There are intermediate steps called proto-testing, says K Thyagarajan, a former program director of Chandrayaan-1, where you test only a “few sub-systems”, like, say, power or electronics. “But the qualification-model-to-flight-model is the correct way because the proof of the pudding is in the eating. We always say [this in Isro] that even poor designs can be made to work if they are properly tested.”
But let’s grant that to cut time, the two models—qualification and flight—are developed near-simultaneously. Rigour in testing also depends on the lifespan of the satellite, which is short in this case. The GLXP only expects teams to land their spacecraft on the moon, move 500 meters and send videos and images. The planned maximum life of its rover, ECA (Ek Chhoti Asha, or One Small Hope), is 320 hours. (We wrote about it earlier.)
Still, even if you shorten the qualification test, there’s yet another test—the acceptance test. Though not as rigorous or intrusive as the qualification test, it does require four-six weeks in which it tests the operational model to check if there are any manufacturing defects. The TeamIndus spacecraft will carry two rovers—its own as well as of the Japanese participant in the GLXP, Team HAKUTO, which is hitching a ride on PSLV—and a few other instruments that it’d accumulate after selling its excess carrying capacity.
In an emailed response (because he wasn’t free for a meeting or a telecon for two weeks), Narayan said, the qualification model testing is a “two-week exercise and the flight model will undergo the same testing in December before being shipped to Sriharikota”. “The integration of the flight model will begin early October, will take approximately 2-3 weeks for structural integration and an additional 3-4 weeks for package integration.”
In plain sight, all this adds up to something space experts have rarely seen. It takes anywhere from four-six weeks for the spacecraft to be fully checked and integrated with the rocket, says S Ramakrishnan, former director of Vikram Sarabhai Space Centre in Thiruvananthapuram. “However, at the end of the day, it depends on the confidence of the spacecraft maker and not dictated by the launch vehicle.”
Soft landing, hard time
For the milestone prize in 2014, TeamIndus constructed a prototype of the lander structure and performed vibration and rock testing. But from that prototype to the actual lander, weighing nearly 600kg with all the payloads on it, and ensuring a perfect controlled landing is several times more challenging. Even Isro hasn’t done a soft landing before and is elaborately testing the descent of its lander-rover module for its second moon mission in early 2018, Chandrayaan-2. While Isro wouldn’t talk about it, people in the know say it is preparing for “a dynamic test that will fully test the controlled descent of the lander using a parachute”.
Where and how is TeamIndus testing the descent of the lander? After all, in Narayan’s own words, “It is a landing mission. The mission duration from launch to touchdown is about four weeks.“ Is it doing a dynamic test—firing the engines to see if the spacecraft lands as envisioned? Or is TeamIndus largely doing a simulation test? Because the 18-second drop test video that it put on its Facebook page this month, hardly demonstrates the full descent.
Why is this significant? Because the lander descent is autonomous, controlled by the onboard computer, and very complex. Which is why only three countries so far have managed a soft landing on the moon. In Narayan’s own words, from a press conference in December in Delhi:
“When we begin descent, the spacecraft is moving at 1.7km a second. The signal to reach Earth is one second. In two seconds, the spacecraft has moved so much. There the descent is completely autonomous. It begins at 12.6km altitude. The spacecraft is moving at 1.7km a second. In 15 minutes—900 seconds—it has to completely kill all its velocity. It’s moving horizontally, it has to go completely vertical, and touch down at less than one metre per second. One metre/second—when a plane touches down on a runway and you feel a jerk, that is 0.25 metres/second.”
Narayan says TeamIndus would do up to 10 more drop tests to “characterise the stability envelope” of the spacecraft. “Full scale landing tests with thrusters firing is a program on its own. We instead intend to use drop-tests to better understand the dynamics of our spacecraft, co-relate our simulation models and conduct those tests inside a software simulation.”