The Indian Space Research Organisation has successfully launched a Reusable Launch Vehicle Technology Demonstrator mission. The flight began at 07:00 Indian Standard Time on 23 May (01:30 UTC), resulting in a successful launch for the winged, reusable space plane that conducted a suborbital mission to gather in-flight data.



Reusable Launch Vehicle Technology Demonstrator:

The quest to develop reusable spaceflight has led the Indian Space Research Organisation (ISRO) to develop a series of Technology Demonstration missions in a phased approach toward the introduction of a new Two-Stage To Orbit, winged, reusable launch vehicle.

As part of this development sequence, ISRO has developed a four flight test sequence that will incrementally test the various flight characteristics its under-development Reusable Launch Vehicle (RLV) will experience during flight as well as the scramjet propulsion engines the RLV is planned to use.

This four flight test sequence includes the Hypersonic Flight Experiment (HEX), the Landing Experiment (LEX), the Return Flight Experiment (REX), and the Scramjet Propulsion Experiment (SPEX).

The mission that launched was HEX, the first test in the four-flight sequence.

The HEX mission used a scaled prototype, called the Reusable Launch Vehicle – Technology Demonstrator (RLV-TD), of the RLV design.

In total, the RLV-TD is a 1.75 ton, 6.5 meter long vehicle that is able to achieve an altitude of approximately 70 kilometers.

The RLV-TD was mounted atop a 1 meter in diameter, 9 ton solid booster (HS9) and launched from the first launchpad at the Satish Dhawan Space Centre, previously known as the Sriharikota High Altitude Range, in southeastern India.

The HS9 booster launched the RLV-TD beyond most of Earth’s lower atmosphere.

After a 90sec burn, the booster delivered the RLV-TD to the proper altitude before separating from the prototype and destructively fall back to Earth in the Bay of Bengal.

Meanwhile, the RLV-TD continued on, falling back into Earth’s atmosphere at hypersonic velocity.

During this hypersonic test, the RLV-TD pitched its nose up relative to the horizon and direction of travel – just as the Space Shuttles did during atmospheric entry.

This allowed engineers to gather valuable in-flight data surrounding the performance of the vehicle’s thermal protection system (600 heat-resistant tiles and a carbon-carbon nose), its aerodynamic characteristics during hypersonic flight, and inform the overall design of the eventual full-scale RLV.

Since this was primarily a hypersonic flight technology demonstration mission, this particular RLV-TD did not carry the capability to return to land for landing.

Therefore, once it performed its primary mission glide through the atmosphere, it splashed down into the Bay of Bengal.

While several sources claimed that the HEX mission RLV-TD was to be recovered, an equal number of sources claim that it wasn’t to be recovered. The latter proved to be correct as officials noted that while recovery ships had been deployed, the recovery of the vehicle was not in the planning for this test.

The entire flight, from liftoff to splashdown was claimed to have lasted approximately 20 minutes.

Regardless of recoverability, HEX carried five distinct mission objectives, including: validating the aerodynamic design characteristics of the RLV during hypersonic flight, characterizing induced loads during hypersonic re-entry into the atmosphere, recovering the vehicle from the sea, assessing the performance of the carbon fibre used in construction of the nose of the vehicle, and demonstrating first stage separation sequencing.

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It should be noted that these mission objectives have not been publically updated since 2009, which initially pointed at the “recovery of the vehicle” objective. However, that has since moved to future tests.

The Road to the HEX mission:

As with all first flight scenarios, several proposed and targeted launch dates have come and gone for the HEX mission.

The initially stated launch date was in 2009, but has slipped incrementally as various testing and support facilities came on line.

By the end of 2014, ISRO stated that the HEX mission would launch in one year’s time.

For a while, that looked like it would hold true, with numerous media events and releases in early 2015 stating that the HEX mission was targeted for the “first half of 2015”.

In March-April 2015, ISRO highlighted the work over the previous year toward RLV-TD, which included studies of 3D heat flux and shear distribution of heat over the test vehicle, software validations, uplink trials of telemetry packages with satellites, and actuator design, fabrication, and acceptance testing.

Moreover, the HS9 solid rocket motor’s Secondary Injection Thrust Vector Control System was tested successfully.

By April 2015, ISRO stated that the HS9 solid booster was at Satish Dhawan Space Centre and that flight of the HEX mission was on track to occur before July 1, 2015.

However, by mid-May 2015, launch had slipped to the second half of 2015, and vehicle integration was still 8-10 weeks away.

This delay was directly linked to ISRO’s decision to prioritize its commercial launches ahead of HEX.

The mission then slipped to early 2016 before settling in May 2016 after a leak was found before final ground testing on the RLV-TD.

The HEX vehicle arrived at the launch site in late-April and was integrated to its HS9 booster without publicized issue.

A Mission Readiness Review was conducted on May 11 and cleared the RLV-TD and booster for launch.

(Images: ISRO)