And that represents a phenomenal amount of fuel.
Because of Tsiolkovsky’s unforgiving rocket equation, the biggest problem facing any craft heading into space is the need to boost “excess” mass in the form of fuel, most of which is the fuel required to transport the fuel it will burn later in the journey. And the spacecraft’s weight problems grow exponentially. The multistage vehicle was invented to soften this problem. In such a vehicle, a relatively small payload—such as the Apollo spacecraft, an Explorer satellite, or the space shuttle—gets launched by huge, powerful rockets that drop away sequentially or in sections when their fuel supplies become exhausted. Why tow an empty fuel tank when you can just dump it and possibly reuse it on another flight?
Take the Saturn V, a three-stage rocket that launched the Apollo astronauts toward the Moon. It could almost be described as a giant fuel tank. The Saturn V and its human cargo stood thirty-six stories tall, yet the three astronauts returned to Earth in an itty-bitty, one-story capsule. The first stage dropped away about ten minutes after liftoff, once the vehicle had been boosted off the ground and was moving at about 9,000 feet per second (more than 6,000 miles per hour). Stage two dropped away about ten minutes later, once the vehicle was moving at about 23,000 feet per second (almost 16,000 miles per hour). Stage three had a more complicated life, performing several episodes of fuel burning: the first to accelerate the vehicle into Earth orbit, the next to get it out of Earth orbit and head it toward the Moon, and a couple more to slow it down so that it could pull into lunar orbit. At each stage, the craft got progressively smaller and lighter, which means that the remaining fuel could do more with less.
From 1981 to 2011, NASA used the space shuttle for missions a few hundred miles above our planet: low Earth orbit. The shuttle has three main parts: a stubby, airplanelike “orbiter” that holds the crew, the payload, and the three main engines; an immense external fuel tank that holds more than half a million gallons of self-combustible liquid; and two “solid rocket boosters,” whose two million pounds of rubbery aluminum-based fuel generate 85 percent of the thrust needed to get the giant off the ground. On the launchpad the shuttle weighs four and a half million pounds. Two minutes after launch, the boosters have finished their work and drop away into the ocean, to be fished out of the water and reused. Six minutes later, just before the shuttle reaches orbital speed, the now-empty external tank drops off and disintegrates as it reenters Earth’s atmosphere. By the time the shuttle reaches orbit, 90 percent of its launch mass has been left behind.
Space Tweet #23
Main shuttle tank in use until orbit – long after atmospheric O2 is available to burn. So must carry its own O2.
May 14, 2010 3:03 AM
Now that you’re launched, how about slowing down, landing gently, and one day returning home? Fact is, in empty space, slowing down takes as much fuel as speeding up.
Familiar, earthbound ways to slow down require friction. On a bicycle, the rubber pincers on the hand brake squeeze the wheel rim; on a car, the brake pads squeeze against the wheels’ rotors, slowing the rotation of the four rubber tires. In those cases, stopping requires no fuel. To slow down and stop in space, however, you must turn your rocket nozzles backward, so that they point in the direction of motion, and ignite the fuel you’ve dragged all that distance. Then you sit back and watch your speed drop as your vehicle recoils in reverse.
To return to Earth after your cosmic excursion, rather than using fuel to slow down, you could do what the space shuttle does: glide back to Earth unpowered, and exploit the fact that our planet has an atmosphere, a source of friction. Instead of using all that fuel to slow down the craft before reentry, you could let the atmosphere slow it down for you.
Space Tweets #24–#27
Discovery Orbiter re-enters today. From 17,000mph to 0mph in an hour. Relies on air resistance (aerobraking) to slow down
Mar 9, 2011 8:30 AM
Will take 3/4 of a trip around Earth for atmosphere to drop Discovery out of the sky & land safely as a glider at Kennedy, FL
Mar 9, 2011 10:54 AM
After the Shuttle drops below sound speed (Mach 1) it’s just a fat, stubby glider coming in for a landing
Mar 9, 2011 11:51 AM
Welcome home Discovery. 39 missions, 365 days & 148,221,675 miles on the odometer
Mar 9, 2011 11:59 AM
One complication, though, is that the craft is traveling much faster during its home stretch than it was during its launch. It’s dropping out of a seventeen-thousand-mile-an-hour orbit and plunging toward Earth’s surface, so heat and friction are much bigger problems at the end of the journey than at the beginning. One solution is to sheathe the leading surface of the craft in a heat shield, which deals with the swiftly accumulating heat through ablation or dissipation. In ablation, the preferred method for the cone-shaped Apollo-era capsules, the heat gets carried away by shock waves in the air and a continuously peeling supply of vaporized material on the capsule’s bottom. For the space shuttle and its famous tiles, dissipation is the method of choice.
Unfortunately, as we all now know, heat shields are hardly invulnerable. The seven astronauts of the Columbia space shuttle were cremated in midair on the morning of February 1, 2003, as their orbiter tumbled out of control and broke apart during reentry. They met their deaths because a chunk of foam insulation had come loose from the shuttle’s huge fuel tank during the launch and had pierced a hole in the leading shield that covered the left wing. That hole exposed the orbiter’s aluminum dermis, causing it to warp and melt in the rush of superheated air.
Here’s a safer idea for the return trip: Why not put a filling station in Earth orbit? When it’s time for the shuttle to come home, you attach a new set of tanks and fire them at full throttle, backward. The shuttle slows to a crawl, drops into Earth’s atmosphere, and just flies home like an airplane. No friction. No shock waves. No heat shields.
But how much fuel would that take? Exactly as much fuel as it took to get the thing up there to begin with. And how might all that fuel reach the orbiting filling station that could service the shuttle’s needs? Presumably it would be launched there, atop some other skyscraper-high rocket.
Think about it. If you wanted to drive from New York to California and back again, and there were no gas stations along the way, you’d have to tug a truck-size fuel tank. But then you’d need an engine strong enough to pull a truck, so you’d need to buy a much bigger engine. Then you’d need even more fuel to drive the car. Tsiolkovsky’s rocket equation eats your lunch every time.
In any case, slowing down or landing isn’t only about returning to Earth. It’s also about exploration. Instead of just passing the far-flung planets in fleeting “flybys,” a mode that characterized an entire generation of NASA space probes, the craft ought to spend some time getting to know those distant worlds. But it takes extra fuel to slow down and pull into orbit. Voyager 2, for instance—launched in August 1977—has spent its entire life coasting. After gravity assists, first from Jupiter and then from Saturn (the gravity assist is the poor man’s propulsion mechanism), Voyager 2 flew past Uranus in January 1986 and past Neptune in August 1989. For a spacecraft to spend a dozen years reaching a planet and then spend only a few hours there collecting data is like waiting two days in line to see a rock concert that lasts six seconds. Flybys are better than nothing, but they fall far short of what a scientist really wants to do.
On Earth, a fill-up at the local gas station has become a pricey activity. Plenty of smart scientists have spent plenty of years inventing and developing alternative fuels that might one day see widespread use. And plenty of other smart scientists are doing the same for propulsion.
The most common forms of fuel for spacecraft are chemical substances: ethanol, hydrogen, oxygen, monomethyl hydrazine, powdered aluminum. But unlike airplanes, which burn fuel by drawing oxygen through their engines, spacecraft have no such luxury; they must bring the whole chemical equation along with them. So they carry
not only the fuel but an oxidizer as well, kept separate until valves bring them together. The ignited, high-temperature mixture then creates high-pressure exhaust, all in the service of Newton’s third law of motion.
Bummer. Even ignoring the free “lift” a plane gets from air rushing over its specially shaped wings, pound for pound any craft whose agenda is to leave the atmosphere must carry a much heavier fuel load than an airplane does. The V-2’s fuel was ethanol and water; the Saturn V’s fuel was kerosene for the first stage and liquid hydrogen for the second stage. Both rockets used liquid oxygen as the oxidizer. The space shuttle’s main engine, which had to work above the atmosphere, used liquid hydrogen and liquid oxygen.
Wouldn’t it be nice if the fuel itself carried more punch than it does? If you weigh 150 pounds and you want to launch yourself into space, you’ll need 150 pounds of thrust under your feet (or spewed forth from a jet pack) just to weigh nothing. To actually launch yourself, anything more than 150 pounds of thrust will do, depending on your tolerance for acceleration. But wait. You’ll need even more thrust than that to account for the weight of the unburned fuel you’re carrying. Add more thrust than that, and you’ll accelerate skyward.
Space Tweet #28
At a fine Italian restaurant this evening. Served grappa at meal’s end. NASA should study it as a replacement rocket fuel
Dec 7, 2010 12:27 AM
The space mavens’ perennial goal is to find a fuel source that packs astronomical levels of energy into the smallest possible volumes. Because chemical fuels use chemical energy, there’s a limit to how much thrust they can provide, and that limit comes from the stored binding energies within molecules. Even given those limitations, there are several innovative options. After a vehicle rises beyond Earth’s atmosphere, propulsion need not come from burning vast quantities of chemical fuel. In deep space, the propellant can be small amounts of ionized xenon gas, accelerated to enormous speeds within a new kind of engine. A vehicle equipped with a reflective sail can be pushed along by the gentle pressure of the Sun’s rays, or even by a laser stationed on Earth or on an orbiting platform. And within a decade or so, a perfected, safe nuclear reactor will make nuclear propulsion possible—the rocket designer’s dream engine. The energy it generates will be orders of magnitude more than chemical fuels can produce.
While we’re getting carried away with making the impossible possible, what we really want is the antimatter rocket. Better yet, we’d like to arrive at a new understanding of the universe, to enable journeys that exploit wormhole shortcuts in the fabric of space and time. When that happens, the sky will no longer be the limit.
• • • CHAPTER TWENTY-TWO
THE LAST DAYS OF THE SPACE SHUTTLE
May 16, 2011: The Final Launch of Endeavour
Space Tweets #29–#36
8:29 am
If camera-coverage enables, six cool things to look for just seconds before ignition of the SolidRocketBoosters...
8:30 am
1) Orbiter’s steering flaps jiggle back and forth – a final reminder that they can angle the way they’re supposed to
8:32 am
2) The Orbiter’s 3 rocket nozzles gimbal to & fro – a final reminder that they can aim the way they’re supposed to
8:33 am
3) Sparks spray onto launch pad – they burn away any potentially flammable hydrogen gathered there from the main engine
8:35 am
4) Water Tower dumps a swimming-pool’s worth onto the launch pad – H2O absorbs sound vibrations, preventing damage to craft
8:37 am
5) “Main Engine Start” – Orbiter’s 3 nozzles ignite, take aim, and force shuttle to tip forward. Bolts still hold her down
8:38 am
6) “3 - 2 - 1 – Liftoff” – SolidRocketBoosters ignite, tipping Shuttle straight upwards again. Bolts explode. Craft ascends
9:18 am
In case you wondered: Space Shuttle Endeavour gets a British spelling because it’s named for Captain Cook’s ship
June 1, 2011: The Final Return of Endeavour
Space Tweets #37–#45
1:20 am
Just an FYI: To land, space shuttle Endeavour must lose all the energy of motion that it gained during launch
1:30 am
Shuttle now executing a “de-orbit burn” dropping its path low enough to meet scads of motion-impeding air molecules
2:00 am
As Endeavour dips into Earth’s atmosphere, the surrounding air heats up, whisking away the Shuttle’s energy of motion
2:10 am
As Endeavour’s speed slows, it drops lower in Earth’s atmosphere, encountering an ever-increasing density of air molecules
2:20 am
Protective Shuttle tiles reach thousands of degrees (F), persistently radiating heat away. Shielding the astronauts within
2:30 am
For most of Endeavour’s re-entry, it’s a ballistic brick falling from the sky. Below the speed of sound, it’s aerodynamic
2:34 am
Kennedy Space Center’s Shuttle’s landing strip is 15,000 feet long. Long enough for the brakeless Orbiter to coast to a stop
2:35 am
Welcome home Astronauts: 248 orbits, 6,510,221 miles. Well done Endeavour: 25 missions. 4671 orbits, 123,883,151 miles
9:10 am
Einstein’s relativity shows that Endeavour astronauts moved 1/2000 sec into the future during their stay in orbit
July 8–21, 2011: The Final Journey of Atlantis & the End of the Shuttle Era
Space Tweets #46–#51
Jul 8 9:54 am
Shuttle mission in the film “Space Cowboys” was STS-200. With the launch of Atlantis, the actual program reaches only STS-135
Jul 8 10:25 am
Space Arithmetic: Mercury + Gemini + Apollo = 10 years. Shuttle = 30 years
Jul 8 10:52 am
Just an FYI: Human access to space doesn’t end with the Shuttle era, only American access. China and Russia still go there
Jul 8 11:24 am
Apollo in 1969. Shuttle in 1981. Nothing in 2011. Our space program would look awesome to anyone living backwards thru time
Jul 21 5:42 am
Worried about privatization of access to Earth orbit? Overdue by decades. NASA needs to look beyond, where it belongs.
Jul 21 5:49 am
Lament not the shuttle’s end, but the absence of rockets to supplant it. Who shed a tear when Gemini ended? Apollo awaited us
• • • CHAPTER TWENTY-THREE
PROPULSION FOR DEEP SPACE*
Launching a spacecraft is now a routine feat of engineering. Attach the fuel tanks and rocket boosters, ignite the chemical fuels, and away it goes.
But today’s spacecraft quickly runs out of fuel. So, left to itself, it cannot slow down, stop, speed up, or make serious changes in direction. With its trajectory choreographed entirely by the gravity fields of the Sun, the planets, and their moons, the craft can only fly past its destination, like a fast-moving tour bus with no stops on its itinerary—and the riders can only glance at the passing scenery.
If a spacecraft can’t slow down, it can’t land anywhere without crashing, which is not a common objective of aerospace engineers. Lately, however, engineers have been getting clever about fuel-deprived craft. In the case of the Mars rovers, their stupendous speed toward the Red Planet was slowed by aerobraking through the Martian atmosphere. That meant they could land with the help of nothing more than heat shields, parachutes, and airbags.
Today, the biggest challenge in aeronautics is to find a lightweight and efficient means of propulsion, whose punch per pound greatly exceeds that of conventional chemical fuels. With that challenge met, a spacecraft could leave the launchpad with fuel reserves onboard, and scientists could think more about celestial objects as places to visit than as planetary peep shows.
Fortunately, human ingenuity doesn’t often take no for an answer. Legions of engineers are ready to propel us and our rob
otic surrogates into deep space with a variety of innovative engines. The most efficient among them would tap energy from a nuclear reactor by bringing matter and antimatter into contact with each other, thereby converting all their mass into propulsion energy, just as Star Trek’s antimatter engines did. Some physicists even dream of traveling faster than the speed of light by somehow tunneling through warps in the fabric of space and time. Star Trek didn’t miss that one either: the warp drives on the starship USS Enterprise were what enabled Captain Kirk and his crew to speed across the galaxy during the TV commercials.
Space Chronicles: Facing the Ultimate Frontier Page 17