Twenty minutes that matter

A scramjet engine has no moving parts and spinning blades. Air rushes through the engine at hypersonic speed and becomes incredibly hot. The fire lights as soon as the fuel mixes with the rushing air inside the combustor. However, since the air is going in with such a speed, keeping the fire burning becomes a challenging task, and that is what India has been working to solve for the past 16 months.

The test proved that the scramjet combustor could be kept a little longer from destroying itself than the last time, using an indigenous endothermic liquid hydrocarbon fuel that absorbs heat as it burns coolant through the engine’s own walls.

According to the DRDO, the test also demonstrated the capability of the Scramjet Connect Pipe Test Facility (SCPT) to reproduce those conditions, validating both the test facility as well as the hardware it was designed to test.

India has successfully flight-tested basic variants of scramjet engines multiple times. The Indian Space Research Organisation (Isro) first demonstrated the technology in flight in August, 2016, achieving stable supersonic combustion for about five seconds at Mach 6 using a sounding rocket, making India the fourth country to do so.

Furthermore, DRDO successfully flight-tested its Hypersonic Technology Demonstrator Vehicle in September 2020, providing sustained scramjet combustion for over 20 seconds at hypersonic speeds.

“You can do 100 different tests on the ground, but if you have one or two good flight tests, the data that you get is always way more valuable than several ground tests. The ground tests are of course an important precursor to flight” Duvvuri Subrahmanyam, associate professor of aerospace engineering at the Indian Institute of Science, Bengaluru, said.

“You can demonstrate a lot of individual subsystems working at their optimum, but can the whole thing be put together to achieve system performance at the desired level? That’s a major challenge with these class of technologies” he added.

Niranjan Oak, research analyst at the Manohar Parrikar Institute for Defence Studies and Analyses, said, “A hypersonic cruise missile uses a scramjet engine. It will take a hypersonic, very high-velocity trajectory from the word go.”

Subramanyam argued that on a pure technological comparison, a scramjet is a significantly more challenging thing to achieve than a typical rocket engine. “The design margins in every system, subsystem, and sub-subsystem are going to be very, very small in something like this.”

Both the United States and the erstwhile Soviet Union spent many decades building up very large scale ground test facilities. Similarly, China has established itself with facilities of the same class in the last few years thanks to a major government push.

“India will not, at least in the short term, be able to match that kind of ground infrastructure,” Subrahmanyam said.

“The approach, in my view, that we should take is to do a lot more flight testing. When the R&D (research and development) time is costed along with opportunity costs, the total cost actually works out to be lower than investing in very large ground facilities, which will take many years to realise,” he added.

“As a nation, if you can afford to have multiple or parallel programmes attempting complex technologies with slightly different approaches, that is always a good thing, because you are in some sense hedging your risk.”

“The Americans are a good example of this. Any major programme usually involves two or three entities doing things in parallel and competing; India being a major economic power today should afford a similar approach” he said.

For nearly two decades, India’s most credible path to a hypersonic cruise missile ran through BrahMos-II, a joint venture with Russia targeting speeds of Mach 7-8. That programme has, by several recent accounts, been stalled by cost and also by Russia’s reluctance to transfer scramjet propulsion technology derived from its own Zircon missile.

Owing to this, the DRDO’s indigenous scramjet effort, internally designated extended trajectory–long duration hypersonic cruise missile (ET-LDHCM), has quietly taken a backseat.

“I think the cruise missile programme is quite lagging behind the HGV programme; the HGV track, linked to the long-range anti-ship missile, has already been flight-tested,” Oak said.

On the hypersonic missile systems, India has a long way to go. Russia remains the most operationally mature hypersonic power by far. Its Kinzhal air-launched missile has seen combat use in Ukraine since 2022.

China has already deployed the DF-17 glide vehicle across multiple Rocket Force brigades and fielded the YJ-21 anti-ship system, alongside newer platforms, including the scramjet-powered YJ-19, unveiled at last year's military parade. The US declared initial operational capability for the Army’s long-range hypersonic weapon in 2023 and has since revived its previously shelved AGM-183A air-launched programme with fresh funding.

Pakistan’s military publicised a system called the Fatah-3 in May, describing it as a ramjet-powered cruise missile, though independent assessments put its top speed at roughly Mach 3.5-4, which is supersonic rather than hypersonic, and no test firing had been publicly confirmed as of the most recent reporting.

For India, though, the concern is not only what Pakistan can develop on its own. “Pakistan has an unlimited and uninterrupted supply of Chinese weaponry, even during wartime. Things are very different on the ground, and Operation Sindoor has shown us that. We should take into consideration the role China is going to play in the next conflict,” Oak said.

That makes the development of hypersonic systems relevant on both the offensive and defensive sides. “Once you know the offensive role, it’s very helpful for scientists working on defensive capability. It’s another side of the coin. This hypersonic missile is purely offensive, but it also helps defensive capability. It’s not a question of whether we can sustain or not.

We should sustain this programme,” he added.

“I think our overall approach to this should be somewhat different than what traditionally the Americans and Soviets have adopted,” Subrahmanyam said. The debate over method and pace is, in the end, two sides of the same coin.