Thrust vs. Specific Impulse: A Rocket's Strength and Fuel Economy, Calculated with NASA Sources, the RS-25 and Falcon 9

Thrust is the force an engine uses to push a rocket, measured in newtons (N). Specific impulse is thrust divided by the weight of propellant used per second, measured in seconds. A NASA document defines it as the average thrust divided by gravitational acceleration times the average mass flow rate. Thrust tells you how hard an engine pushes; specific impulse tells you how efficiently it uses propellant (its fuel economy). For example, the RS-25, the engine of the Space Shuttle and NASA's SLS Moon rocket, produces 512,300 lbf (about 2,280 kN) of thrust in vacuum with a specific impulse of 452.3 seconds.

Two numbers always come up when engines are compared: thrust and specific impulse. In Merlin vs. Raptor we compared thrust, but thrust alone does not tell you whether an engine is good. Here we use NASA's definition and real engine figures to work out the difference between the two.

The short answer: thrust is strength, specific impulse is fuel economy

• Thrust is the force with which an engine pushes the rocket. Unit: newtons (N). More thrust lets you lift a heavier rocket.
• Specific impulse measures how efficiently the engine uses propellant (fuel plus oxidizer). Unit: seconds. Higher means the same propellant can make the rocket go faster. It is the rocket's "fuel economy".
• A NASA document defines specific impulse as average thrust ÷ (gravitational acceleration × average mass flow rate). Mass flow rate is the mass of propellant expelled each second.
• Thrust decides whether you can get off the ground quickly; specific impulse decides how far you can go. A rocket needs both.

Background: force, weight, recoil and speed

The newton, and weight

A force is a push or pull that changes how something moves. Its unit is the newton (N). One newton is the force that makes a 1 kg object go 1 m/s faster every second.

Let's build an everyday feel for it. On Earth everything is pulled downward (gravity). The rate at which gravity speeds things up, gravitational acceleration, has a standard value of 9.80665 m/s², roughly 9.8. So a 1 kg object weighs about 9.8 newtons. Turned around, 1 newton is the weight of about 102 g, roughly a small apple resting on your palm.

The key point is that mass and weight are different things. Mass (kg) is the amount of stuff and does not change in space. Weight (N) is the pull of gravity on it, and on the Moon it drops to about one sixth. Keeping the two apart makes rocket talk much clearer.

American documents often give force in pounds-force (lbf). 1 lbf is about 4.448 N. There is also the tonne-force (tf): 1 tf is about 9.8 kN, the weight of one tonne. Whatever the unit, converting to newtons lets you compare.

Moving by recoil: throwing balls off a boat

A rocket does not move by pushing against something. In the vacuum of space there is no air to push on. So how does it move?

Imagine standing on ice holding an armful of balls. Throw one forward hard and your body slides backward. As much "motion" as the ball carries forward, your body carries backward. Physicists call this conservation of momentum. Momentum is mass × velocity, a measure of how much motion something has. Before the throw the total is zero, so after the throw the ball's momentum and yours still add to zero.

A rocket does this continuously, in enormous quantity and at enormous speed. The "balls" are hot gas from burning propellant. The NASA/MSFC paper (Gregory, 2004) derives rocket thrust from exactly this conservation of momentum.

Speed and acceleration

Speed is distance per second (m/s). Acceleration is how much the speed increases each second (m/s²). Force and acceleration are linked by force = mass × acceleration: the same force speeds up a light object more. A rocket gets lighter as it burns propellant, so the same thrust accelerates it harder later in the flight.

What thrust is: mass expelled per second × exhaust speed

From conservation of momentum, the NASA/MSFC paper arrives at:

In the boat picture: how many kilograms of balls you throw per second, and how fast. Throwing more, or throwing faster, both increase the recoil.

Try numbers. Expel 500 kg of gas per second at 4,000 m/s and the thrust is 4,000 × 500 = 2,000,000 N (2,000 kN, or 2 meganewtons), equal to the weight of about 204 tonnes.

What specific impulse is: propellant "fuel economy"

You can raise thrust as much as you like by expelling more. But then the propellant runs out quickly. Specific impulse captures how much push you get from the same propellant.

In words, the NASA/MSFC definition is:

The denominator, gravitational acceleration × mass flow rate, is the weight of propellant used per second. So specific impulse says how many newtons of thrust you get per newton of propellant weight burned per second, expressed as a time.

And since thrust = exhaust velocity × mass flow rate, substituting gives:

(equation 13 in the paper, assuming constant exhaust velocity and flow rate). Specific impulse is just the exhaust velocity divided by 9.8. A high-specific-impulse engine is an engine that throws its gas out fast.

Why the unit is seconds

Velocity (m/s) divided by acceleration (m/s²) leaves seconds. In plain terms:

"For how many seconds can 1 kg of propellant produce a force equal to 1 kg's weight (9.8 N)?"

An engine with 450 seconds of specific impulse can produce 9.8 N for 450 seconds from 1 kg of propellant; a 300-second engine, for 300 seconds. Like a car's miles per gallon, it says how long a given amount can keep working. Because it is based on weight, it comes out as the same number of seconds in metric or imperial units, which is handy.

Calculating with a real engine: the RS-25

The RS-25 is NASA's SLS Moon rocket engine, derived from the Space Shuttle main engine. According to manufacturer L3Harris's specification sheet (July 2024), it burns liquid hydrogen and liquid oxygen and flew on all 135 Space Shuttle flights. Figures are at 109% power level.

A Military Aerospace article (January 15, 2015) carrying Aerojet Rocketdyne's information gives the same thrust and a specific impulse of 452 s in vacuum and 366 s at sea level.

From these numbers we can work out the exhaust speed and how much propellant is used each second (our calculation):

1. Exhaust velocity (vacuum) = 452.3 × 9.80665 ≈ 4,436 m/s, about 13 times the speed of sound (about 340 m/s).
2. Mass flow rate = thrust ÷ velocity = 2,279,000 ÷ 4,436 ≈ about 514 kg per second. One engine pushes out roughly half a small car's mass of propellant every second.

Why the values differ at sea level and in vacuum

In the table, the same RS-25 produces 418,000 lb at sea level and 512,300 lb in vacuum. The reason is outside air pressure.

At the nozzle exit the gas is trying to expand outward. At sea level, the surrounding air (about 1 atmosphere) pushes back on the exit. In vacuum nothing pushes back. The thrust equation in NASA Glenn's educational pages adds or subtracts exactly this term: (exit pressure − outside pressure) × exit area.

Checking with numbers (our calculation):

• Thrust ratio: 418,000 ÷ 512,300 ≈ 0.816
• Specific impulse ratio: 366 ÷ 452 ≈ 0.810

They drop by almost the same proportion. The engine burns about the same propellant per second either way; at sea level, the air pushing back costs about 20% of both thrust and specific impulse.

SpaceX's figures reflect the same idea. Falcon 9's first-stage Merlin produces 845 kN, while the Merlin Vacuum used on the second stage produces 981 kN (SpaceX official page). The vacuum version is a separately designed engine, so this is not a like-for-like comparison, but the principle holds: engines used in space are designed for vacuum. See What Is Falcon 9? for more.

Why a rocket needs both

Thrust: you can't lift off unless you beat your weight

To leave the ground, thrust must exceed the rocket's weight. The ratio is called the thrust-to-weight ratio.

For Falcon 9 (SpaceX official figures, our calculation):

• Thrust: 845 kN × 9 Merlins = about 7,605 kN
• Weight: 549,054 kg × 9.80665 ≈ about 5,384 kN
• Thrust-to-weight ratio: 7,605 ÷ 5,384 ≈ 1.41

The excess thrust (about 2,221 kN) accelerates the rocket upward. Initial acceleration is 2,221,000 ÷ 549,054 ≈ about 4.0 m/s², like a car from a standstill gaining about 14 km/h (9 mph) every second. As propellant burns off and the rocket lightens, the acceleration grows.

No matter how high the specific impulse, a rocket with a thrust-to-weight ratio below 1 will not leave the pad. That is why a first stage has to be a strongman.

Specific impulse: how much speed you end up with

How much speed a rocket can ultimately gain is set by the rocket equation (Tsiolkovsky's equation). In words:

(ln is the natural logarithm; a standard equation). Exhaust velocity is specific impulse × 9.8, so the speed gained is proportional to specific impulse.

An example (our calculation, mass ratio 10, ln 10 ≈ 2.30):

1.5 times the specific impulse gives 1.5 times the speed. Reaching low Earth orbit takes about 7.8 km/s (a standard figure), so a difference in specific impulse can be the difference between reaching orbit and not. The equation also explains why rockets have stages: dropping heavy empty tanks shrinks the burnout mass and increases the speed gained.

Why both are hard to get at once

My own view: rocket design is a tug-of-war between the two. Liquid hydrogen, for instance, gives a high specific impulse like the RS-25's, but it is a very light (low-density) liquid, so carrying the same mass needs big tanks. Falcon 9's choice of kerosene (RP-1) and Starship's choice of methane weigh tank size, handling and reuse, not specific impulse alone. Fuels are covered in Falcon 9 vs. Starship and What Is Starship?.

Common misconceptions

• "More thrust means a better engine." Thrust grows simply by making an engine bigger or adding more of them. How well it uses propellant is measured by specific impulse.
• "The seconds in specific impulse are burn time." No. It has nothing to do with burn duration; it is an efficiency figure: how many seconds 1 kg of propellant can produce a force equal to its own weight.
• "Rockets push against the ground or the air." No. They move by the recoil of gas thrown backward. That is why they work in vacuum, and in fact produce more thrust there.
• "A catalog thrust figure is fixed." It differs between sea level and vacuum. Always check which condition a number refers to.

Frequently asked questions

Summary

• Thrust = exhaust velocity × mass expelled per second. Unit: N. The rocket's "strength".
• Specific impulse = thrust ÷ (9.8 × mass flow rate) = exhaust velocity ÷ 9.8. Unit: seconds. The propellant's "fuel economy".
• The RS-25 produces about 2,279 kN in vacuum at 452.3 s, which works out to gas leaving at about 4,400 m/s and about 514 kg expelled per second.
• At sea level the air pushes back, cutting both thrust and specific impulse by about 20%. Always check whether a figure is sea level or vacuum.
• Thrust gets you off the ground (thrust-to-weight above 1); specific impulse gets you far. Falcon 9's liftoff thrust-to-weight is about 1.4.

To explain it to a friend: "Thrust is horsepower, specific impulse is fuel economy. A rocket moves by throwing gas, so an engine that throws it faster gets better mileage."

A question to carry forward: throwing gas faster needs high chamber pressure and a nozzle that expands the gas well. So why does a nozzle narrow first and then widen, like an hourglass? Costs are covered in Rocket costs.

Read next: Merlin vs. Raptor compares SpaceX engine thrust using official figures.

Sources (checked 2026-10-27)

• Don A. Gregory, "Specific Impulse and Mass Flow Rate Error" (NASA Faculty Fellowship Program, Marshall Space Flight Center, 2004): https://ntrs.nasa.gov/api/citations/20050215305/downloads/20050215305.pdf
• L3Harris, "RS-25 Propulsion System" specification sheet (July 2024): https://l3harris.com/sites/default/files/2024-07/l3harris-ar-rs-25-spec-sheet.pdf
• Military Aerospace, "RS-25 engine from Aerojet Rocketdyne" (January 15, 2015): https://www.militaryaerospace.com/commercial-aerospace/article/14228081/rs-25-engine-from-aerojet-rocketdyne
• NASA Glenn Research Center, "Rocket Thrust Summary": https://www.grc.nasa.gov/www/K-12/airplane/rktthsum.html
• NASA Glenn Research Center, "Specific Impulse": https://www.grc.nasa.gov/www/BGH/specimp.html
• SpaceX, "Falcon 9": https://www.spacex.com/vehicles/falcon-9

This article is for learning about science and technology and is not investment advice. Calculations are ours, based on the figures in the sources. Figures are AI-generated images.