It may be recalled that the RLV-TD had undergone a review in 2012 itself and had been cleared for launch in late 2013. According to Dr. K. Sivan, director of the Vikram Sarabhai Space Centre (VSSC), there were essentially three reasons for the delay of three years. One was though ISRO had planned for a certain schedule of launches, because of some unforeseen failures in GSLV launches, the entire schedule got affected.

“All the engineering effort had to be diverted to make the indigenous cryo-engine successful,” Dr. Sivan said.

Another chief reason was the realisation of the necessity of carrying out additional tests of the RLV system. Among these was a very important test called “Iron Bird” simulations. What is done here is to connect the entire RLV system mechanically as in flight condition and carry out the simulations. Here the actual flight profiles are simulated using mechanical actuators (complete with hydraulic plumbing lines), control electronics and the entire Navigation and Guidance Control (NGC) hardware and software developed at ISRO.

This test was not included in the earlier Loop Simulation runs for RLV-TD.

The second is related to the use of “hot structures” in RLV. In conventional launch vehicles, the structural elements are called cold structures because temperature is not a criterion there. Here, however, thermo-structural stability of the elements becomes important, Dr. Sivan pointed out. This was also not there in earlier considerations. A conventional LV, for instance, undergoes structural and thermal tests separately.

“These are very time-consuming tests,” pointed out Dr. Sivan, “wherein most intricate and detailed data have to be obtained.” For “hot structures”, such as the nose cap, the RLV uses carbon-carbon composite structure.