Chapter 38 — Key Takeaways (SpaceX and the Reusability Revolution)
A one-page reference. Reread before an exam, or before you cost a launch or judge a reuse claim.
The one idea
Reusability does not repeal the rocket equation — it spends mass ratio (payload) to buy back the vehicle (dollars). The physics is unchanged; the economics collapse by roughly an order of magnitude, and that is what redraws the map of what is affordable in space.
The development method
| Traditional aerospace | SpaceX iterative / hardware-rich |
|---|---|
| Design exhaustively on paper, fly once, perfectly | Build, fly, fail, fix, repeat |
| Vehicle too precious to lose → slow, rare flights | Cheap, uncrewed prototypes → fast, frequent flights |
| Learning rate: low | Learning rate compounds; a landed booster is a returned test article |
Works only when each failure is inexpensive, survivable (uncrewed), and diagnosable (instrumented). Fly crew only after reliability is earned.
Falcon 9, in one row each
| Block | ~Year | Bought you |
|---|---|---|
| v1.0 | 2010 | first flights; expendable |
| v1.1 | 2013 | Merlin 1D, octaweb (engine-out), stretch; first soft ocean landings |
| Full Thrust | 2015 | densified propellant (+mass ratio); first successful landing |
| Block 5 | 2018 | design-for-reuse: cheap turnaround, 10+ (→20+) flights |
Definitions to own: Falcon 9 — partial-reuse 2-stage kerolox, 9 Merlins, booster recovered by propulsive landing (~17 t LEO recovered, ~22.8 t expended). block upgrade — a numbered revision bundling improvements while keeping the vehicle broadly similar.
The cost of reuse (the mass-ratio bill)
Reserved landing propellant leaves with the stage → it raises burnout mass $m_{f,1}$ → lowers mass ratio → cuts ascent delta-v: $$\Delta v_1 = v_e \ln\!\left(\frac{m_{0,1}}{m_{f,1} + m_{\text{reserve}}}\right).$$
| Recovery mode | Reserve | Payload cut (model & real) |
|---|---|---|
| Expended | 0% | baseline |
| Droneship | ~10% | ~25% |
| RTLS (adds boostback) | ~18% | ~40% |
Choose RTLS when faster/cheaper turnaround (operations, $F$) outweighs the extra payload lost.
Reusability economics
$$C_{\text{reuse}}(N) = \frac{M}{N} + R + F \;\xrightarrow{N\to\infty}\; R + F \;\;(\text{the floor}).$$
| Symbol | Meaning |
|---|---|
| $M$ | reusable-hardware build cost (amortized over $N$ flights) |
| $R$ | per-flight refurbishment |
| $F$ | fixed per-flight cost reuse never recovers (expended upper stage + ops) |
| $N$ | flights per vehicle |
Cost per kg (Tier 2, order-of-magnitude): Falcon 9 ~$2,700/kg (full) to ~$3,600/kg (recovered) vs Shuttle ~$54,500/kg → ~20× drop. Always ask: price or cost? full or typical payload? marginal or fully-burdened accounting?
The floor is the plot: first-stage reuse bottoms out at the expended second stage + ops. Only full reuse (both stages) crosses it → Starship.
Starship — trace every choice
| Choice | Traced to |
|---|---|
| Stainless steel | strong when hot (~1,100+ K) → structure is the heat shield → less TPS (Ch. 7, 24); cheap, weldable, strong at cryo (Ch. 23) |
| Methane (methalox) | clean reuse (no coking) + mid-density (Ch. 17/18) + Mars-makeable via Sabatier (Ch. 34) |
| Full reuse | crosses Falcon 9's cost floor (§38.4) |
| Catch the booster | delete legs = save mass where the rocket equation is stingiest + fast turnaround (Ch. 22) |
| Orbital refueling | full reuse eats the upper-stage mass ratio → arrives at LEO ~empty → refill in LEO for beyond-LEO (Ch. 34) |
| Belly-flop re-entry | flat to airflow = low ballistic coefficient → shed energy high, less heating (Ch. 7) |
Definition to own: Starship — fully reusable 2-stage methalox super-heavy (Super Heavy booster + Ship), stainless steel, ~100–150 t LEO (design goal, Tier 3).
Raptor and full-flow staged combustion (use Ch. 17's term)
Definition to own: Raptor — SpaceX methalox full-flow staged combustion engine; first FFSC engine to fly (2019); ~300 bar; built for mass production and reuse.
Why full-flow serves reuse (not just efficiency):
| Feature | Reuse payoff |
|---|---|
| Gas–gas injection | clean, stable combustion → less wear |
| Each turbine sees one propellant | no interpropellant seal (a historic failure point deleted) |
| Full flow through turbines | cooler turbines at high $p_c$ → engine longevity |
| Autogenous pressurization | no helium/COPVs (Ch. 22); nothing to ship from Earth → Mars-compatible |
Why 33 engines: liftoff $T/W \approx 1.5$ (and ~1.5 with one out → engine-out, Ch. 22) + one production line (cheap) + land on a few throttled engines. One choice, four wins.
Decision aids
| Question | Rule |
|---|---|
| Does reuse pay for my mission? | High flight rate → yes; a one-off probe → expend (recovery just costs payload) |
| Droneship or RTLS? | RTLS costs more payload but faster/cheaper turnaround; pick by delta-v budget |
| Why not reuse stage 2 now? | Its recovery mass (TPS/flaps/landing prop) wrecks its mass ratio → needs refuel; only worth it fully reusable |
| Steel or composite for reuse? | Re-entering many times → steel (hot-strength + cheap) often wins despite density |
Numbers worth remembering (all Tier 2/3)
- Reuse costs ~25% of payload (droneship); ~40% (RTLS).
- Falcon 9 ~$2,700–3,600/kg** vs Shuttle ~**$54,500/kg → ~20× drop.
- Reuse break-even ≈ 2 flights; cost floor = expended upper stage + ops.
- Raptor ~300 bar; Super Heavy 33 engines, liftoff $T/W \approx$ 1.5.
- Starship target ~100–150 t to LEO; a Mars mission ≈ ~11 launches (with tanker refueling).
Mission / astrotools additions this chapter
- MDR: a launch-economics note — does cheaper/reusable launch change your architecture, flight rate, and cost per kilogram? Feeds the launch-vehicle selection of Chapter 30 and the Chapter 40 capstone.
mission.py:reuse_cost_per_flight(M, R, F, N)andcost_per_kg(cost_musd, payload_kg).