Thrust is how hard a rocket engine pushes; specific impulse measures how much push it gets from each kilogram of propellant — its fuel efficiency.
Two numbers describe a rocket engine. Thrust tells you how hard it pushes. Specific impulse tells you how efficiently it uses its propellant. A good rocket needs the right mix of both.
Thrust is a force, measured in newtons. Rocket engines are usually quoted in kilonewtons (kN). India's engines cover a wide range. PSLV's solid first stage, the S139, gives up to 4,800 kN. The Vikas engine on PSLV's second stage gives up to 799 kN. The CE-20 cryogenic engine on LVM3's upper stage gives a nominal 200 kN. PSLV's fourth stage uses two small engines of about 7.3 kN each.
At lift-off, thrust must be bigger than the rocket's weight, or it will not leave the pad.
SSLV weighs about 120 tonnes at lift-off. Its weight is mass × g = 120,000 kg × 9.81 m/s² ≈ 1,177 kN. Its first stage, SS1, gives a maximum vacuum thrust of 2,496 kN. That is about twice its weight, so it climbs away firmly.
Upper stages are different. LVM3's C25 stage carries 28 tonnes of propellant and weighs 4,310 kg empty. With a satellite on top it weighs far more than the CE-20's 200 kN can hold up against gravity. That is fine: by the time it lights, the rocket is already high and moving fast, so it does not need to hover. It only needs to keep adding speed.
Specific impulse (Isp) is the thrust an engine produces divided by the weight of propellant it uses each second. Its unit works out to seconds, in metric and imperial units alike.
The higher the Isp, the more speed a rocket gains from each kilogram of propellant. That matters most for upper stages, where every kilogram saved can become a kilogram of satellite. GSLV and LVM3 use cryogenic upper stages for this reason. An estimate from ISRO's own figures for LVM3's C25 stage gives an Isp of about 470 s (see below).
Notice the range of thrust in one rocket: a 2,496 kN first stage at the bottom and 50 N trimming thrusters at the top. See SSLV's flights
NASA's thrust equation for a rocket is F = ṁ·Ve + Ae·(pe − p0). It is often tidied into one term using an equivalent exhaust velocity Veq, so that F = ṁ·Veq.
Specific impulse follows directly:
Isp = F / (ṁ·g0) = Veq / g0
The units are (m/s) ÷ (m/s²) = seconds. Dividing by g0 = 9.80665 m/s² is why Isp has the same value in metric and imperial units.
Worked estimate for LVM3's C25 stage, using ISRO's figures:
This is an average over the burn, not ISRO's own Isp figure, but it shows why hydrogen–oxygen upper stages are prized.
Thrust-to-weight. For SSLV at lift-off: weight = 120,000 × 9.80665 ≈ 1,177 kN. SS1's maximum vacuum thrust is 2,496 kN, so T/W ≈ 2.1. Sea-level thrust is a little lower than vacuum thrust because of the Ae(pe − p0) term, so the real lift-off value is somewhat below 2.1.
Total impulse is I = F × Δt. An upper bound for SS1 is 2,496 kN × 94.3 s ≈ 235 MN·s. For SS3 it is 160 kN × 106.9 s ≈ 17 MN·s — about fourteen times less, because the upper stage pushes a much lighter rocket.
Isp = F / (ṁ × g0) = 200,000 / (43 × 9.81) ≈ 474 s.
It is thrust divided by the weight of propellant used per second: newtons ÷ (newtons per second) leaves seconds. Equivalently, exhaust velocity divided by g0 gives (m/s) ÷ (m/s²) = s.
Yes. It fires when the rocket is already high and moving fast, so it does not need to hold itself up against gravity at a standstill. It only needs to keep adding speed.