to the motor, and two connect to the main high-power power supply which feeds the motor.

This motor requires a 60 V power supply to be operated at full speed. The continuous current draw of the motor is 5 A, although the peak motor current (the stall current) is 29.7 A.

Building any kind of power supply which can supply 5 A at 60 V is expensive and difficult, and especially so in a launch vehicle where batteries must supply all the electric power, with significant limits to volume and mass. We have decided that using 5 x 12 V lead-acid batteries in series is probably the easiest way to supply 60 V to the motor with the current required for testing and development on the ground. In the launch vehicle, the 60 V will probably be supplied by a custom made high-capacity (dangerous and expensive!) lithium polymer battery pack.

These lead-acid batteries may be 7 Ah gel cells, or car batteries, or anything in between. They do not need to be new batteries; it's possible that we can acquire used but still servicable batteries in order to keep our costs down.

The Rutex R2020 driver board is rated for a maximum current of 40 A, so under even the worst case motor stall condition (at 27.5 A) it should never be expected to be damaged. However, the main motor power supply should be fused, for example to protect the Rutex board against a motor short and to protect the relatively thin wires connected to the motor. What rating should this fuse have? 10 A perhaps?

The position encoder mounted on the motor is an S-50-250-IEG? (check this) from US Digital. This encoder outputs 50 pulses per revolution (check this). Therefore, at a rotational speed of 3000 rpm at the motor, the output is 15000 pulses per minute, or 2500 Hz.

Assuming that one "step" on the driver board increments the motor to the point where one "step" has been incremented on the feedback encoder, this means that a "step" clock signal to drive the Rutex board needs to have a frequency of 2500 Hz in order to drive the motor at that speed required. What should the duty cycle of this pulsetrain be? 10%? 50%?

ii. Motor and mechanical powertrain

This comprises the electric motor itself, the mechanical powertrain between the motor and the propellant valve, and the electromechanical feedback mechanism which provides feedback of the valve position back to the electronic controller and to the main AUSROC flight computer (via the microcontroller and the CAN-bus).

The motors used are MCG ID23900 brush motors. Here is a datasheet for this motor.

(To document: are the existing gearboxes COTS units? If so, what is their manufacturer and model? Is a datasheet available?)

The ID23902 datasheet above says that its continuous torque is 0.42 Nm. This is "stepped up" by the 50:1 gearbox, so you get 21 Nm at the valve. 21 Nm is equal to 2.14 kg-m, or 214 kg-cm. However, the peak instantaneous torque available may be higher.

I would like to throw down the tentative possible idea of replacing the motor and gearbox with something like a very large, high torque RC aircraft servo. This would make the drive electronics much simpler, and would completely solve the position feedback problem. It would be much more compact and lighter too.

If you look here for example, there is a servo that does 42 kg-cm of torque.

Hence, I'd like to document quantitatively how much torque is needed to operate the valves. Which brings us to the valves:

iii. Propellant valves

(To document: are the valves COTS units? If so, what is their manufacturer and model? Is a datasheet available?)

If we assume that from fully closed to fully open corresponds to 90 degrees of angular motion, we can define 0 degrees to be 0% propellant flow and +90 degrees to correspond to 100% propellant flow. The propellant valves are required to transit from fully closed to fully open within not more than 250 milliseconds.

90 degrees of valve travel within 0.25 s corresponds to an angular speed of 60 rpm, at the valve. Since that speed is stepped down by the 50:1 gearbox, this corresponds to 3000 rpm at the motor.

The motor's maximum rotational speed is quoted as being 5500 rpm, at a motor voltage of 60 V. Assuming that the motor speed scales linearly with the voltage, then, the minimum voltage required for a rotational speed of at least 3000 rpm is 33 V.

The propellants used in AUSROC 2.5 are liquid oxygen (LOX) and kerosene. Specifically, I believe the fuel used in past AUSROC firings has been commonly available Jet A1 (jet fuel), as opposed to RP-1.

RP-1 is a very highly refined kerosene-like fuel, with chemical and physical properties which are superficially similar to Jet A1, which has a carefully controlled composition for optimum performance and minimal wear and tear on the rocket engine. It is used in the first stages of the large American launch vehicles such as Titan, Delta, Atlas, Thor and Saturn systems, including the Saturn V (systems where large amounts of infrastructure and money exist, and the most exacting standards of performance are demanded, for example for manned spaceflight). RP-1 is very difficult to obtain and is very expensive - hence, in the AUSROC 2 series vehicles, commercial jet fuel is used instead.

The two throttle valves, for kerosene and LOX, must be compatible with those respective materials. (See R.PR.56 and R.PR.62.)

In the case of the kerosene valve unit, this means that any organic materials within the valve (such as polymers or elastomers, for example in gaskets or O-rings) must be compatible with prolonged contact with a kerosene-like fuel. Given the ubiquitous industrial use of petroleum fuels, checking the materials within the valve to ensure such compatibility, and changing them if necessary, is straightforward.

In the case of the LOX valve unit, this is a much more challenging problem. Firstly, any organic materials within the valve must be chemically compatible with liquid oxygen. Many organic materials are, in general, incompatible with liquid oxygen, and can easily ignite or even explode in contact with LOX.

Of course, if both valves are identical, and both valves are all-metal construction without any organic materials, then this means that there are no problems in this regard, for either valve.

The second challenge concerns the physical compatibility of the valve materials with cryogenic liquid oxygen. Oxygen boils at 90 K (-183 degrees C), and the valve assembly needs to not only withstand this temperature but also to withstand the thermal shock associated with the temperature change when the liquid oxygen is initially introduced into contact with the valve assembly, rapidly cooling it down from room temperature to cryogenic temperatures.

The valve assembly needs to be mechanically stable under these conditions and it needs to be able to operate, and operate reliably with the required performance, under these thermal conditions.

Since the failure LOX valve to open properly due to icing was one of the main factors which led to the destruction of the first AUSROC II in 1992, it's certainly important to pay special attention to these issues surrounding the LOX valve assembly.

Perhaps some form of heating blanket or heater assembly may be used to keep the valve assembly at an acceptably high temperature at all times after the vehicle is filled with LOX, if so required? (Alternatively, perhaps it would be possible for hot Tridyne exhaust gas from the Tridyne pressurisation system to be circulated through some kind of heat exchanger system around the LOX valve, keeping it warm? Although this seems like a very ambitious, complicated solution.) 

Miscellaneous links and notes

http://www.asri.org.au/web/system/files/private/ASRI-A25-M-Sponsor-Package%20060519-A.pdf

http://www.sworld.com.au/steven/space/ausroc/ausroc2/a2_monash.pdf

http://www.sworld.com.au/steven/pub/ASRI99.pdf