SCRAT, the Spherical Compact Rechargeable Air Thruster, is an experiment realized by a team of students of the University of Padova in the framework of the REXUS-BEXUS program. SCRAT is low-thrust cold gas actuator, using as propellant the air collected by a compressor from the surrounding atmosphere. The experiment, completely designed, tested and integrated by the students, flew on board the BEXUS10 stratospheric balloon, a cold night between the 8th and the 9th of October, 2010, from ESRANGE, the Swedish launch base above the polar circle.
SCRAT was designed to demonstrate a new technology: using the external air as propellant for a cold gas actuator, avoiding the transportation of fuel or heavy tanks. A greener technology, for both aeronautical (small aerobots) and space (extraterrestrial atmospheres probes) applications.
As a secondary objective, SCRAT sampled the atmosphere during the whole flight, allowing the reconstruction of temperature, pressure and density models in the sub-polar region.
SCRAT collects directly from the surrounding environment the propellant it needs. This solution allows avoiding the on-board storage of gas, making a great gain in terms of volume and weight destined to mission payloads. As visible in figure, air is collected from the atmosphere and then energized by two micro-compressors; the pressurized air fills a two-stage tank before it is ejected from a nozzle to deliver the thrust.
The experiment main elements are:
- the pneumatic circuit, dedicated to the air collection, energization and storage
- the measurement subsystem, consisting in a load cell connected to a rotating shaft
- the electronics, dedicated to the data acquisition and the experiment monitoring
In the tank, on the main components and exposed to the external environment, a set of pressure and temperature sensor are dedicated to continuously sample the experiment and the environment.
During the flight, SCRAT collected data from 101 thrusts, in the range 10-1 N (ground level) - 10-4 N (20 km of altitude), with a total impulse in the range of 10-2 – 10-3 Nm.
The discrepancy of about an order of magnitude between the flight data and the theoretical model was explained considering the load effect induced by air drag into the air pipe used to connect the thruster nozzle to the measurement system.
Compared to standard cold gas thrusters, it was found that the rechargeable actuator is able to save about 1 kg in a four-hour stratospheric flight and more than 2.7 kg on lower altitude equivalent flights.
FRIENDS & FUN: The SCRAT life
SCRAT is more than an experiment and a team of students of the university of Padova.
SCRAT allowed people from different backgrounds to reunite and realize a small, real space mission, enjoying their activity and creating friendship, fun and incredible adventures.
SCRAT was the right occasion to visit Amsterdam and the Nederlands, Stockholm, the north of Sweden above the polar circle. It permitted us to meet Space Experts from ESA as much as friends from all of Europe, learning how to design a space mission and how to have party at -25°C.
SCRAT taught us some more important notions, like “Shit Happens” and, most important, “don’t eat yellow snow!”