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A couple of days ago, someone in India noticed that Russian mission started later than its Indian counterpart, but it's going to land first, because Indians chose an extremely long and involved trip path, taking 40 days instead of 10 days. With national prestige on the line, Hindustan Times made the following interesting claim:

To send Luna-25 to the moon, Russia is using Soyuz 2.1, which is a powerful rocket that can give the necessary thrust to the spacecraft to reach the moon’s surface, instead of having to wait in earth’s orbit.

The Indian space agency, which launched its craft on-board the Launch Vehicle Mark-3, earlier known as the GSLV MK3, has a far less fuel capacity and thrust. It also has limitations of the payload capacity.

However, anyone who knows anything about space rockets will immediately recognize that Indian LVM3 rocket is in fact about twice as big and powerful as Russian Soyuz-2. Its launch mass is about 650 t vs 320 t, and the GTO performance is about 4 t vs 2 t for Soyuz.

Leaving aside just how journalists get away with boldly posting such false claims, what went wrong? Why did a far more powerful Indian rocket underperform in this mission? The expert consensus answer is a combination of two factors: poor mission planning and architecture of LVM3.

Indian rocket is indeed very powerful, but calculations (not mine) show that it can only inject 2100 kg to a direct transfer orbit to the Moon. This happens because LVM3 is designed for comsats and uses a large 3rd stage, which ignites relatively early. For a Moon trip it contributes 3 km/s! Because of its size it is heavy, and that handicaps its performance in beyond-Earth missions.

Still, 2100 kg is more than mere 1750 kg that Soyuz-2 managed. However, Indians chose to split the mission into orbital and lander segments. The orbiter does pretty much nothing except providing a backup for communications, but it eats the mass budget. The mass of scientific instruments is about 30 kg on both missions, although 26 kg is taken by the rover in Indian case.

Of course, Indians will be the last to laugh if Luna-25 lands into a crater that precludes direct communication with the control station at Earth.

Posted by: Pete Zaitcev at 08:25 AM | No Comments | Add Comment
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