On June 22, the Indian Space Research Organisation surpassed itself by launching 20 satellites in one go, on board a PSLV XL rocket. All 20 satellites were launched into a Sun-synchronous orbit at an altitude of 509 km. Before the launch, K. Sivan, director of the Vikram Sarabhai Space Centre (VSSC), had said that the challenge was to constantly reorient the fourth stage of the rocket – designated PS4 – while deploying the satellites so they wouldn’t crash into each other. The mission, called C34, was a flawless success.
However, a more significant success going ahead came later. Once the satellites were launched, PS4 wasn’t discarded. Instead, it was kept in orbit around Earth by the onboard computer for 50 minutes, going from near the south pole toward the north pole. Once it was close to the latter, PS4 was made to fire its engines for five seconds and then shut off, continuing on its way toward the south pole. Once it got there, its engines were again fired for five seconds and then shut off. It was there that the mission ended.
Why did this happen?
The C34 mission launched 20 satellites into a single orbit. However, a future mission might require the PS4 to launch many satellites into multiple orbits, and this in turn would require the launcher to jump between orbits. ISRO tested just this capability by starting and shutting off the PS4’s engines while it was in orbit around Earth, increasing or decreasing its altitude. And that it was a success means the PSLV can, in the future, launch satellites into different orbits in the same launch.
Without this capability, ISRO will have had to launch multiple PSLV missions, each targeting a different orbit. And with this capability, ISRO now stands to be able to reduce its launch costs further as well as launch more satellites faster.
Starting and shutting off the engines only sounds simple; such a description glosses over the fact that the PS4 is equipped with two liquid-fuel engines that have to be operated in sync. The engines also heat up considerably while in operation, so if they have to be restarted, they need to be cooled down rapidly in the interim period. Additionally, because the manoeuvres are being performed in space, gravity is extremely weak and the flow of fuel to the engines wouldn’t be as efficient as it is on Earth’s surface, when the PSLV takes off. As ISRO put it: “… appropriate measures were taken with overall mission planning and management, augmentation of control requirements, manoeuvring and controlling the vehicle into different orbits, reconditioning of propulsion systems and propellant management under micro-gravity conditions.”
The significance and acclaim attending the feat can’t be diminished; that said, ISRO had already demonstrated the PS4’s ability to restart its engines during the PSLV C29 mission, on December 16, 2015. At the time, S. Somnath, director of the Liquid Propulsion Systems Centre, had told IANS that the technique – of restarting the engines in space as well moving between orbits – would come into play in the C35 mission. The C35 mission is scheduled to happen in July 2016 to launch the ResourceSat 2A satellite into a low-Earth orbit and the ScatSat 1 satellite into the higher geostationary orbit.
The final launch date hasn’t been announced yet.
Speaking of reducing launch costs, another important test is to be conducted soon – of the scramjet engine being developed by ISRO for its own reusable launch vehicle. Though the engine won’t yet power a full-fledged flight in the test, VSSC’s K. Sivan has said that a smaller version of it will be installed on a sounding rocket to get it going at 6-8 times the speed of sound. In a conventional rocket, an engine is supplied with oxygen from a tank that the rocket carries in order to combust the fuel. A scramjet, on the other hand, is a type of engine that sucks in oxygen from the atmosphere itself, and that too at hypersonic velocities. So, as Sivan went on to say: “What we wish to demonstrate is hypersonic ignition and sustainability of combustion at high Mach numbers. We also want to see whether we are getting the expected thrust value.”
The reusable launch vehicle that a scramjet proper will eventually power hasn’t yet been built. On May 23, ISRO experimented with a scaled-down version of the vehicle to check if the heat-resistant tiles installed on it performed as they ought to and if the onboard computer was able to manoeuvre the flight without major errors. In the next test, a date for which hasn’t been announced, ISRO will drop the vehicle from an airplane so the former can execute a runway landing.
A working vehicle is expected to be ready only around 2030. It will be able to lift between 10,000 kg and 20,000 kg to the low-Earth orbit. Apart from a scramjet, the vehicle may be powered by five semi-cryogenic engines as well as might be made of lighter materials. Altogether, ISRO expects all efforts to culminate with a reduction in launch costs by a factor of 10. It pays to consider this number in context: by 2030, the organisation will face stiffer competition from its peers around the world in terms of lowering the cost of access to space as well as in payload capacity (e.g., Elon Musk expects to land humans on Mars by 2024).
Good luck, ISRO!