# Fly Astra Launch Architecture

## Baseline concept

**Fly Astra is a reusable, air-breathing hypersonic first stage paired with a separate rocket-powered orbital stage.**

The aircraft is a launch-assist stage, not an orbital vehicle. It takes off and lands on a runway, uses atmospheric oxygen during powered flight, accelerates the orbital stage in the correct downrange direction, and releases it above the dense lower atmosphere. The orbital stage carries its own oxidizer and supplies the remaining velocity needed for orbit.

```text
Runway takeoff
      |
      v
Reusable air-breathing carrier / first stage
      |
      | high-altitude, downrange separation
      +------------------------> carrier returns to a runway
      v
Rocket-powered orbital stage (or compact multistage stack)
      |
      v
Low Earth orbit
```

This architecture deliberately avoids the least credible parts of an air-breathing single-stage-to-orbit vehicle:

- The air-breathing engine is not expected to operate in vacuum.
- The carrier does not accelerate its wings, landing gear, and air intakes all the way to orbital velocity.
- Rocket propulsion performs the vacuum and orbital-insertion portion of flight.
- Each propulsion system operates in the environment where it is effective.

## First-principles design constraint

A circular 300 km orbit requires approximately 7.73 km/s of horizontal velocity and 32.7 MJ/kg of ideal mechanical energy. Altitude accounts for only about 2.8 MJ/kg; most of the requirement is orbital kinetic energy.

| Carrier release condition | Approximate speed | Fraction of orbital speed | Fraction of orbital kinetic energy |
| --- | ---: | ---: | ---: |
| Subsonic carrier, Mach 0.8 | 0.24 km/s | 3% | 0.1% |
| Supersonic carrier, Mach 3 | 0.9 km/s | 12% | 1.4% |
| Hypersonic carrier, Mach 5 | 1.5 km/s | 19% | 3.8% |

These energy fractions do not equal propellant savings. Rocket propellant requirements follow the exponential rocket equation, so removing the first 1.5 km/s can improve the orbital stage's mass ratio substantially. The improvement must be balanced against the mass and cost of the carrier's wings, landing gear, hypersonic propulsion, thermal protection, and recovery systems.

## Subsonic versus Mach 5 carrier

### Subsonic air launch

A conventional carrier aircraft releasing near Mach 0.8 and 11-12 km provides only a small direct energy contribution. Its real advantages are operational:

- Existing aircraft, engines, maintenance practices, and airports can be used or adapted.
- The launch point can be moved to accommodate weather, range safety, and orbital inclination.
- The rocket begins above much of the weather and dense atmosphere.
- The carrier is recovered through ordinary runway operations.
- Development risk and capital requirements are much lower than for a hypersonic aircraft.

The disadvantages are equally important:

- Nearly all orbital velocity must still come from the rocket.
- A single rocket stage is unlikely to provide enough practical mass margin; a compact two-stage orbital stack is more credible.
- The carrier's payload and external-load limits constrain launcher size.
- Air launch does not inherently make the service inexpensive; aircraft modification, integration, range operations, and a flight crew add cost.

### Mach 5 air launch

A Mach 5 carrier releasing around 25-30 km offers a meaningful propulsion advantage:

- Approximately 1.5 km/s is supplied without carrying first-stage oxidizer.
- The orbital stage begins above almost all aerodynamic drag.
- The required rocket mass ratio can be materially lower.
- A high-performance single orbital stage becomes more credible, although not automatically practical.

An illustrative rocket-equation comparison, assuming an effective rocket specific impulse of 370 seconds, shows the leverage. These are not vehicle performance predictions:

| Simplified case | Rocket delta-v | Initial/final rocket mass ratio |
| --- | ---: | ---: |
| Ground-like ascent | 9.3 km/s | 13.0 |
| Subsonic-assisted ascent | 9.0 km/s | 12.0 |
| Idealized Mach 5 assistance | 7.8 km/s | 8.6 |

Mach 5 therefore can save much more than 4% of rocket propellant even though it supplies only about 4% of final kinetic energy. However, realizing that saving requires a new class of aircraft:

- Turbine/ramjet or another combined-cycle propulsion system.
- Variable hypersonic inlet and nozzle geometry.
- High-temperature structure and thermal protection.
- Safe stage separation in hypersonic flow.
- Large restricted test and operating corridors.
- Specialized fuel, inspection, and maintenance infrastructure.
- A recovery trajectory and runway compatible with the launch azimuth.

This is not conventional-airline technology merely because it uses a runway.

## Program decision

The recommended development sequence is:

1. **Subsonic demonstrator:** Prove airborne integration, release, autonomous ignition, range safety, payload deployment, and carrier reuse with a small orbital stack.
2. **Operational subsonic service:** Establish launch cadence and determine whether mobile air launch provides a valuable market advantage.
3. **Mach 2-3 research carrier:** Evaluate whether a turbine-based reusable first stage improves total system economics without requiring a scramjet.
4. **Mach 5 carrier only after propulsion demonstration:** Proceed only if full-scale engine, inlet, thermal, separation, and turnaround tests show a lifecycle cost advantage.

For near-term reliability and logistics, the subsonic carrier is the better product. For maximum payload fraction and reduced onboard oxidizer, Mach 5 is better in theory. The Mach 5 system becomes economically preferable only at a launch cadence high enough to repay its much larger development and maintenance costs.

## Success criteria

The architecture should be selected using total lifecycle cost rather than fuel consumption alone:

- Development cost and schedule
- Payload delivered per flight
- Carrier and orbital-stage turnaround labor
- Inspection hours per flight hour
- Abort and loss-of-vehicle probability
- Required launch-range closures
- Propellant and consumables
- Fleet size required for the target cadence
- Cost per successful kilogram delivered to the target orbit

## References

- [SpaceX Falcon User's Guide](https://www.spacex.com/assets/media/falcon-users-guide-2025-05-09.pdf)
- [NASA: Performance Advantages and Disadvantages of Air Launch](https://ntrs.nasa.gov/api/citations/20170005245/downloads/20170005245.pdf)
- [NASA: X-43A Hyper-X](https://www.nasa.gov/reference/x-43a/)
- [ESA: SABRE-powered reusable launcher system study](https://www.esa.int/Enabling_Support/Space_Transportation/Future_space_transportation/System_study_results_for_SABRE-powered_reusable_launcher)