Rockets are usually compared by how much mass they can place into orbit, but how they are used after launch is just as important. An expendable rocket is designed to fly once, drop empty stages, and lose most hardware after the mission. A reusable rocket recovers major parts, such as the first-stage booster, so they can be inspected, refurbished, and flown again.
This difference affects launch cost, schedule, engineering design, and the amount of space hardware that is discarded.
Key Facts
- Tsiolkovsky rocket equation: Δv = ve ln(m0 / mf)
- Payload fraction = payload mass / liftoff mass
- Cost per kilogram = launch cost / payload mass
- Reusable boosters need extra mass for landing fuel, legs, grid fins, and thermal protection.
- Expendable rockets can use nearly all available propellant for ascent, often giving higher payload for the same vehicle size.
- A reusable system becomes most valuable when refurbishment cost and turnaround time are much lower than building a new booster.
Vocabulary
- Expendable launch vehicle
- A rocket designed for one use, with stages or major parts discarded after launch.
- Reusable launch vehicle
- A rocket system designed to recover and fly major components again after inspection and refurbishment.
- Staging
- The process of dropping empty rocket sections so the remaining vehicle has less mass to accelerate.
- Retropropulsion
- The use of rocket engines firing opposite the direction of motion to slow a vehicle for descent or landing.
- Turnaround time
- The time needed to prepare a recovered rocket component for its next flight.
Common Mistakes to Avoid
- Assuming reusable always means cheaper. Reuse only reduces cost if recovery, inspection, repairs, and lost payload capacity cost less than building a new stage.
- Ignoring landing propellant in payload estimates. A reusable booster must reserve fuel for entry burns, landing burns, and control, so it cannot use all propellant for reaching orbit.
- Thinking the entire rocket usually lands back on Earth. In many reusable systems, only the first stage or booster is recovered while upper stages may still be expendable.
- Comparing launch price without comparing payload mass. A lower total launch price may still have a higher cost per kilogram if the rocket carries much less payload.
Practice Questions
- 1 An expendable rocket launch costs $90 million and carries 22,500 kg to orbit. What is its cost per kilogram?
- 2 A reusable rocket launch costs $67 million and carries 16,750 kg to orbit after reserving propellant for landing. What is its cost per kilogram, and how does it compare with the expendable rocket in Question 1?
- 3 Explain why adding landing legs, grid fins, and reserve propellant can make a reusable booster harder to design even though it may lower long-term launch costs.