Vicor Contributes to European Space Agency-Funded Sounding Rocket Program Experiment

October 23, 2014
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Sounding rockets, which are named after the nautical term “to sound,” or take measurements, have been used since 1959 to test instruments for satellites and spacecraft. The University of Pisa selected Vicor power components for an experiment to develop improved cooling technology for electronics systems found in spacecraft payloads as part of the international Sounding Rocket Program promoted and sponsored by the European Space Agency and other European countries’ space agencies.

Better thermal management is the primary issue for operators seeking to improve data transmission rates from satellites or increase the electrical power available to their space applications. Passive systems such as heat pipes are becoming the most popular choice for high heat power dissipation in electronics; they can supply sufficient cooling capacity while heat can be transported away efficiently through a combination of phase change and capillary forces without requiring extra equipment. Their thermal performance can be similar to that of a more complex active cooling system.

Figure 1 - Pulsating Heat Pipe

Fig.1: Closed Loop Pulsating Heat Pipe

PHP, or Pulsating Heat Pipes are passive systems that look particularly promising. These two-phase passive heat transfer devices offer high performance, low cost, simple geometry and an ability to cover wide heat transfer surfaces. A closed loop PHP system is shown in Fig. 1.

The sealed serpentine capillary tube is evacuated and then partially filled with a working fluid. When the pipe’s diameter is less than a certain critical value, the liquid slugs act as pumping elements when heated. The diameter’s critical value is gravity dependent; it is known that in space conditions and reduced gravity environments the liquid slugs can fill larger channels.

The university’s project team of 12 students had to design and manufacture a PHP with an internal diameter greater than earth-critical (PHP300) with one of nearly earth-critical value (PHP162) onto the sounding rocket to perform micro-gravity experiments that verify the activation of the fluid motion and the heat transfer effectiveness. The Experimental Box containing the hardware is shown in Fig.2.

Fig.2 Experimental Box for PHP Cells

Fig.2 Experimental Box for PHP Cells

Power for the experiment is delivered by a battery pack comprising fourteen Li-Ion cells, comprising two parallel sets of seven in series. As each cell voltage is 3.75 V, the nominal voltage of each series is 26.25 V. This supply feeds two power management PCBs, one each for the PHP300 and PHP162 heating cables. Both systems require the same power profile, as shown in fig.3. The batteries need to supply 400 W total for 20 s, then 260 W for 100 s. The change between peak and nominal power level is handled by a microcontroller-driven MOSFET switch.

Fig.3 Power profile for the two cells

Fig.3 Power Profile for the Two Cells (click to enlarge)

 

The peak current at 26.25 V would be nearly 15 A, and the battery voltage is not stabilized. Therefore, a power conversion train is needed to provide voltage stabilization. It must also boost the voltage to allow a reduced current to flow through the load. The load is purely resistive; the group calculated a suitable resistance value to ensure the right current and required heat dissipation resulted when using a stabilized, fixed 48 V supply. The group decided to employ commercial components for these functions, so they needed DC/DC step-up converters that met their criteria for efficiency, size and mass.

Given these constraints, finding suitable off-the-shelf components proved to be quite a challenging task. However a solution was identified in the form of Vicor’s PRM Regulator and VTM Current Multiplier modules, which provide the required power density and efficiency together with high reliability. The PRM performs regulation and conversion to 48 VDC, while the VTM provides isolation.

Five PRMs are used, as each can handle 120 W. Three are used in parallel to supply the PHP300, while two supply the PHP162. As the VTMs are rated to 300 W, only two are required – one each for the PHP300 and PHP162 power supplies.

The group said that Application Notes from Vicor were extremely useful in helping to set up the system configuration and choose the necessary component values. The configuration includes a master-slave arrangement for the PRMs, allowing them to share current safely and correctly. Evaluation boards were accordingly assembled. The group soon discovered that they had to use soldered connectors to eliminate any problem of high frequency noise, but once this was done the boards worked first time.

Although overall efficiency is high, at 91% – 92% for 400 W, air convection is not available in space conditions. Instead, cooling is successfully managed conductively by using thermal pads between the Vicor chips and the experiments’ enclosure.

The project is scheduled to fly on the REXUS 18 Sounding Rocket, due to be launched from the ESRANGE Space Center in Kiruna, Sweden during May 2015.

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