Somewhere in northern Sweden there is a small crater, and at the bottom of that crater are the remains of triumph of student engineering: WUSat2.
It was designed to free-fall from space at up to twice the speed of sound, gathering atmospheric data on its 100km journey, sending it live to an earth station because WUSat had almost no chance of survival.
The triumph was that the ground station got the data, all except the last little bit.
“The European Space Agency said this was the first time they had had a free-falling unit with communication that actually worked,” Warwick Satellite Programme director Dr Bill Crofts told Electronics Weekly. They didn’t get contact all the way down. We think the antenna may have burned up.”
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Crofts is director of the University of Warwick’s satellite programme, and WUSat2 is a stepping stone on the University’s plan to get a satellite in orbit.
The first step, if you discount six years of work on an ESA moon-orbiting space programme that got shelved in 2012 through no fault of Warwick’s, was WUSat1, dropped from 30km altitude (see photo) over England.
Its journey started slightly less glamorously than WUSat2′s rocket-assisted trajectory, on a weather balloon launched near Welshpool.
“This was the first test, for the systems and tracking,” said Crofts.
Both WUSat1 and WUSat2 were CubeSats, based on the 10x10x10cm 1kg format that has proved so popular amongst university space shots.
WUSat1 was mostly COTS (commercial, off the shelf) components, but for WUSat2 things had to get more serious.
The launch was on an ESA programme called Rexus (rocket experiments for university students) which is jointly run by the German and Swedish space agencies, from which Sweden makes some of its payload slots available to other European countries. Launches are from Esrange, north of Arctic Circle.
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To get on a flight, in this case on Rexus 17, Warwick’s team of undergraduates had to compete with 11 other teams to show that they had a viable mission. “It is quite rigorous programme, you have to show simulation and modelling,” said Crofts.
You also have to have a viable science experiment, which was thought up by Warwick physicist Professor Don Pollacco – more of this later.
Winning the launch slot was only the first hurdle. The team had to show their spacecraft could both survive the launch, and not ruin the launch for others.
“It’s a pretty rigorous process, going through ESA. They set pretty high standards and you get treated pretty as much as a commercial sub-contractor,” said Crofts.
Several commercial companies, including RS Components and Thales, helped nudge the students in the right direction.
One of them was Portsmouth-based connector maker Harwin.
The electronics in CubSats is generally distributed over several PCBs plugged together in a stack.
“Having high quality high-reliability connectors was pretty much part of the design,” said Crofts.
Ben Green of Harwin takes up the story.
“We have worked with Warwick for a number of years. They had been using our DataMate connectors – they used them for the balloon launch, but for the rocket launch we recommended the Gecko line. It’s got very small pitch [1.25mm], very good high vibration and high shock”, he said, adding that many people use Micro-D connectors in CubeSats, but “Gecko is much smaller and much lighter. The Surrey Space Centre is using them in its QB50 CubeSats as well.”
For vibration resistance, the Gecko female part has four-point contact through beryllium copper springs: “There is no discontinuity of current up to 20G vibration and 100G shock,” said Green.
Another thing that swung the choice was that the Gecko range has cable-to-board connectors alongside board-to-board types, and that pins are available pre-assembled with PTFE-insulated flying leads of 150, 300 or 450mm, in both 26 and 32AWG sizes.
The other specs are: -65 to +150°C, 250Vdc or 250Vac peak at altitude, 2A/pin on all pins (2.8A for a single pin), and finally a life of 1,000 disconnections. “I don’t think they got anywhere near that,” said Green.
Science
Pollacco’s scientific experiment, dubbed Solspec, measured the Sun’s spectrum at different atmospheric path lengths as an analogy to the study of exoplanet atmospheres. It gathered data on oxygen and sodium concentrations using photodiodes to analyse light through various optical filters – bought to the experiment through optical fibres from translucent domes on either side of WUSat2. Received data was “pretty much consistent” with known atmospheric data, said Crofts.
From post-flight analysis of Doppler shift of the radio signals, the researchers are also attempting to get a speed profile for the decent.
The optical domes protruded outside the traditional 1U CubeSat profile, precluding the use of a standard ‘P-POD’ CubeSat ejector, so the team built a custom spring-loaded ejector to separate WUSat from its attachment beneath the rocket’s nose cone.
Unlike its more glamourous companion, the ejector made it back in one piece, stuffed with snow but unharmed. It was still attached to the rocket, which is saved for re-use by its own recovery parachute.
Originally it was planned that WuSat2 would have its own parachute, but it became obvious there was not enough room – particularly as sophisticated parachutes are required to brake from high speed.
“There was no guarantee we would get WUSat back at all, which put a great onus on communication,” said Crofts.
As the spacecraft could only have a tiny aerial, link budget was improved by building two large custom helical antennas for the ground station – whose equipment was also created by the team.
And just in case the comms link still couldn’t make it, sensor data was backed-up on flash memory, which is probably safe in that hole in Sweden.
To avoid any chance of messing with the rocket’s systems, WUSat had fly totally un-powered, booting-up 15 seconds after the pyro-cutters had initiated separation.
Processing came from a PIC microcontroller, “selected by the 2013-14 team as they felt it represented the simplest and lowest power consuming control architecture”, said Crofts.
Power and comms for charging and checking the CubeSat whilst installed in the ejector were are available through an umbilical that disconnected on ejection.
Parts including the domes, battery casing, and some of the flat-packed antenna were 3d-printed, and much of the rest was machined from aluminium.
Everything was designed and made at Warwick for the satellite and ejector, and most of the testing was done inside the university – which has its own vibration facility, said Crofts.
He is proud of his student-led satellite team, which each year comes from the fourth year Warwick’s MEng course. “Nine years in existence and eight years on ESA projects. That’s quite something for undergraduates,” said Crofts.
Already WUSat3 is in the pipeline. With the credibility it has built up, the university is looking for a low earth orbital launch this time, and there is even a possibility it will be from the International Space Station.
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