He joined ISRO in the year 1966.In the early 1970s, he invented the liquid fuel rocket technology. He knew that India would need liquid fuelled engines for the upcoming ISROs civilian space programmes.He was backed by his successor U.R. Rao and the then ISRO chairman Satish Dhawan. Nambi first built the successful 600-kg thrust engine; leading to the development of liquid propellant motors.He was assigned the charge of the cryogenics division (the behaviour and the production of materials at low temperature).He has worked with prominent scientists of India like Satish Dhawan, Vikram Sarabhai, Dr APJ Abdul Kalam, U.R. Rao, etc. and has served ISRO for 35 years.He was so tormented and agitated by the false charges that he threatened the Intelligence Bureau Officers that he would make them pay for it. In an interview, he revealed that one of the officers replied that Sir, we are doing our duty. If what you are saying is the truth and you are vindicated, you can slap us with your slippers..Nambi and another scientist D Sasikumaran were accused of transferring the ISRO secrets for millions. Nambi Narayanan Arrested During the Probe in the ISRO espionage Case.At that time, ISRO did not support Nambi as ISRO could not interfere in a legal matter, stated Krishnaswamy Kasturirangan (the then chairman of the ISRO.).In 1996, all the charges against him were dismissed by the CBI.In 1998, he was acquitted by the Supreme Court of India.