Glossary of Spaceflight Terms

Term	Definition
T- or T+	Countdown time to Launch (-) or Elapsed time after launch (+), expressed as T-minus or T-plus.
Hold	A scheduled or unscheduled pause in the countdowns’ T- time.
L-	Time until launch, not subject to holds
NET	No earlier than
Scrub	When a countdown is aborted
MECO	Main Engine Cutoff
SECO	Secondary Engine Cutoff
Max Q	Period during ascent phase of maximum aerodynamic pressure on the rocket
RUD	Rapid Unscheduled Disassembly – Slang for rocket broke up or exploded
GEO	Geostationary Orbit. A satellite in GEO always maintains its position relative to a fixed point on the earth’s surface.
LEO	Low Earth Orbit – Orbital altitudes less than 1200 miles high
Bird	Slang for a space vehicle or rocket
LOX	Liquid Oxygen – a type of rocket fuel
RP-1	Kerosene-based rocket fuel
Kerolox	Liquid Oxygen and Kerosene-based rocket fuel
Methalox	Rocket fuel based on a combination of methane and liquid oxygen
Hydrolox	Hydrolox rocket fuel combines liquid hydrogen and liquid oxygen
Hypergolic, or Hypergol	Hypergolics are rocket propellant combinations used in a rocket engine, whose components spontaneously ignite when they come into contact with each other.
Solid Rocket Engines	A solid fuel rocket motor – cannot be started and stopped like liquid fuel based engines
LES	A solid fuel rocket motor – cannot be started and stopped like liquid fuel-based engines
SRB	See Solid Rocket Engines.
OMS	Orbital Maneuvering System – small thrusters used to make fine adjustments to a spacecraft’s position or orientation in space.
POV	Probability Of Violation (of weather)
EVA	Launch Escape System – rocket motor that propels crew module to safety in case of a launch phase malfunction.

More Information

T-0 or T-minus, T plus

The “T” is time and the dash is a minus. It’s the planned time for a liftoff on the countdown clock. It is usually listed in days, hours, minutes and seconds in a clock format. The clock runs backwards to zero, and at zero, the rocket is scheduled to fire up and fly off of the pad. When you hear an announcer giving the famous “10, 9, 8, 7” call, they are actually counting the last few seconds of t-minus.If there is a stoppage (see: hold) the clock stops.

At liftoff, the clock transitions to a “T plus” format, commonly seens a T+. T+ is used to track mission elapsed time all the way to the mission’s end. “T times” are pretty obvious, but there is a similar term, L-0 that isn’t exactly the same.

L-0 or L-minus

The “L” is launch and like t-minus, it is said as L-minus. When we reach L-0, the rocket is scheduled to launch. This one is almost the same as T-0, and it also counts to zero in the form of days, hours, minutes and seconds. Unlike T-0, the clock continues to count down even during holds. Many launches have planned holds in the countdown — Saturn V launches did, Space Shuttle launches did and the recent ULA Vulcan launch did. The T-minus clock stops, the L-minus clock continues.

NET

No Earlier Than — this is often used on launch schedules, and is meant to say that a given mission will not launch until a certain date and time.

Scrub

When a launch countdown is halted and that particular launch attempt is cancelled, it is “scrubbed.”

MECO

Main engine cutoff. This usually refers to when the first stage’s (the booster’s) engines turn off, meaning the first stage has completed its powered part of the flight.

SECO

Second (stage) cutoff. This usually refers to when the second stage of a rocket has completed its work and has been turned off. There are often two or three second-stage burns in an orbital flight, so there will be multiple SECO’s. For example, a typical SpaceX Starlink mission has one SECO when the second stage has reached orbital velocity and a second burn to change the orbit to its intended altitude. That in mind, Starlink missions usually have two SECOs.

Max-Q

This is a critical point in a launch. Max-Q is the point at which the rocket’s structure undergoes maximum mechanical stress. On one end, you have the rocket’s engines pushing it upwards through the atmosphere, and on the nose of the rocket, the air in the atmosphere is pushing back. This puts the vehicle under a lot of stress, and Max-Q is the point where those forces are the highest. As the rocket climbs past Max-Q, the air is thinner, so there is less stress on the rocket.

Think of it this way: if you are riding in a car on the highway and you open the window and stick your hand out in the rushing air, the air tries to push your hand backwards. To counter that, you have to push your hand forward to keep it in the same place. The same principle applies in spaceflight, except instead of staying the same place the rocket is accelerating. That resistance to that acceleration reaches a maximum point, or Max-Q.

RUD

Rapid Unplanned Disassembly. We never want to see that — it’s a nice way of saying that the rocket failed, or exploded. A good example of a RUD is SpaceX’s Starship Heavy’s second flight, where the company lost communications with the second stage and as a result the rocket automatically self-destructed, meaning it blew itself up so as not to endanger the public. That was a RUD.

GEO or Geostationary Orbit

When a satellite is placed in orbit, it circles the planet. At the same time, the Earth is spinning on its axis, which gives is day and night. If a satellite is placed high enough over the Earth, its orbit and the speed of the Earth turn more or less match, and the satellite never seems to move over a position on the ground.

Those orbits are roughly 22,236 miles high (35,785 km) and are handy for communications and observation satellites. Since the satellite is close to “stationary,” as seen from the ground, an antenna can be aimed once and left in that position to maintain communications. If you’ve ever seen a DirecTV or Dish satellite dish on the side of someone’s house, that’s what’s happening: the dish was aimed at a geostationary satellite and doesn’t need to be re-aimed to maintain TV service for that home.

LEO or Low Earth Orbit

They are orbits with an altitude of 2,000 km (1,200 mi) or less, and this is the region where most satellites and the International Space Station are located. Unlike a geostationary orbit, the satellite or ISS will appear to move across the horizon from the ground.

By definition, a satellite will complete a LEO orbit in 128 minutes or less.

Bird

In space terms, a bird is a space vehicle or rocket. This one is not used as much as it was in the 1960s, but you might hear it occasionally.

LOX

Liquid oxygen. LOX is mixed with some kind of fuel, for example, RP-1, a highly refined form of kerosene and the result is fire. SpaceX’s Falcon 9 uses RP-1 and LOX has its propellants.

RP-1

RP-1 is a highly refined form of kerosene outwardly similar to jet fuel (sometimes called JP-1), used as rocket fuel. One of the major differences between RP-1 and jet fuel is that RP-1 is refined to remove sulfurs even more than jet fuel. This is done because sulfurs don’t burn completely and create a waxy residue that decreases engine efficiency. That residue is often called “coke,” and that coke is definitely not the refreshing soda that you know.

Kerolox

Kerolox is the combination of RP-1 (a kerosene) and LOX, and is usually used to describe the propellant combination a given engine uses. SpaceX Falcon 9’s Merlin engines are kerolox engines because they use RP-1 and LOX as propellants.

An interesting thing about Kerolox engines is that their flame plume is usually a yellowish-red combination. Saturn V’s first stage engines (the F-1 engine) were the kerolox type, giving a bright yellow flame. Same thing with Falcon 9.

Methalox

Methalox is the combination of methane and LOX and is usually used to describe the propellant combination a given engine uses. Blue Origin’s BE-4 engines are methalox-based, as is SpaceX’s Raptor engines that they are using on their new Starship rocket.

Methane is also known as natural gas, though it is highly refined for spaceflight. Like a gas stove or a gas grill, the flame from a methalox engine is blue.

Hydrolox

Hydrolox combines liquid hydrogen and liquid oxygen, another set of propellants used to power rocket engines. The Space Shuttle and SLS main engines are hydrolox.

Hydrolox engines have nearly a clear flame, with some blue-ish tint.

Hydrolox is the most powerful propellant combination we currently use, but it can be problematic because the hydrogen molecule is the smallest molecule in normal chemistry. That being said, liquid hydrogen creates hydrogen gas, and that tiny molecule can and does find even the smallest of space to leak out of, creating a hazard for the rocket on the launch pad or in flight. You might recall SLS having those issues during its first few launch attempts. Because of that problem, many engine and rocket designers will use kerolox of methalox engine designs — they are easier to seal fully.

Hydrolox engines are also the “cleanest” engines in use. They combine hydrogen and oxygen and as such, the byproduct of their combustion is water in the form of steam.

Hypergolic, or Hypergol

Hypergolics are rocket propellant combinations used in a rocket engine, whose components spontaneously ignite when they come into contact with each other. Other propellant combinations like kerolox and methalox require an external ignition source to start the engines, while hypergolic ones do not.

The most common hypergolic fuels, hydrazine, monomethylhydrazine and unsymmetrical dimethylhydrazine, and oxidizer, nitrogen tetroxide, are all liquid at ordinary temperatures and pressures. Those chemicals are all highly toxic, many are carcinogenic, and all of them are highly unstable and, therefore, extremely dangerous. They are just waiting for a chance to start reacting, and as a result they can be highly explosive.

The reason that hypergolic propellants like hydrazine have been used in spaceflight since their inception is that they are extremely reliable because they are eager to react with each other and also because they don’t require highly specialized insulation to keep them from turning into a gas and escaping. That makes them perfect for control thrusters on a space capsule or for the re-entry engines, for example. Another use for hypergolics is for launch escape motors — an emergency where the capsule needs to get away from a failing rocket is one where instant escape thrust is vital, and that’s something that hypergolic motors excel at.

SpaceX uses hypergolic fuel combinations in its SuperDraco engines on the Crew Dragon space capsule for launch-escape capabilities. They also use the smaller hypergolic-based Draco engines for Dragon and Crew Dragon maneuvering thrusters. The reliability and instant response of those propellants is perfect for orienting the capsule in space.

Solid Rocket Engines

Most rocket boosters are liquid-based, that is, they use kerolox, methalox or hydrolox combinations to power the engines. Notably, Titan II, which powered the Gemini program in the 1960, was hypergolic based, but it too was a liquid-fueled rocket. Another class of engines use an entirely different set of propellants that can be combined with a polymer and then solidified into the shape that the rocket designers prefer.

A simplified solid rocket motor consists of a casing, nozzle, grain (propellant charge), and igniter. This is basically the design of a model-rocket booster, such as an Estes rocket that many people played with as kids. A more sophisticated solid rocket motor was the Space Shuttle’s SRB (solid rocket boosters) — the two side boosters on either side of the orange tank on the shuttle.

Solid rocket boosters provide a lot of power, but once ignited, they cannot be turned off or as easily throttled as a liquid-fueled Merlin engine on a Falcon 9. Strictly speaking, modern advanced solid boosters can be throttled to some degree and even extinguished, but those are relatively recent developments and are uncommon in most mainline production rockets.

They are mainly used as secondary “strap-on” boosters to give the rocket a big push off of the launch pad. You’ll see them quite often on ULA’s Atlas V and Vulcan rockets, but SpaceX and Blue Origin do not use them.

LES – Launch Escape System

Crewed launches almost always have an LES or launch escape system that will rapidly ignite and move the capsule (and astronauts) out of harm’s way. An exploding rocket is not where you want to be if you are an astronaut and the LES is what gets you away from it.

POV – Probability of Violation

The US Space Force has dedicated weather teams that forecast launch conditions for their and their clients’ missions. Unlike a typical weather forecast with a 30% chance of rain in the area tomorrow, they forecast the Probability Of Violation of launch weather criteria. That includes surface winds, winds aloft, the chance of lightning in the launch area during the launch window, the chances of thick cloud cover, the chances of the rocket flying through a rainstorm, and so forth.

If the USSF calls for a 10% POV, they think there will be a 90% chance that weather conditions will be acceptable.

EVA – Extra-Vehicular Activity

An EVA is generally considered to be when an astronaut dons a spacesuit and leaves their vehicle or space station to perform some sort of task — a spacewalk, walking on the surface of the moon, performing a repair or something similar. Keep in mind that a spacesuit is a spacecraft in its own right — it has to provide life-support for its occupant while they are doing an EVA!

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