While the testing of a prototype Reusable Launch Vehicle (RLV) on Monday is no doubt an important milestone for ISRO, it is easy to see why the space organisation itself has been calling it only a “baby step” towards the objective of acquiring a launcher that can deliver satellites into space just like airplanes transport passengers and cargo.
The RLV technology is at least four decades old, and several nations, and even private space firms, have experimented with it. However, only NASA has put it to any practical use until now, in its much-acclaimed space shuttle programme that ran from 1981 to 2011.
The main rationale for developing a reusable system is to bring down the costs of satellite launch, and to increase the frequency of launches. Satellites and scientific instruments need to ride on rockets to go into space. These are of the use-and-throw kind, which mostly fall into the sea after doing their job, or sometimes float uselessly in space, adding to space debris. Reusable rockets can save the costs of building a new vehicle for every launch, and also the manufacturing time, thus enabling more frequent launches.
No quick cost cuts
It is estimated that RLV, once fully developed in about a decade, could bring down launch costs 8-10 times. Currently, it costs Rs 6-8 lakh to send a 1 kg payload into a low earth orbit. The PSLV and GSLV carry payloads of 1,000-2,500 kg per flight.
Since no RLVs have been used except in NASA’s space shuttle programme, there is little direct evidence for cost reductions. Besides, the space shuttle programme was driven more by the need to frequently carry astronauts and logistics to the International Space Station than to cut launch costs. In 30 years, the shuttle flew 135 missions — or 4.5 times a year on average. The programme budget was about $ 209 billion (2011 $), making the average mission cost more than $ 1.5 billion (approx Rs 10,000 crore today). This cost included expenses over buildings, salaries and other logistics. And though the shuttle was reusable, albeit only partially, the incremental cost per flight was estimated to be about $ 450 million (approx Rs 3,000 crore). This cost can be an indication of the service, repair and replacements needed before every flight. There is no estimate of how much additional money NASA would have spent if it had used an expendable launch vehicle instead of a partially reusable one.
Also, the cost advantage of a reusable vehicle can become evident only over several launches. That is because the development cost of RLV far exceeds the manufacturing cost of an existing launch vehicle. ISRO has spent about Rs 90 crore on developing the prototype RLV. The likely cost of an operational RLV over the next 10 years is expected to be substantially more than the average cost of a PSLV, which is about Rs 120 crore. A GSLV costs about Rs 170 crore. The development of an experimental fully reusable vehicle called X-33 by NASA, aborted in 2001, cost $ 1.3 billion (about Rs 9,000).
Several studies have compared the costs of a fully reusable system and continued use of an expendable vehicle. The results vary depending on the parameters chosen.
Degree of reusability
The cost advantage also depends on the degree of reusability built into the vehicle. There are different stages to the flight of a launch vehicle. At each stage, a part of the rocket breaks off, while providing thrust to the remainder to keep going. There are designs of Single-Stage-To-Orbit (SSTO) vehicles that would not require booster parts. But most vehicles in use today are multi-stage rockets.
A fully reusable vehicle would deliver payloads into orbits and return to Earth completely intact. Even a Two-Stage-To-Orbit (TSTO) vehicle, which ISRO’s tested prototype was, can be fully reusable if both the parts are made to fly back to Earth. In the case of the prototype tested on Monday, only the second stage, a winged structure that looks like an airplane, re-entered the atmosphere and landed in the Bay of Bengal for possible reuse.
In the NASA shuttle, the final stage, called the orbital vehicle, was reusable while some components of the boost stages were also recovered and reused after servicing. Studies have shown that recovery and reuse of the final stage, or the orbital vehicle, has more cost benefits compared to the boost stages.
On the degree of reusability depends the repairs and servicing required before reuse.
Not the first time
In January 2007, ISRO launched a 555-kg space capsule aboard PSLV-C7 that remained in orbit for 12 days before re-entering the atmosphere and crashing into the Bay of Bengal. In December 2014, ISRO carried out the Crew Module Atmospheric Re-entry Experiment, sending a heavy payload to a height of 126 km on the inaugural experimental flight of GSLV-Mk III, an advanced launch vehicle still under development. The payload separated and re-entered the atmosphere, and fell into the Bay of Bengal after a nearly 21-minute flight.
However, both these re-entries were meant to result in crash landings. The vehicle could be recovered but not reused. The RLV prototype was structurally very different — the winged structure intended to be able to make a soft landing like an airplane, and thus much more challenging. The actual RLV, when it is developed, would have to land on a runway. ISRO has said a 5-km runway, more than double the length of the longest in the country, would have to be built for it.