68 votes
Accepted

Was the payload bay of the Space Shuttle depressurized before launch?

Not before launch, during launch. The structure of the payload bay (and the other Orbiter compartments except for the crew compartment) was not strong enough to withstand either crush loads from a ...
Organic Marble's user avatar
59 votes
Accepted

What "actually" happens at T-minus-0

T minus zero appears to generally indicate the moment of booster liftoff, but can also indicate something different. To borrow Cort Ammon's wording, it's an action or event that results in a ...
user's user avatar
  • 7,322
59 votes
Accepted

Why did the JWST solar array deploy early?

JWST solar panels have limits on what angular rate of rotation they can be deployed at. After separation, there is rotation introduced into the spacecraft, which it has to dampen with thrusters until ...
Barry Jenakuns's user avatar
37 votes

Was Starship’s “launch window” administrative, rather than due to orbital mechanics?

Besides the "physical" constraints of launch windows (orbital parameters, technical limits like fueling), keeping a launch window "open" is also associated with a large fiscal and ...
Dragongeek's user avatar
  • 18.6k
30 votes
Accepted

Were the Space Shuttle's Main Engines ever off while the Solid Rocket Boosters were firing?

Hydrogen-oxygen engines produce a relatively faint blue flame, with visible blue-white shock discs or diamonds under certain circumstances, particularly low altitudes where air pressure confines the ...
Russell Borogove's user avatar
28 votes
Accepted

What is the rocket engine chilldown?

Chilldown is needed for engines using cryogenic propellants -- liquid oxygen as the oxidizer most commonly, sometimes in conjunction with liquid hydrogen as the fuel. Pumping large amounts of very ...
Russell Borogove's user avatar
28 votes
Accepted

What does 'Rocket is in self align' mean?

Up until that point in time, ground control has been regularly telling the vehicle where it is in inertial space relative to the center of the Earth, and in which direction it is pointing in inertial ...
David Hammen's user avatar
  • 74.2k
27 votes

Were the Space Shuttle's Main Engines ever off while the Solid Rocket Boosters were firing?

No, they were never off. Here is my explanation: have you ever noticed that the top engine is tilted down? This is us to keep the center of thrust more or less aligned with center of mass of the ...
le_daim's user avatar
  • 2,060
20 votes
Accepted

In the Saturn V launch sequence, what defined COMMIT, and what happened between COMMIT and LIFTOFF?

This article might offer some clarity. The Saturn V was held down to the pad until T=0. The engines started up several seconds before. The engines took a few seconds to build up to and stabilize at ...
Anthony X's user avatar
  • 17.4k
19 votes

What exactly is the purpose of connection links strapped between the rocket and the launch pad

In general, umbilicals are provided from the launch pad to the vehicle for any services that need to be provided after the vehicle is installed on the launch mount, and to remove hazardous gases from ...
Organic Marble's user avatar
19 votes
Accepted

How (and when) was the RTG in the lunar modules installed?

Partial answer (everything but exactly when in the timeline it was done): The fuel capsule was installed at the launch pad "through a ten-inch access port in the spacecraft structure". The fabulous ...
Organic Marble's user avatar
16 votes

How much throttleback to keep acceleration under 3g?

Shuttle: tl;dr - Shuttle throttled down based on the sensed acceleration. The "bobbling" about the 3g limit you see is because a proportional-integral control scheme was used and because the ...
Organic Marble's user avatar
13 votes

Why was Jason-3 the last launch we heard the T-13 readiness poll during a SpaceX Webcast?

The go/no go poll is the last major hurdle before the launch. If one of the polled stations says 'no go', the launch will be delayed. If the rocket is full of subcooled propellant, that delay means ...
Hobbes's user avatar
  • 127k
13 votes

Weaponizing the Starlink constellation

I'm going to restrict my answer to only the question of "can Starlink satellites be used as kinetic weapons." I'm pretty sure the answer is "not in any practical sense." The ...
Erin Anne's user avatar
  • 10.8k
12 votes

How much throttleback to keep acceleration under 3g?

The engines of the Saturn V were not individually throttlable. Instead, in the first two stages, the center engine of the five-engine cluster was shut off entirely -- for the first stage, this is done ...
Russell Borogove's user avatar
11 votes

How was the Centaur stage fueled in the Space Shuttle?

It was filled through the "T-0 umbilicals" (referring to the time of disconnect). LO2 through an umbilical on the Orbiter boattail, LH2 through one in the midbody. This schematic shows the plumbing ...
Organic Marble's user avatar
11 votes

What "actually" happens at T-minus-0

The awkward pause is mostly due to the commentator. He counts along with an on-screen counter until T-0. Then he waits until he sees the rocket move, or he hears ground control confirm liftoff, before ...
Hobbes's user avatar
  • 127k
10 votes

Were the Space Shuttle's Main Engines ever off while the Solid Rocket Boosters were firing?

Offered as a supplement to the other answers - here's a frame from an SRB mounted camera (post-separation) showing the SSMEs running in the absence of SRB plumes. As stated by others, there's no real ...
Organic Marble's user avatar
10 votes

Why should the roll program position the shuttle's nose to the east?

The phrase 'with the nose to the east' is a shorthand. The direction depended on the orbital inclination chosen for the mission. Missions to the ISS have to match the 51.6° inclination of the ISS ...
Hobbes's user avatar
  • 127k
9 votes
Accepted

What does "all systems are internal" mean?

This can refer to a number of things, the specific source (mission and point in time of the countdown) would be helpful. It can refer to all systems running on internal power sources rather than ...
DarkDust's user avatar
  • 12.6k
8 votes
Accepted

What does CLG INIT mean?

"CLG INIT" stands for "closed loop guidance initiation", and means that the launcher is switching into a mode where it actively steers based on its current trajectory rather than a fixed, ...
Russell Borogove's user avatar
8 votes
Accepted

Are rocket tanks being filled up until the last seconds before liftoff?

Neither Ariane nor the Space Shuttle are refueled while the engines are running. I don't know of other launch vehicles that have extended periods where the engines are running before liftoff. The ...
Rikki-Tikki-Tavi's user avatar
7 votes
Accepted

What's the first recorded use of a countdown associated with a rocket launch?

According to Rockets, Missiles, and Space Travel by Willy Ley, the Fritz Lang movie Girl in the Moon (1926) featured a fictional rocket launch countdown for dramatic purposes and it was adopted by the ...
MAGolding's user avatar
  • 211
7 votes
Accepted

Why the long (19 hour) mission profile for SpaceX's trip to ISS?

To achieve a 4 hour docking time, you have to really have everything working perfectly. Crew Dragon is still a very experimental spacecraft, and won't do such a quick docking for a few reasons. Of ...
PearsonArtPhoto's user avatar
  • 121k
7 votes
Accepted

Space shuttle MECO condition: velocity threshold or fuel level?

Either. For the first part, you already answered your question. The main criterion was velocity, because the SSMEs on the orbiter are the second (and orbit insertion) stage of the Shuttle. There's no ...
TooTea's user avatar
  • 1,585
6 votes

How can the elements involved in loading a LOX tank be involved in an explosion?

Just to mention one facet of this multifaceted problem, any electrical field potential that exists (i.e. items that aren't on a common bias electrically) can and usually will at some point create an ...
Russ Reed's user avatar
  • 119
6 votes
Accepted

What event determined the transfer of control from Kennedy Space Center to Houston?

For Space Shuttle: In the flight software, "tower clear" was defined as the end of the vertical rise phase when it was OK to start the Single Axis Rotation (aka "roll program"). This happened when ...
Organic Marble's user avatar
6 votes
Accepted

How were the pins for Saturn V controlled release tested?

This page describes in some detail the design of the holddown mechanism: https://www.hq.nasa.gov/office/pao/History/SP-4204/ch13-4.html It doesn't say how they were tested but it says they were ...
Seth Kurkowski's user avatar
6 votes

Simple alternative to Powered Explicit Guidance for orbital insertion?

I have also been unable to implement PEG and struggled to find an adequate approach in my orbital launch sim. I have tried simply steering proportional to fraction-of-orbital-altitude-reached in the ...
Russell Borogove's user avatar
6 votes

Which orbital elements are normally targeted at launch?

Plane changes are expensive in terms of fuel consumption, making it essential that any launch intended for rendezvous with the ISS must target an orbit whose orbital plane is very close to that of the ...
David Hammen's user avatar
  • 74.2k

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