Vikram was in touch till 335m above lu­nar surface: ISRO

The New Indian Express - - FRONT PAGE - SV KR­ISHNA CHAI­TANYA @ Chen­nai

IT it is learnt that Chan­drayaan-2’s Vikram lan­der was much closer than 2.1 km to the lu­nar surface when it went in­com­mu­ni­cado. ISRO sources said the lan­der com­mu­ni­ca­tion link was in­tact till it was 335 me­tres above the moon’s surface.

ISRO failed to soft-land the lan­der safely dur­ing its last 15 min­utes of ‘ter­ror’, denying In­dia the credit of be­com­ing the fourth na­tion to soft-land on the moon. Ini­tially, ISRO chair­man K Si­van said: “Vikram lan­der de­scent was as planned and nor­mal per­for­mance was ob­served up to an alti­tude of 2.1 km. Sub­se­quently, the com­mu­ni­ca­tion from the lan­der to the ground cen­tre was lost.” This sug­gested com­mu­ni­ca­tion was lost at an alti­tude of 2.1 km. ISRO is yet to re­veal what caused the glitch.

Sources said the first phase of brak­ing of the lan­der — from a 30 km-alti­tude to 400 me­tres — was nearly suc­cess­ful. The lan­der’s ori­en­ta­tion was changed from hor­i­zon­tal to ver­ti­cal. Four-corner thrusters were op­er­ated to brake and the cen­tral thruster was switched off. At height of 400 me­tres, the sec­ond phase of brak­ing started and com­mu­ni­ca­tion was lost when the lan­der was 335 me­tres above the lu­nar surface.

ISRO se­nior ad­viser Ta­pan Mishra, who was in­stru­men­tal in build­ing In­dia’s radar satellites, on Wed­nes­day, fur­ther ex­plained what may have happened.

Writ­ing a de­tailed post on his Face­book page, Mishra said Chan­drayan 2 had five big (800 New­ton) thrusters and eight small thrusters. Thrusters are es­sen­tially small rock­ets, usu­ally mono or bi-pro­pel­lant based. Big thrusters are kept for brak­ing/hov­er­ing and small thrusters are meant for ori­en­ta­tion change and hov­er­ing. With­out di­rectly say­ing thrusters of the lan­der mal­func­tioned, Mishra said: “Five big thrusters (four at cor­ners and one in the cen­tre), if fired equally, will com­bine in the ver­ti­cal di­rec­tion, pro­vid­ing op­pos­ing force and the re­sul­tant ver­ti­cal axis of vec­tor will pass through cen­tre of grav­ity, pro­vid­ing stability. If an im­bal­ance is cre­ated by throt­tling four en­gines, ie by vary­ing fuel in­jec­tion rate, the re­sul­tant un­com­pen­sated hor­i­zon­tal force will spin the lan­der in the hor­i­zon­tal plane. If spin­ning in two or­thog­o­nal planes goes out of con­trol, it will es­sen­tially tum­ble down the lan­der. Tum­bling of lan­der with thrusters on, will make things very com­plex, like fire­works burnt in Di­wali, called spin­ning wheel or ‘Charki’. The re­sult will be si­mul­ta­ne­ous tum­bling and zigzag ran­dom mo­tion of lan­der, be­yond the con­trol of on-board con­trol sys­tem. So throt­tling of the four thrusters is a crit­i­cal ac­tiv­ity,” Mishra said.

He also in­di­cated de­ple­tion of fuel in the lan­der’s fuel tank. “A very large com­po­nent of lan­der is its fuel tank. When the lan­der ac­cel­er­ates, de­cel­er­ates, be­cause of in­er­tia, the liq­uid fuel gets into slosh­ing, akin to splash­ing of wa­ter in a tub. Slosh­ing be­comes se­vere as more and more fuel de­pletes in the fuel tank, mak­ing life dif­fi­cult. It may so hap­pen that the en­gine noz­zle feed will be starved of fuel, re­sult­ing in un­con­trolled throt­tling.”

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