Chandrayaan-4 changed its rocket mid-development. Here is what that means for the sample return

India's first attempt at bringing lunar soil home is not going the way it was designed to go, and the change is in the rocket rather than the spacecraft. Chandrayaan-4, targeted for October 2027, will have to wait on a launch vehicle that ISRO has decided to rebuild in the middle of development.

Why two rockets in the first place

The mission has five modules — a Descender and an Ascender that go down to the surface and back, and a Propulsion, Transfer and Re-entry unit that stays in lunar orbit for the return leg. Together they weigh roughly 9,200 kg. LVM3, India's heaviest operational launcher, can put about 8,000 kg into low Earth orbit. One rocket therefore cannot do it, so the modules fly separately and rendezvous in Earth orbit before the integrated spacecraft leaves for the Moon.

That two-launch architecture was always the plan, and it is genuinely unprecedented — no country has returned samples from the lunar south pole. The sequence is that the first LVM3 carries the landing stack to Earth orbit, and a second, shortly afterwards, carries the propulsion, transfer and re-entry units up to dock with it.

The reason for the switch is a semi-cryogenic engine, which burns liquefied gas as an oxidiser with a liquid hydrocarbon fuel. India has cryogenic engine experience — the CE-20 that powers LVM3's upper stage is one — but a semi-cryogenic core stage is a different proposition, and the first fully integrated hot test is still ahead.

For context on what is being asked of the launcher: LVM3 stands 43.5 metres tall with a gross lift-off weight of about 640 tonnes, using two S200 solid boosters, a liquid L110 core stage and a C25 cryogenic upper stage, and is rated for 4,000 kg to geostationary transfer orbit. Its track record is unblemished. What it has not yet done is fly with the engine Chandrayaan-4 now requires.