It may differ from one launch vehicle to another, but most orbital rockets follow a similar procedure during a launch sequence. This article takes a closer look at the steps involved before and during a rocket launch.
Before and during any major orbital rocket launch, a series of calls are made between the Launch Control Center and various other parties involved in the launch vehicle’s mission. Much of this communication can often be heard by TV and online audiences.
It may sound like a longwinded and tedious process consisting of numerous calls, checks, and confirmations that starts hours before actual liftoff and continues well into the rocket’s flight. But as the following section will describe, these steps are very necessary & important.
Why So Many Steps Are Needed In A Rocket Launch Sequence
A modern launch vehicle stands on average approximately 58 meters (190 feet) high with a weight of 1063 metric tons or 2.34 million pounds. More than 85% of this mass consists of highly flammable liquid propellants.
(The average height and weight were determined by calculating the average length and mass of the 30 biggest and most widely used launch vehicles ever produced, past and present. Learn more by reading the original article here.)
A vehicle of these dimensions and mass, filled with highly combustible propellants, can have catastrophic consequences if a failure in any part of the craft leads to an explosion while on the launchpad or in flight.
An explosion on the launchpad will not only result in the destruction of the launch vehicle but can also damage or completely destroy the launchpad and support structure. If all personnel is not vacated at this point, it may also lead to serious injury or loss of life.
(If a vehicle is crewed, extra measures need to be taken to ensure the safety of all astronauts/cosmonauts onboard, both before and after launch.)
If a rocket is already airborne when an explosion occurs, debris can fall on the surface below and potentially cause widespread damage and injury. A rocket does not only travel straight up but also covers a horizontal distance of hundreds of miles as it starts to pitch over.
Apart from the danger it poses, a rocket consists of millions of unique components and thousands of moving parts. It also uses strong, lightweight materials like titanium and aluminum, which are expensive, especially when used over a rocket’s large surface area.
As a result, orbital rockets typically cost several hundred million dollars (a conservative estimate since NASA estimates that its SLS rocket will cost 4.1 billion dollars per launch). Losing such an expensive launch vehicle in an explosion is an extremely costly exercise.
A failure and resulting explosion will not only result in a massive financial loss but also valuable time. The time it takes to plan and execute the manufacturing of a launch vehicle spans more than a year and can take multiple years in some cases.
It should come as no surprise, then, that so many measures are in place, with the status and conditions of a multitude of components and conditions in and around the launch vehicle’s launchpad and its flight path that needs to be verified before and after liftoff.
Steps In A Rocket Launch Sequence
As the previous section clearly highlighted, it is essential that all the steps in a rocket’s launch sequence are followed to ensure the safety of ground staff, protect the vehicle and its surroundings, and prevent any malfunction from occurring or causing a potential failure.
(Needless to say, if the vehicle is crewed, the safety of crew members takes center stage.)
However, not all orbital rockets follow the same steps or have the same launch sequences leading up to and following liftoff. Even the same type of launch vehicle may have different launch sequences, depending on the payload, designated orbit & location of the launch site.
There are several steps and procedures, though, that are common to all orbital rocket launches that are followed at each event, even if they do not occur at precisely the same time or in the same order.
The following events are the primary steps in a rocket launch. The sequence of events is generally in the correct order, but it has to be noted that it will differ from one launch vehicle to another, including the time at which each event occurs:
- Rocket Transported To Launchpad
- Vehicle Powered Up
- Weather Briefing
- Launch Area Evacuation
- Commence Propellant Loading
- Vehicle On Internal Power
- Final Flight Readiness Poll
- Roll & Pitch Maneuver
- Vehicle Goes Supersonic
- Maq Q
- First Stage Separation
- Second Stage Ignition
- Payload Fairing Separation
- Payload Deployment
One will be able to get a better understanding of what each step entails by taking a closer look at them individually.
1) Rocket Transported To Launchpad
From 12 hours up to a day before launch, the rocket is transported from the assembly facility to the launchpad. Some launch vehicles are already in their vertical position before final transportation to the launchpad, while others are transported horizontally.
For example, SpaceX Falcon 9 rocket is transported horizontally aboard a Transporter Erector (TE) to the launchpad, where the “Strongback” hoists it into a vertical position. Russia’s Soyuz rocket is also transported via rails in a horizontal position to its launch site.
The Space Shuttle, Delta IV, and Atlas rockets, though, were already assembled in a vertical position inside the Vehicle Assembly Building (VAB) or Vertical Integration Facility (VIF) before being transported on top of a Mobile Launcher Platform (MLB) to the launchpad.
2) Vehicle Powered Up
The vehicle is powered up as long as a day before launch. This is to run a wide range of system checks to ensure all electrical, electronic, and other equipment is functioning correctly and prepare the vehicle for several pre-launch events, including propellant loading.
3) Weather Briefing
For any rocket launch, it is crucial to have regular weather updates during the period leading up to & during liftoff. It does not only apply to the area surrounding the launch site but also the downrange region and possible emergency landing sites in the case of a crewed flight.
The weather plays a major role in any launch event and can have a significant impact on a launch vehicle while on the launchpad or in flight. Several weather conditions, specifically wind and lightning, can adversely affect a rocket and potentially destroy it.
(Going into detail about the various effects of weather on a rocket launch falls beyond the scope of this article, but you can read the full, in-depth article here.)
As a result, space agencies have put in place a number of measures, including regular weather updates from all locations involved in an orbital rocket launch. The first of these official weather updates take place as early as T-12 hours (12 hours before launch).
Meteorologists from various agencies reporting to launch control continuously monitor atmospheric conditions, and a final weather briefing is given approximately 10-15 minutes before liftoff.
4) Launch Area Evacuation
Hours before liftoff, it may look quiet and serene as the rocket stands motionless on the launchpad, waiting for the countdown clock to reach zero, but looks can be deceiving. There is actually a lot going on in and around the launchpad itself.
Fuel tanks, pipes, and other equipment involved in the fueling process are pressurized and prepared for fueling. Equipment involved with the transport and storage of cryogenic fuels, including the launch vehicles’ tanks, are also cooled down or “chilled.”
Various pneumatic and hydraulic systems are also pressurized on the pad, and in some cases, the safety mechanisms of explosive charges are also disabled during this period.
These are just a few of a large number of potentially dangerous processes that occurs at a launch site in the hours leading up to launch. As a result, a launchpad is not a safe environment for any human being during this period.
Non-essential ground crews are already evacuated from the launch area up to a day before liftoff, and all personnel leaves the site no later than approximately 2-3 hours before launch.
5) Commence Propellant Loading
This section covers the loading of propellants into a liquid-fueled launch vehicle, which is actually a series of events that involves the loading of fuel and oxidizer into their designated tanks before launch.
To simplify reading, these events are grouped under one section, but it has to be noted that it involves multiple events occurring at different times in the leadup to liftoff.
Fueling at the launch site is limited to liquid-fueled rockets since solid-fueled rocket boosters are already filled with solid propellant at the manufacturing facility and transported as complete units to the launch site after final assembly.
The primary liquid propellants used in modern orbital rockets are a highly refined form of kerosene (RP-1), liquid hydrogen (LH2), and liquid methane (CH4). Liquid Oxygen serves as the common oxidizer used for the three fuel types.
(Learn more about the different fuels orbital rockets use, their characteristics, and the advantages/disadvantages in this article.)
Depending on the launch vehicle, fuel loading into the launch vehicle starts approximately 1-3 hours before launch. For example, SpaceX’s Falcon 9 commences RP-1 loading at T-1 hour and ten minutes, while a Delta IV starts its fuel loading as early as T-4 hours and 50 minutes.
The lengthy fueling process involves first pressuring all tanks and cooling the ones filled with cryogenic fuels. The fuel and oxidizer are then pumped into their respective tanks in separate events into both the first stage and second/upper stage sections of the vehicle.
(An orbital rocket uses at least two stages to allow it to reach space and establish an orbit around the Earth or travel to other celestial bodies in the Solar System. Learn more about rocket staging and why it is needed for a launch vehicle to get to orbit in this article.)
Another important prelaunch event that coincides with propellant loading is called an engine chill, where a small amount of cryogenic propellant is allowed to flow to the turbopumps and other parts of a rocket engine to cool them down to prevent any thermal shock.
(Learn more about a rocket engine chill or prechill and why it forms such a crucial part of any orbital launch in this in-depth article.)
6) Vehicle On Internal Power
In the hours leading up to the launch, rockets get their power supplied from either their mobile launch platforms (MLB) or the support structure on the launchpad.
At approximately T-5 minutes, the launch vehicle disconnects from all external power sources and starts using its internal fuel cells. At this point, the rocket is fully fueled and independently powered.
7) Final Flight Readiness Poll
With literally hundreds of people involved with the various systems and processes that form part of a rocket launch, including weather updates from the launch site/range, it is crucial to get an update on the status of each from the responsible individuals on a regular basis.
A flight readiness poll, where the flight director calls and gets feedback from all responsible parties regarding the status/readiness of their particular system or process, is held on more than occasion. A first poll is held up to 12 hours before launch.
At approximately T-10 minutes (a time which can vary significantly), the flight director gets the final go/no go from the various parties before giving the “go for launch” and commencing with the final countdown.
Seconds before the launch, the various first stage boosters (and or solid rocket boosters) on the rocket ignite, and at T-00 minutes, the vehicle lifts off and starts to accelerate into the atmosphere.
9) Roll And Pitch Maneuver
In only a couple of seconds, after the rocket has cleared the towered and started to gain altitude and speed, the rocket performs its roll and pitch maneuver.
The roll maneuver is for the rocket to align itself to its launch azimuth while the rocket pitches over to perform a gravity turn (or zero-lift turn) which allows the rocket to set it on its intended trajectory and assist it in accelerating to orbital speed.
(Learn more about why rockets roll after launch and what a launch azimuth is in this article.)
10) Vehicle Goes Supersonic
At approximately T+80 seconds (1 minute and 20 seconds after launch), the vehicle goes supersonic, meaning it is traveling faster than the speed of sound or has surpassed Mach 1.
11) Max Q
At T+90 seconds, the vehicle passes through the area of maximum dynamic pressure. During this period, the rocket experience the biggest gravitational pull, most drag while achieving maximum thrust, which put an incredible amount of stress on the vehicle’s structure.
The launch vehicle powers its thrusters slightly down during this period to reduce the stress on the vehicle.
Shortly after Max Q, at approximately T+3 minutes, the propellants of the first stage are depleted, and the vehicle powers down its first stage boosters in preparation for First Stage Separation. It is commonly referred to as MECO (Main Engine Cut Off).
13) First Stage Separation
After MECO, the First Stage section of the rocket separates from the rest of the vehicle at T+3 minutes and 30 seconds by detonating explosive bolts or pneumatic systems on the vehicle’s interstage. (A rocket’s interstage connects the different stages of a launch vehicle.)
14) Second Stage Ignition
After safely jettisoning the first stage away, the launch vehicle’s second stage ignites its engine to carry its payload to its intended destination at T+4 minutes. Depending on the payload and mission, the second stage thrusters may shut down and reignite several times.
After performing a burn for a predetermined period of time, the second stage engine shuts down, a process referred to as SECO (Second Engine Cut Off).
If only one burn was required, this event would be followed by the delivery of its cargo to orbit or releasing it to continue on its predetermined trajectory.
16) Payload Fairing Separation
Before an orbital rocket can release or deploy its cargo, it needs to jettison its payload fairing (PLF) away through a process known as Payload Fairing Separation. The payload fairing is the outer covering or shell of the cargo bay and often also forms the nosecone of a rocket.
It typically consists of two halves that separate in a horizontal fashion away from the vehicle. Although it needs to be ejected before the cargo can be deployed, it is often jettisoned away as early as T+4 minutes and 30 seconds.
(This can take place since the rocket is out of the thick atmosphere at this point and not subject to atmospheric influences like drag and air particles that can damage the payload.)
17) Payload Deployment
The final part of the launch vehicle’s mission is delivering its payload to its intended orbit (or putting it on a trajectory to another celestial body or allowing it to travel to the edge of the Solar System and beyond, as is the case with the Voyager 1, launched in 1977.)
This final leg of an orbital rocket’s mission can last hours, days, or even months, depending on the mission, and may involve several burns, orbital adjustments, and getting gravity assists from other celestial bodies in the Solar System.
With the payload fairing out of the way, the launch vehicle’s cargo is released from its upper stage to continue to establish itself in orbit around the Earth or continue on its journey to its intended destination.
There is an almost overwhelming amount of steps involved in the launch sequence of a rocket, but as this article highlighted, every single step is crucial to ensure the safety and success of an orbital rocket launch.
This article mainly focused on the primary steps involved in both the pre-launch and launch sequences and what processes are involved with each step. It is by no means an exhaustive list of all the steps but highlighted the most crucial ones.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.