Sub-orbital trajectories are a fundamental concept in Kerbal Space Program (KSP), especially for beginners who are just starting to explore the mechanics of rocketry and spaceflight. Whether you’re launching your first rocket or testing early prototypes, understanding how sub-orbital flights work is critical for mission success. In KSP, a sub-orbital trajectory means your craft will leave the lower atmosphere, reach space, and return to Kerbin without completing a full orbit. Though it may sound simple, mastering this flight path involves knowledge of thrust, gravity, drag, and proper flight planning.
What Is a Sub-Orbital Trajectory in KSP?
Basic Definition
A sub-orbital trajectory occurs when a spacecraft reaches space but lacks the horizontal velocity needed to stay in orbit. The flight path forms an arc, with the vessel eventually falling back to the planet due to gravity. In KSP, any flight that reaches above 70,000 meters (the edge of space on Kerbin) without achieving a stable orbit is considered sub-orbital.
Visual Indicators
On the map screen in KSP, a sub-orbital trajectory is shown as an arcing line that intersects the surface of Kerbin. Unlike orbital paths, which form complete ellipses or circles around the planet, a sub-orbital trajectory ends with a predicted impact point. If the periapsis is below 70 km, your trajectory will intersect Kerbin, confirming it’s sub-orbital.
Common Uses of Sub-Orbital Flights
Science and Contracts
Early in a career mode save, sub-orbital missions are a key way to complete contracts and gather science. Many early contracts ask players to:
- Reach a certain altitude
- Conduct crew or mystery goo experiments in space
- Test parts under specific conditions
- Perform atmospheric reentry for heat shielding
These objectives often don’t require a full orbit and can be achieved using a basic sub-orbital rocket design.
Tourist Missions
Tourist contracts frequently request sub-orbital flights. These missions are profitable and allow players to gain funds and reputation while building experience. Tourists may want to reach space, feel G-forces, or perform a splashdown recovery.
Reentry Testing
Practicing sub-orbital flights is useful for understanding reentry dynamics. You can test heat shield effectiveness, parachute deployment altitudes, and center of mass behavior. Sub-orbital missions provide a low-risk environment to gain this experience without investing in a full orbital craft.
How to Plan a Sub-Orbital Trajectory
Designing a Suitable Rocket
For a basic sub-orbital flight, your rocket should consist of:
- A command pod (Mk1 is the standard early choice)
- Science instruments like mystery goo or barometers
- Fuel tank and engine (Reliant or Swivel are common)
- A decoupler to separate the spent stage
- Parachutes for recovery
- Optional heat shield and control surfaces
You don’t need a lot of delta-v for sub-orbital launches. Around 3,000 m/s is typically enough to reach space and return safely.
Launch Procedure
Start by launching straight up or with a slight gravity turn. At around 10,000 meters, begin tilting eastward at about 10-15 degrees to ease the transition. You want your apoapsis to exceed 70 km, but not so high that you waste fuel or lose control.
Cut your engines once the apoapsis is where you want it, usually between 75-120 km. You’ll then coast to the top of the arc before reentering the atmosphere.
Factors That Affect Sub-Orbital Flights
Thrust-to-Weight Ratio (TWR)
Maintaining a high enough TWR (greater than 1.2) is essential to escape the thick lower atmosphere. If your TWR is too low, the rocket may burn too much fuel fighting gravity and never reach space.
Aerodynamics
Drag plays a big role in sub-orbital flights. Streamlined rockets are more efficient, while bulky designs may lose velocity quickly. Nose cones, fairings, and properly stacked parts help reduce drag losses.
Reentry Heating
While less intense than orbital reentry, sub-orbital heating can still destroy parts, especially on steeper trajectories. Use a heat shield for protection and angle your reentry path to avoid overheating.
Tips for Successful Sub-Orbital Missions
Monitor Apoapsis and Periapsis
Use the map view to track your apoapsis as your rocket ascends. Once it’s above 70,000 meters, you’ve officially reached space. If the periapsis is still inside Kerbin, your trajectory is sub-orbital.
Stage Efficiently
Don’t carry extra weight after you’ve completed your boost phase. Use decouplers to discard empty fuel tanks and engines, reducing mass and making parachute deployment safer and more stable.
Recover for Funds
Sub-orbital flights are a great way to conserve money. If you land near the KSC, you can recover up to 100% of your rocket’s value. Consider using radial parachutes or drogue chutes for better recovery control.
Transitioning from Sub-Orbital to Orbital Flight
What’s the Difference?
The key difference between sub-orbital and orbital flight is horizontal velocity. In sub-orbital flights, your horizontal speed isn’t high enough to prevent falling back to Kerbin. To orbit, you must boost laterally until your periapsis is above 70 km.
Making the Jump
Once you’re comfortable with sub-orbital launches, you can begin experimenting with orbital insertions. After reaching a high enough apoapsis, wait until you reach the top, then perform a prograde burn to raise your periapsis out of the atmosphere. When both values exceed 70 km, you’ve achieved orbit.
Common Mistakes and How to Avoid Them
- Burning Too Late: Waiting too long to initiate your ascent can result in low altitude apoapsis and wasted fuel.
- Not Accounting for Reentry: Coming in too steep can destroy your craft. Always angle your reentry path for safety.
- Unbalanced Designs: Rockets with off-center mass can tumble during ascent. Keep designs symmetrical when possible.
- Too Much Speed Too Soon: Rapid acceleration in the lower atmosphere causes excessive drag and overheating. Throttle gradually.
Mastering Sub-Orbital Trajectories
Sub-orbital trajectories in KSP are an excellent way to learn the core principles of rocketry and spaceflight. They serve as a stepping stone between simple launches and complex orbital mechanics. By mastering sub-orbital flights, players develop critical skills such as throttle control, staging, reentry planning, and efficient fuel usage. Whether you’re playing in sandbox or career mode, these early missions offer rewarding opportunities for science, contract completion, and experience. With consistent practice, you’ll soon be able to transition from short hops into space to full orbital missions and interplanetary travel.