Concepts A–Z
Every idea, A to Z.
One plain line on each idea our models and films lean on, and every page where it comes up: the model to change, the film, the Journal post and the guide. 24 terms so far.
A
Acoustic loads
Also: Launch noise
The shaking a rocket and its payload get from the rocket’s own roar, worst at liftoff, when the sound bounces off the pad.
- Try it
- Tall Tower Launch
Air pressure
Also: Atmospheric pressure
The weight of all the air above a point, so it also tells you how much of the atmosphere is still overhead.
- Topic guides
- Ways to spaceTesla Roadster
- Atmospheric pressure see Air pressure
Average and peak acceleration
Also: Peak acceleration
A steady push to a speed v over a length L averages a = v²/2L; real guns push hardest early, so their peak is several times higher.
- Try it
- Space Gun Lab
- Topic guides
- Ways to space
C
Centripetal acceleration
Also: Centripetal force
The inward acceleration, a = v²/r, that keeps anything moving on a circle; double the speed and it quadruples.
- Topic guides
- Ways to space
- Centripetal force see Centripetal acceleration
Circularization burn
The rocket burn near the top of a climb that turns a thrown, falling arc into an orbit that stays above the air.
- Topic guides
- Ways to space
D
Dynamic pressure
Also: Ram pressure
The push of the air on something moving through it, q = ½ρv², which grows with the square of the speed.
- Topic guides
- Ways to space
E
- Energy concentration see Tapered whip
- Expansion ratio see Nozzle expansion ratio
F
- Flow separation see Nozzle expansion ratio
G
G-force
Also: g-load
An acceleration measured against gravity’s 9.81 m/s²: at 10 g, a 1 kg part presses on its mount like a 10 kg one.
- Topic guides
- Ways to space
- g-load see G-force
H
Hardened electronics
Circuits built to keep working through the huge acceleration of a gun or sling launch.
- Topic guides
- Ways to space
- Hydrogen gun see Light-gas gun
I
L
- Launch noise see Acoustic loads
Light-gas gun
Also: Hydrogen gun
A gun driven by hydrogen or helium, light gases that can push a projectile several kilometres a second.
- Try it
- Space Gun Lab
- Topic guides
- Ways to space
M
- Mach cone see Sonic boom
Mach number
A speed divided by the speed of sound: Mach 1 is sound’s own speed, about 343 m/s in air at 20 °C.
- Topic guides
- Ways to space
Max Q
Also: Maximum dynamic pressure
The moment in a rocket’s climb when the air pushes on it hardest, and the load its structure is built for.
- Try it
- Tall Tower Launch
- Maximum dynamic pressure see Max Q
Multi-injection gun
Also: V-3 cannon
A gun that adds pushes along its barrel, from side charges or gas tanks fired in turn behind the projectile.
- Try it
- Space Gun Lab
- Topic guides
- Ways to space
N
Nozzle expansion ratio
Also: Expansion ratio, Flow separation, Vacuum bell
How much wider a rocket nozzle’s exit is than its throat; a bigger ratio speeds the exhaust up, but only works in thin air.
- Try it
- Tall Tower Launch
- Read
- Why launch rockets from a 20 km tower?Towers, slings, whips and space guns: Neal Stephenson’s launch ideas
- Topic guides
- Ways to space
- Explainers
- Rocket Nozzle Lab by Ryan Sael
O
Orbital speed
The sideways speed that makes a spacecraft fall around Earth instead of into it: about 7.8 km/s in low orbit.
- Topic guides
- Ways to space
P
- Payload fraction see Rocket equation
- Peak acceleration see Average and peak acceleration
R
Ram accelerator
A gun whose tube is filled with fuel and air that burns behind the projectile, so it rides its own combustion wave.
- Try it
- Space Gun Lab
- Ram pressure see Dynamic pressure
Rocket equation
Also: Tsiolkovsky rocket equation, Payload fraction
Tsiolkovsky’s Δv = vₑ ln(full mass ÷ empty mass): every extra bit of speed costs exponentially more fuel, so any head start saves a lot.
- Topic guides
- Ways to space
S
- Shadow photography see Shadowgraph
Shadowgraph
Also: Shadow photography
A way to photograph air by the shadows its density changes cast, which shows a shock wave as a sharp line.
- Try it
- Whip Crack Lab
- Shock wave see Sonic boom
Slingatron
Derek Tidman’s spiral-track launcher: the spiral is gyrated, so a projectile sliding along it speeds up on every turn.
Sonic boom
Also: Mach cone, Shock wave
The shock wave from something moving faster than sound, which trails it as a cone and is heard as a bang or a crack.
- Topic guides
- Ways to space
Specific impulse
Also: Isp
How much push an engine gets from its propellant, in seconds; times 9.81 m/s², it is the effective exhaust speed.
- Try it
- Tall Tower Launch
- Topic guides
- Ways to space
Speed of sound
How fast pressure waves cross the air: about 343 m/s at 20 °C, and slower in colder air.
- Explainers
- Sound by Bartosz Ciechanowski
T
Tapered whip
Also: Energy concentration
A whip that thins toward the tip, so a loop rolling down it carries its energy in less and less mass and speeds up.
- Try it
- Whip Crack Lab
- Topic guides
- Ways to space
Travelling loop
The rolling bend a flick sends down a whip; the whip at the loop moves twice as fast as the loop itself.
- Try it
- Whip Crack Lab
- Topic guides
- Ways to space
- Tsiolkovsky rocket equation see Rocket equation
V
- V-3 cannon see Multi-injection gun
- Vacuum bell see Nozzle expansion ratio
Vacuum chamber
The sealed, air-free chamber SpinLaunch spins its projectile in, so the spin-up meets almost no drag.
- Try it
- Spin Launch Lab
- Topic guides
- Ways to space
Each model page explains its concepts with its own numbers and the research behind them, under “Concepts in this film”. For whole questions, start with the topic guides.