What went wrong with ISRO moon lander?

It has been one year since the Vikram Lander (Pragyan Rover) crashed near the Lunar south pole. Although the Orbiter is doing well in its orbit and brand new science is being generated, the failed landing still stings to many Indians. Lets look at the Dark Side (obvious pun with dark side of the moon(:-)) of the Mission!

DARK SIDE (The failed landing)

Well to understand why the lander may have crashed we needa do a short crash course(pun noted!)on propulsions.

How to land on moon?

Step 1: Go to moon! (I mean enter its orbit by use of Retrograde propulsion after reaching moon at the apogee of earth bound orbit (perilune) i.e. your trans-lunar injection. And yes do all the necessary Homan transfers in-between.)

Fig: Descent Trajectory

Step 2: Fire your rocket engines in the opposite direction wrt the direction of your motion. This will reduce your velocity wrt moon and lower your orbit. Do this for long enough and you will have a crash course to the moon.

Step 3: Do precision breaking. Fine tune the vertical and horizontal velocity of your lander with RCS and attitude control motors (here motor refer to rocket engines).

Step 4: Powered decent. Try to land with almost a constant velocity by counteracting gravity with reverse thrust. Then ( Soft/hard/crash ) Landing!

HOW TO CONTROL THE SPATIAL MOVEMENT OF YOUR LANDER

1. Use Thrust Vectoring : you basically use a single rocket engine and off center its thrust to achieve directional control.

Fig: Thrust vectoring/ gimbaling.

2. Use multiple engines with Throttle-able Thrusts. Here you have 4 or more engines and you try to balance it on them and selectively throttle them to move in any direction. This is technologically more challenging and requires more software power.

Fig: See the 4 engine configuration of Vikram

OK SO WHY DID WE CRASH?

These are the speculations:

1. Well rocket science is already too complicated. Isro had choosen even more complicated approach to land Vikram. It went for the more complicated multi-engine approach to landing + switch to the central engine just before touchdown ! (Double Dhamaka). A former official of the ISRO said this.

How the Problem arose: Originally the plan was to launch the Chandrayaan-2 spacecraft along with the lander using GSLV-Mk II that can carry a weight of two tonne. Later on, the fifth engine at the centre of the lander was added so as to prevent the dust from the Moon surface from flying up and damaging the lander while it was landing. This, in turn, increased the spacecraft’s weight and changed the other specifications that GSLV-Mk II was not capable of carrying. It was then decided to launch Chandrayaan-2 with higher capacity GSLV-Mk III.

After the fifth engine was introduced, there was a software change. How well it was tested is not known said the same official. ISRO should also check out the extent to which various failure modes were simulated, he added.

2. Somersault Theory: Around 11 minutes after Vikram began its descent, things went haywire.

• At that point, Vikram was supposed to rotate slightly so that its cameras could map the lunar surface for a suitable landing site.
• During this crucial movement, Vikram unexpectedly and inexplicably performed a somersault.
• For a brief moment the Chandrayaan-2 lander was upside down over the lunar surface.

Fig: Live telecast of ISRO suggesting the upside down Vikram Lander. Noted this just before ISTRAC lost communication wih the lander @ 2.1 km altitude.

What this meant was that the reverse thrust-producing engines, which were slowing Vikram down, faced the sky for some time. And so, instead of slowing the craft down, the engines actually pushed the Vikram lander down towards the lunar surface.

This was clearly visible in the final readings sent by Vikram — at 11 minutes and 28 seconds after beginning its descent, Vikram’s vertical velocity (the speed with which it was descending on to the Moon) was 42.9 metres per second. A minute and a half later, the speed dramatically increased to 58.9 metres per second.

ISRO officials also said that the lander experienced conditions beyond what it was ment to experience

3. Software/technical Glitch: There could have been been some issue with the software that could have caused to feed wrong data into the controls. Or maybe we have some faulty part in the engine which might have caused the lander to under perform. Maybe a software caused the above mentioned errors.

CONCLUSION

A failed landing on moon. Bunch of Sad or Angry Indians. Hope we land with our next missions.