The Indian Space Research Organisation (ISRO) has provided more details about its Gaganyaan programme, including new stages for its GSLV Mk III launch vehicle, through – of all things – a tender notice. Such surreptitiousness is par for the course for India’s spaceflight organisation, which has often done next to nothing to publicise even its most high-profile space missions.

According to Google’s timestamp, the notice has been available online since at least August 2017; another version was online on January 25. In it, ISRO has invited quotations for a slew of infrastructure upgrades that will prepare its second launchpad (SLP) at the Satish Dhawan Space Centre, Sriharikota, to support a rocket that can lift humans to space, as well as heavier satellites. The last date to submit proposals is listed as February 20, 2019.

Perhaps the more tantalising details concern two rocket stages, called SC120 and SC200. The Mk III is a three-stage rocket. The first stage comprises two boosters called S200 attached to the sides of the rocket. The second stage is powered by the L110 stage, powered by liquid propellants combusted by a pair of Vikas 2 engines. The ‘S’ and ‘L’ denote solid and liquid, and the numbers denote the total propellant mass they carry.

The third stage is powered by a cryogenic engine, C20. The stages are ignited in the order of their numbering.

The SC in ‘SC120/200’ stands for semi-cryogenic, a type of engine ISRO had already been developing for its reusable launch vehicle programme. Both of them seem to be alternatives for the Mk III rocket’s second stage, the L110.

Also read: Is ISRO Working on Three Reusable Rocket Designs at Once?

A discussion on Reddit suggests adapting the GSLV Mk III to be able to use them would require enough changes for the modified version to differ significantly from the original. Such a rocket is then expected to be able to lift over 5,000 kg to the geostationary transfer orbit – a goal that former ISRO chairman A.S. Kiran Kumar spelled out in 2017. With the L110, the Mk III can currently lift up to 4,000 kg.

According to a technical document describing the trailer system used to transport rocket stages, the SC120 stage will be 4 m wide, 17.29 m tall and weigh 11,500 kg. A more futuristic variant is likely to see the SC120 replaced by the SC200 system. Using both together would be infeasible because of their combined weight.

The tender notice also describes a new and heavier cryogenic upper stage called C32, a variant of the C20 engine that the Mk III uses at present. In the Indian space programme, a rocket stage powered by a cryogenic engine carries liquefied oxygen and liquefied hydrogen, a combination shortened in industry parlance as hydrolox. The C32-powered upper stage, according to the transport system specs, will be 4 m wide, 14.75 m long and weigh 7,400 kg, which is 400 kg more than the C20.

The stage with the semi-cryogenic configuration will carry liquefied oxygen and a highly refined form of kerosene called RP-1 – a.k.a. kerolox. Kerolox has a lower specific impulse than liquefied hydrogen. Specific impulse is a measure of “how much more push accumulates as you use that fuel” (source).

However, to its significant credit, RP-1 is 10-times denser, which means the same volume of kerolox will generate more thrust than the same volume of hydrolox (same source: thrust is “the amount of push a rocket engine provides to the rocket”). RP-1 is also cheaper, more stable at room temperature and presents much less of an explosion hazard. A well-known launch vehicle that uses kerolox is the SpaceX Falcon 9.

Additionally, kerolox engines are harder to ignite than hydrolox engines, more so when the propellant flow rate increases as the engine fires for longer. As a result, they are sometimes ignited on the ground itself, where the process can be better controlled. This is unlike the L110 engine, which switches on over 110 seconds after liftoff.

Beyond the crewed spaceflight programme itself, ISRO will need to continue its march to a heavier lift launch vehicle. Many commercial satellites and India’s own GSAT communication satellites are starting to weigh near 7,000 kg, especially as the latter is tasked with bringing more transponders online to sate India’s growing bandwidth demand.

India currently relies on launch vehicles operated by the French company Arianespance, such as its Ariane 5 rocket, to launch such heavy missions. These contracts are very expensive (over Rs 400 crore per launch). On the other hand, using a homegrown and home-operated vehicle is likely to provide better control over the expenditure, support local manufacturing and keep vehicles ready as and when necessary.

Moreover, ISRO has a programme-wise approach to science missions, which means it typically announces opportunities based on the availability of launchers in the future, and not the other way round. In this paradigm, having a heavier lift launch vehicle, akin to China’s giant Long March 5, will present correspondingly greater opportunities to India’s scientific workforce.

Also read: ISRO Will Work on Electric Propulsion System, Says A.S. Kiran Kumar

At the same time, it is also important that ISRO undertake launches more frequently. This isn’t something the Mk III can help with because – unlike the Polar (PSLV) and Small Satellite Launch Vehicles (SSLV) – it is a much more complex machine, and will be even more so in the SC120/200 configuration. It can’t be setup and launched with as much ease.

This in turn requires a launchpad able to support such an intense workload, together with the logistical requirements for transporting and loading different fuels. As the notice states (lightly edited):

For servicing of semi-cryo stage at the SLP, it is necessary that new facilities and/or augmentations are established apart from augmentation of existing cryo and gas systems together with associated instrumentation and control systems. 1. Isrosene system

2. Liquid oxygen storage and filling system (LOFS)

3. Nitrogen storage and filling system (NSS)

4. Gas storage and servicing system (GSSF)

5. Instrumentation and control systems

6. Cable trench and pipe trench

7. Augmentation of [liquid oxygen] storage at SLP, etc.

(Isrosene is a grade of kerosene that ISRO has developed as a ‘greener’ fuel to be used on future missions.)

A launchpad upgraded in this fashion will also be useful for the reusable launch vehicle programme, expected to be ready by 2030. Its current design envisages a launch vehicle powered by four or five kerolox semi-cryogenic engines during its ascent (and a scramjet engine during the descent phase).