the problem with ground launch
to reach low earth orbit, you need ~7.8 km/s of velocity.
but launching from the ground, you actually need ~10 km/s.
where does the extra 2.2 km/s go?
| loss type | delta-v cost |
|---|---|
| gravity drag | ~1.0-1.5 km/s |
| aerodynamic drag | ~0.3-0.5 km/s |
| steering losses | ~0.1-0.2 km/s |
that’s 20-25% of your total delta-v budget wasted fighting the atmosphere and gravity.
the rockoon concept
“rockoon” = rocket + balloon. first proposed in 1949 by van allen (yes, the radiation belt guy).
the idea: float a rocket to 35 km altitude on a stratospheric balloon, then ignite.
at 35 km:
- atmosphere is 1000x thinner than sea level
- 99% of aerodynamic drag eliminated
- air density: ~1% of sea level
the numbers
| parameter | ground launch | 35km launch |
|---|---|---|
| air density | 1.225 kg/m³ | ~0.01 kg/m³ |
| drag losses | 0.3-0.5 km/s | ~0 km/s |
| gravity drag | 1.0-1.5 km/s | ~0.5-0.8 km/s |
| max Q stress | high | minimal |
| engine type | sea-level optimized | vacuum optimized |
| Isp gain | baseline | +10-15% (vacuum nozzle) |
vacuum-optimized engines have better specific impulse because the nozzle can expand fully without fighting back-pressure.
why this matters
delta-v savings cascade through the rocket equation:
Δv = Isp × g₀ × ln(m₀/m₁)every 1% improvement in Isp or mass ratio compounds.
if you save 0.5 km/s on drag and get 10% better Isp from vacuum engines, that’s potentially 15-20% less propellant mass needed.
less propellant = smaller tanks = lighter structure = even less propellant.
the tyranny of the rocket equation works both ways.
current players
- b2space - first successful balloon-rocket launch
- zero2infinity - bloostar concept
- jp aerospace - project tandem (balloon-based space platform)
these are real companies doing real tests. not paper concepts.
the catch
scaling is hard.
to lift an orbital-class rocket (~10,000 kg), you need a massive balloon. we’re talking football-field sized.
balloon stability at max altitude is tricky. winds in the stratosphere can exceed 100 m/s during certain seasons.
ignition at altitude requires robust systems - no abort once you’re floating at 35 km.
why it’s still worth pursuing
ground launch infrastructure costs billions.
a balloon launch platform can theoretically launch from anywhere - ocean, desert, equator.
equatorial launches save another ~0.4 km/s from earth’s rotation velocity boost.
the combination: balloon + equatorial + vacuum engines could save 1-2 km/s total delta-v.
that’s the difference between a 2-stage and 3-stage rocket.
prior art declaration
this post documents publicly known physics and engineering concepts as of 2026-01-30.
the rockoon concept dates to 1949. the physics is well-established. several companies are actively developing commercial versions.
rune.みんな