Putting objects into orbit is expensive because a rocket must give its payload enough speed and altitude to keep falling around Earth instead of falling back to the ground. Low Earth orbit requires a sideways speed of about 7.8 km/s, and rockets need even more speed in practice because of gravity and air resistance losses. The cost is often measured in dollars per kilogram, which lets engineers compare different rockets and missions fairly.
Lower launch cost makes satellites, space stations, science probes, and future human exploration more practical.
Key Facts
- Launch cost per kilogram = total launch price / payload mass delivered to orbit.
- A satellite in low Earth orbit travels at about v = 7.8 km/s.
- Specific orbital energy near a circular orbit is approximately E/m = v^2/2, not counting losses.
- The rocket equation is Delta v = ve ln(m0/mf), where ve is exhaust velocity.
- Reusable rockets lower cost by flying expensive engines, tanks, and structures more than once.
- Total mission cost includes rocket production, fuel, labor, testing, operations, insurance, and payload integration.
Vocabulary
- Launch cost per kilogram
- The price to deliver each kilogram of payload to a specified orbit.
- Payload
- The useful mass carried by a rocket, such as a satellite, spacecraft, crew capsule, or scientific instrument.
- Low Earth orbit
- An orbit relatively close to Earth, usually between about 160 km and 2000 km above the surface.
- Delta v
- The total change in velocity a spacecraft or rocket must produce to complete a maneuver or mission.
- Reusability
- The ability to recover and fly rocket hardware again, reducing the cost assigned to each launch.
Common Mistakes to Avoid
- Confusing fuel cost with launch cost. Rocket propellant is only one part of the price, while engines, manufacturing, testing, workers, launch operations, and risk are often much larger costs.
- Using dollars per kilogram without naming the orbit. A kilogram to low Earth orbit is much cheaper than a kilogram to geostationary orbit, the Moon, or Mars because each destination needs different energy.
- Assuming heavier rockets are automatically less efficient. A larger rocket may have a lower cost per kilogram if it carries much more payload or reuses expensive parts.
- Forgetting gravity and drag losses when estimating launch energy. The simple kinetic energy calculation gives a lower limit, but real rockets need extra Delta v while climbing through the atmosphere and fighting gravity.
Practice Questions
- 1 A rocket launch costs $67 million and delivers 22,800 kg to low Earth orbit. What is the launch cost per kilogram?
- 2 A small launcher costs 90 million and can deliver 16,000 kg to orbit. Which has the lower cost per kilogram, and by what factor is it cheaper?
- 3 Explain why recovering and reusing only the first stage of a rocket can still greatly reduce launch cost, even though the upper stage and fuel are not reused.