Shuttle Countdown and Launch FAQ

 

Why do you drop water under the shuttle as the engines start?

Has the shuttle ever been rolled back to the Vehicle Assembly Building due to bad weather conditions?

Why isn't there a sonic boom when the shuttle launches?

How hot are the flames from the shuttle engines?

How is the shuttle transferred from the crawler to the launch pad?

Why does the space shuttle constantly increase its speed at lift-off, even though its thrust is the same?

Why must the shuttle be launched vertically?

How long before liftoff does the crew board the space shuttle?

Why is it easier to launch the shuttle when closer to the equator than it is closer to the poles?

How long is a countdown?

Are the shuttle's elevons used at all in the roll maneuver?

How does the local fire department assist in the launch activities?

What keeps the flame from creeping back into the oxygen tank on the Space Shuttles Main Engines?

How many shuttle launch sites are there and where are they?

What are the sparks you see before the main engines starts?

What does your body feel like when you take off?

How noisy is the shuttle inside the flight deck when the shuttle is taking off?

Why doesn't the shuttle get as hot when it leaves the atmosphere as it does when it reenters the atmosphere?

How many people are working when a liftoff takes place?

After a launch, how much repair is needed to prepare the pad for the next launch?

What is the reason for the roll maneuver after launch?

Why is it important to have an accurate launch window?

 

 

QUESTION:

Why do you drop water under the shuttle as the engines start?

 

ANSWER from Michael Hogue on January 5, 1999:

The water deluge just prior to launch of the Shuttle is to reduce acoustic (sound wave) forces on the Shuttle. Prior to installing the water spray, videos of the first Shuttle launch (STS-1) showed the Shuttles' tail structure vibrating greatly from the reflected acoustic load from the pad surface and structures. A static body of water would not work as well since sound waves would still be reflected from the flat surface with little absorption. A turbulent water spray has a tremendous and broken up surface area for wave reflection and absorption.

 

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QUESTION:

Has the shuttle ever been rolled back to the Vehicle Assembly Building due to bad weather conditions?

 

ANSWER from Ed Mango on October 3, 1998:

There has been four occasions in which the Shuttle had to be rolled back to the VAB due to Hurricane forces. I have listed below a Web page where you can find all the specific requirements for rollback. STS-79 was rolled back twice in 1996 for Hurricane Fran and Bertha. STS-69 was rolled back for Hurricane Erin, and STS-35 was rolled back for Tropical Storm Klaus. The key reason to roll back is to protect the Shuttle from damaging winds and the strong probability of tornado's associated with hurricanes.

 

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QUESTION:

Why isn't there a sonic boom when the shuttle launches?

 

ANSWER from Francis J. Merceret on January 5, 1998:

The Shuttle approaches sonic speed at an altitude of about 25,000 feet and at that point it is moving up and away from the launch site. The shock waves (which cause the sonic booms) propagate upward and away from the launch site as well since they move in the same general direction as the vehicle. The result is that shock waves generated during the supersonic portion of the launch trajectory do not intersect the launch site -- so no sonic boom there.

 

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QUESTION:

How hot are the flames from the shuttle engines?

 

ANSWER from Greg Katnik on April 16, 1997:

The temperature inside a solid rocket motor (combustion chamber) is 5,630 degrees Fahrenheit. The temperature has cooled a little (5,400 degrees F) at the nozzle throat and nozzle exit. Those temperatures are hot enough to melt metal, yet the insulation inside the steel motor cases and graphite layers inside the nozzle are able to contain the heat. The Orbiter main engines are fed with fuel (hydrogen) and oxidizer (oxygen) from the External Tank. The hydrogen is at a -423 degrees F while the oxygen is -297 degrees F. When these are mixed for burning in the engine combustion chamber, the temperature is 6,000 degrees F. To keep the nozzle itself from melting, liquid hydrogen is circulated through small capillary tubes inside the wall of the nozzle. This keeps the nozzle cool and warms up the hydrogen prior to the combustion process.

 

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QUESTION:

How is the shuttle transferred from the crawler to the launch pad?

 

ANSWER from Grant Cates on January 19, 1998:

The Space Shuttle Vehicle (SSV) is built-up upon the Mobile Launcher Platform (MLP) in the Vehicle Assembly Building (VAB). The MLP rests upon pedestals in the VAB. The Crawler/Transporter (CT) is driven into the VAB and positioned under the MLP. The CT then raises its load-bearing platform until it makes contact with the bottom side of the MLP. The CT platform continues to raise the entire MLP/SSV until it is clear of the pedestals. The CT then carries both the MLP and the SSV (which is atop the MLP) to the launch pad. When the CT - with its MLP and SSV - arrives at the launch pad, it is very precisely positioned over the launch pad pedestals. Then the CT lowers the MLP onto the pedestals so that the pedestals bear the weight of the entire MLP/SSV. The CT continues to lower its load-bearing platform until it is clear of the MLP. The CT is then driven out from under the MLP and returns to the CT parking area.

 

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QUESTION:

Why does the space shuttle constantly increase its speed at lift-off, even though its thrust is the same?

 

ANSWER from Glen Davis on July 7, 2000:

The increase in speed is do to the decrease in weight. The shuttle weighs approximately 4.5 million pounds at lift off. The shuttle by it self (less fuel, external tank and solid rocket motors) weights less than 100,000 pounds. As the fuel is burned off the weight is dramatically reduced thus the speed increases.

 

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QUESTION:

Why must the shuttle be launched vertically?

 

ANSWER from Ed Mango on April 3, 1998:

The answer is in the physics. In order to get an object to orbit the earth, it must be travelling at over 17,000 mph. Using today's technology, the only way to generate enough energy to get the shuttle to orbit is with engines that are more powerful then jet engines. Those engines use fuels like liquid oxygen, liquid hydrogen and solid fuels like in a bottle rocket. The Space Shuttle generates about 6,000,000 pounds of energy (thrust) to get the orbiter to orbit at the right speed. There are no known wheels, tires or runways that can handle that much energy. So, we designed the shuttle to launch vertically.

 

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QUESTION:

How long before liftoff does the crew board the space shuttle?

 

ANSWER from Rebecca Aubin on March 28, 1999:

The crew enters the Shuttle on the launch pad between 2-3 hours before lift-off. Once inside, the astronauts have to be seated properly, with good harness fittings and suit comfortably for their ride. They also have to go through procedures, such as testing out the voice quality of the crew to Florida's Control Center and Houston's Mission Control Center. They have to make sure that there is clear communication to/from the Shuttle. The astronauts also have to make sure that all the switches onboard work, as well as emergency lights. While things are taking place inside the Shuttle, there are also things taking place outside of the Shuttle, such as the fueling of the orange External Tank attached to the Shuttle and the moving of the launch pad structure away from the Shuttle for launch.

 

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QUESTION:

Why is it easier to launch the shuttle when closer to the equator than it is closer to the poles?

 

ANSWER from Eric Hammer on January 26, 1999:

It uses more fuel to launch at the poles than at the equator. When we launch a rocket, we like to do it so it does use the least amount of fuel, because that makes the mission less expensive. Since the earth rotates to the east, we always launch roughly towards the east so that we can use the velocity of the earth's rotation to help us get to orbit. This allows us to get to orbit with less gas. The maximum gain in velocity is realized at the equator. However, all NASA space shuttle launches occur at Cape Canaveral in Florida. This is not at the equator, but it is the closest we could get to the equator while still having an acceptable launch site for safety reasons. If we were to launch at the poles, there is no eastward rotation, and therefore we would need more gas to get to the same orbit than if we launched at the equator.

 

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QUESTION:

How long is a countdown?

 

ANSWER from Glen Davis on October 14, 1999:

There are two countdowns happening simultaneously at every launch. One is referred to as L- (minus) the other is called T- minus. The L- starts 72 hours before lift off and counts down continually till the orbiter launches. T- time has built in holds. You may have heard the term T-nine minutes and holding. At nine minutes we put in a ten-minute hold to take care of any issues that might surface.

 

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QUESTION:

Are the shuttle's elevons used at all in the roll maneuver?

 

ANSWER from Bryan Lunney on August 11, 1998:

The answer to this question is yes and no. The elevons are moving during the roll just after launch, but this is done only to relieve aerodynamic loads on the elevons. In no way are they being used to control the vehicle during the roll maneuver, or any other time during ascent. The SRB and SSME nozzles are moved during ascent to maintain vehicle control.

 

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QUESTION:

How does the local fire department assist in the launch activities?

 

ANSWER from Bill Gary on October 15, 1997:

The KSC fire department assists in launch activities by providing a stand by crew of firefighters and fire fighting equipment for space shuttle launches and landings. Special equipped trucks and other fire fighting vehicles are parked near the shuttle landing facility when the shuttle returns from orbit to quickly get the astronauts to safety should there be a problem while the shuttle is landing. At the launch pad there is a "Firex" water deluge system that is capable of around 200,000 thousand gallons of water a minute to help extinguish any fire that might develop, along with a crew of firefighters in specially designed heat-resistant suits and fire fighting equipment. These suits have a self-contained breathing device to allow the firefighters to operate in toxic environments.

 

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QUESTION:

What keeps the flame from creeping back into the oxygen tank on the Space Shuttles Main Engines?

 

ANSWER from Mike Wilhoit on March 31, 1997:

The liquid oxygen that gets injected into the main combustion chamber (MCC) of the Space Shuttle Main Engine (SSME) enters at a pressure of several thousand pounds per square inch. By the time it passes through the injectors and is burned with the liquid hydrogen, the resultant mixture pressure in the chamber is less than that. That burned mixture is in a real big hurry to exit the nozzle to expand to a lower pressure (making the thrust), so there's no flame going backwards from the MCC during engine operation. During engine shutdown, a shutoff valve stops the oxygen flow to the MCC.

 

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QUESTION:

How many shuttle launch sites are there and where are they?

 

ANSWER from Frank Merceret April 4, 1997:

There is only one Shuttle launch site. It is here at Kennedy Space Center on the East Coast of Florida at Cape Canaveral. There are two Shuttle launch pads here. We call them Launch Complex 39A (or LC-39A for short) and LC 39B. They are about two miles apart. By having two pads, we can launch one Shuttle while preparing another at the other pad. It also allows us to continue launching Shuttles even when a launch pad is being repaired or refurbished.

 

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QUESTION:

What are the sparks you see before the main engines starts?

 

ANSWER from Glenn Davis on June 23, 1997:

These sparks are from devices called hydrogen burn igniters. These devices are mounted below the main engines on the mobile launch platform. The are ignited just prior to main engine start (approximately 10 seconds before launch and 4 seconds before the main engine start). Their purpose is to ignite and burn off any hydrogen gas that may have collected under the engines in a controlled way. In other words, we really don't want a lot of gas collecting under the engines when they start up. The closest thing you probably could compare the hydrogen burn igniters to are roman candles or perhaps sparklers. They are replaced before every launch.

 

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QUESTION:

What does your body feel like when you take off?

 

ANSWER from Sean Kelley on August 11, 1997:

The maximum number of g's the body feels on the shuttle during launch is 3. A g is the equivalent force (acceleration) of gravity, so at 3 g's you body feels like it's three times as heavy. Since the crew is also in a seated position, lying on their back, and wearing a pressure suit, the forces are unlike anything else they have experienced. Most crewmembers report that it is more difficult than the thought to move around during ascent. On entry, the g forces are much smaller, but their bodies have adapted to 0-g (weightlessness), so the forces feel much more than the actual 1.5 g's.

 

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QUESTION:

How noisy is the shuttle inside the flight deck when the shuttle is taking off?

 

ANSWER from Wayne Ranow and Joe Tanner on September 23, 1997:

The Shuttle is real noisy on the inside during launch when the big boosters are burning for the first two minutes. It's not that loud to us because we have headphones over our ears and our helmets on. After the boosters are gone it's a pretty quiet ride. The only thing you can hear when the Shuttle is in orbit is the noise made by the fans that circulate air for us to breathe. They are pretty loud so there is a constant hum.

 

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QUESTION:

Why doesn't the shuttle get as hot when it leaves the atmosphere as it does when it reenters the atmosphere?

 

ANSWER from Jeff Lauffer on October 20, 1998:

The Shuttle is moving a lot slower through the atmosphere during ascent than during reentry. When you hear the call for throttle up, the vehicle has gone through the thickest part of the atmosphere. This occurs at about 70 sec into the flight with the vehicle traveling at only 3500 mph or so. When the orbiter deorbits and lands, it is contacting the thick part of the atmosphere while slowing down from its orbital velocity of 17,000 mph. That is why you see more heat during reentry than during ascent.

 

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QUESTION:

How many people are working when a liftoff takes place?

 

ANSWER from John Guidi on January 26, 1999:

There are about 90-100 folks (depends on the payload) that are actively working a launch countdown from the Launch Control Center with another 200-300 supporting personnel providing engineering guidance, rescue capability, security, safety, quality, etc which are monitoring systems performance across the various critical launch systems and are in a standby mode -- ready to react if needed. These personnel provide the redundancy and capability for timely resolution of problems needed in a manned spaceflight system.

 

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QUESTION:

After a launch, how much repair is needed to prepare the pad for the next launch?

 

ANSWER from Andy Warren on November 16, 1998:

After launch there is typically some blast damage that has to be repaired in order to support the next launch. A large portion of the repairs (approximately 50 to 75%) are performed on the Mobile Launch Platforms (MLP's) with much of the work involving welding. This includes repairs of the solid rocket booster (SRB) holddown posts, heat shields, blast deck and the structure of the MLP. Occasionally repairs are required on the fixed service structure (FSS) at the launch pads. This work includes repairing handrails, repairing steel grating and decking, and replacement of lights. These repairs typically last about 2 to 3 weeks. Approximately $7000 worth of materials and labor is required to repair the structural damage in order to support the next mission. Much effort is expended to ensure that areas, which may have been damaged by the launch blast, do not create debris which could damage the launch vehicle.

 

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QUESTION:

What is the reason for the roll maneuver after launch?

 

ANSWER from Tony Ceccacci on April 11, 1997:

The roll to the "heads down" position starts at pad clear and is completed at approximately 10 seconds. There are two reasons why the shuttle performs the roll maneuver. The first reason is that the "heads down" configuration minimizes the aerodynamic loads that are put on the shuttle as it moves through the atmosphere (air gets pretty heavy when you go that fast with that big a vehicle). The second reason is that it provides the shuttle crew an orientation point (so they know where they're going). Then, if the crew needs to perform any type of contingency Orbit raising burns, they can use the horizon as their "guide" (place to point the shuttle) to maintain the proper burn attitude.

 

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QUESTION:

Why is it important to have an accurate launch window?

 

ANSWER from Jim Draus on October 25, 1999:

The launch window for each Shuttle mission is determined by factoring in a number of technical limitations. The Shuttle carries a limited amount of fuel, which gives it a certain range to intercept or rendezvous with a target in space. This range is based partly on how heavy the Shuttle is for each particular mission. As the earth rotates, these rendezvous points move further away from where the Shuttle can intercept them. So, the launch window is the period of time that the Shuttle can liftoff and still meet up with it's rendezvous target. The exact time of lift-off is based on vehicle performance, satellite deployment, and/or rendezvous requirements. The optimum launch time is one which requires the least amount of fuel consumption to get from the launch pad to the planned rendezvous point in space; a straight line course requiring little or no course corrections during ascent. If for some reason the launch at the optimum T-0 is delayed, then more fuel will be required to steer the Shuttle back to the flight path that will allow the Shuttle to reach its destination in space. If the launch occurs outside the calculated launch window, the Shuttle will not have enough energy to meet its rendezvous target in space. Launch window times for Hubble and Space Station missions are more critical because they are based on Shuttle performance and the actual rendezvous requirements.

 

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