The missions planned in the first half of 2018 from Sriharikota spaceport are: GSLV-FO8 (GSAT-6A satellite), GSLV -MkIII (second development flight), Chandrayaan-2 and PSLV (IRNSS-1I navigation satellite), he said.
The Bengaluru-headquartered space agency has also awarded a contract to Arianespace to launch the 5.7-tonne GSAT-11 satellite from the European space consortium's spaceport at Kourou in French Guiana by June.
"Right now, our hands are full", Sivan, also Secretary in the Department of Space and Chairman of the Space Commission, told PTI.
India currently has a fleet of 45 orbiting satellites and the space agency chief said the demand is for a similar number of spacecraft.
"We are planning to have 15-18 launches per year from next year," he said, but hastened to add that it's an "ambition and target".
"In reality, there are so many other issues. Currently, it (ISRO's launches per year) is less than half of that (goal of 15-18 launches)".
The GSAT-11 is widely perceived to be the last heavy satellite to be launched by a foreign launcher, but Sivan cautioned against jumping to conclusions.
"We cannot say we will not go (for a foreign launcher). Right now, we have the capability to launch satellites of up to 4 tonne," he pointed out.
"We are trying to reach that (higher) capability. We are thinking of reducing the size of the satellite. Whatever we are getting through 5.7-tonne satellite, the same thing we can get through 4-tonne (satellite) if we use electric propulsion.
"And it will be within the capability of our GSLV. Different strategies are being worked out," he said.
The noted space scientist said the second developmental flight of GSLV-MkIII is scheduled before June. To declare this "fat boy" operational needs two consecutive successful developmental flights.
The space agency, Sivan said, has taken up the work on design and development of a small rocket to cut cost, and ISRO officials said it's a compact launch vehicle, weighing around 500 kg.
The ISRO Chairman, however, said it's early days as far as this small rocket initiative is concerned, and it would take time.
"Work is going on; it will take some time...design work, realisation...qualifying model....that work is going on. We are trying to manage (work relating to small rocket) with existing money itself. We are not taking it as a project."
On the benefits of such a small rocket, ISRO officials said it cuts cost and waiting period of small satellites for launch would shrink significantly.
"Small satellites need not wait for bigger vehicles (for launch). Waiting will not be there. The ISRO does not have to use bigger, dedicated launch vehicles to orbit small satellites", one of them noted.