The engine will eventually power the first indigenous reusable launch vehicle, expected to be ready by 2030, alongside five semi-cryogenic engines.

Advanced Technology Vehicle – that’s the name of the ‘rocket’ that ISRO will be testing in July, in the second in the series of experiments the organisation is conducting en route to developing India’s first reusable launch vehicle (RLV).

On May 23, ISRO tested a scaled-down prototype of the RLV, dubbed RLV-TD for ‘technology demonstrator’. It comprised an airplane-shaped upper stage fixed to a solid rocket booster and hoicked to 70 km. There, the booster fell off and the RLV-TD was manoeuvred by an onboard computer to glide down into the Indian Ocean, where it disintegrated. (Note: the technical details of this mission were published by ISRO a few days ago.)

A six-times-larger version of the RLV-TD is eventually expected to be built by ISRO by 2030. Once ready, it will be able to launch satellites into the low-Earth orbit and sail back down to land, like a conventional airplane, on a custom runway. With it, ISRO hopes to be able to reduce launch costs by 10-times as well as reduce payload cost from the prevailing $5,000/kg to $2,000/kg. The global average is much higher.

The RLV’s economy is enhanced it if it is able to lift heavier payloads. One way to achieve this is by reducing the amount of oxidisers that the vehicle will carry. The PSLV rocket carries 225 tonnes of propellants when it launches, and each mission can lift 3,250 kg to the low-Earth orbit at a cost of Rs 90 crore. About 70% of the propellants are oxidisers: compounds that allow the combustion of the fuel itself. However, ISRO plans to use a kind of engine onboard the RLV that will be able to suck in oxygen from the atmosphere, so the vehicle won’t have to carry an oxidiser for that engine.

The Advanced Technology Vehicle (ATV) to be tested in July will test precisely this component. And for this, ISRO engineers have designed an indigenous scramjet engine.

In a regular engine, a fuel-oxygen combination is pumped into a combustion chamber, where a spark plug (or equivalent) ignites the mixture to release energy. In a vehicle powered by a scramjet engine, the vehicle carries only the fuel. Once it has been launched using a conventional engine and goes supersonic, the scramjet engine will inhale oxygen from the atmosphere to ignite the fuel. In effect, it’s a modified ramjet engine but with the ability to function at supersonic velocities.

ISRO plans to affix an ATV bearing a scramjet engine on a two-stage sounding rocket during the July test. The first stage of the rocket will – like in the first test – take the ATV to a height of about 70 km and fall off. Then, the ATV will coast for a bit before starting to plummet. During the coasting phase, the ATV will be travelling at Mach 6 (2 km/s). That’s when ISRO plans to test the scramjet engine for five seconds.

Sounding rockets are typically one- or two-stage projectiles that are used to probe the upper atmosphere. According to ISRO, the ATV will be the second stage of an RH-560 sounding rocket. ‘RH’ stands for the Rohini class of sounding rockets and 560 is its diameter in millimetres.

The RH-560 was previously tested in March 2010, when it was qualified as being India’s heaviest sounding rocket at three tonnes. During that test, designated ATV-D01, the rocket successfully maintained a velocity of Mach 6 for seven seconds as well as attained a dynamic pressure of around 80 kilopascals. A statement released apropos ATV-D01 had said, “It carried a passive scramjet engine combustor module as a test bed for demonstration of air-breathing propulsion technology. … These conditions are required for a stable ignition of active scramjet engine combustor module planned in the next flight of ATV.”

Though the ATV test slated for July is the second in a series that will lead up to the building of the RLV, it is not one of the four ‘broader’ experiments some of whose details ISRO had already announced. The first, called HEX 1, was conducted on May 23. Its purpose was to test the guiding computer onboard the RLV-TD and the performance of heat-resistant tiles affixed to the vehicle. In the second test, called a landing experiment (LEX), ISRO will attempt to land an RLV-TD on a custom runway parked in the middle of the Indian Ocean.

In the third test, an RLV-TD will be launched by a sounding rocket and made to land on the runway in a single mission. In the fourth, the vehicle will be actively powered by a scramjet engine. The dates for the last three tests haven’t been announced.

Apart from using a scramjet engine, ISRO has said that the RLV will also be fit with five semi-cryogenic engines. Its payload capacity to the low-Earth orbit is expected to be in the range of 10,000-20,000 kg. To compare, India’s heaviest extant rocket, the GSLV Mk-III, uses one cryogenic engine to be able to lift 8,000 kg to the same orbit. These specifications may pall in comparison to those offered by other spaceflight players, especially SpaceX, by the time the RLV is ready in 2030. SpaceX’s Falcon 9 rocket can already lift 13,000 kg to the low-Earth orbit for about $57 million (Rs 382 crore).

However, ISRO chief A.S. Kiran had told Frontline in an interview in May this year, “In principle, even if the cost comes down by 50 per cent, it is worth it. After factoring in the logistics of recovering it, etc., whatever it can bring down is worth it.”