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Once a general idea for an interplanetary space mission has been formulated and accepted, the real hard work begins. The initial idea, which may be nothing more than a couple of pages with scientific objectives and requirements, needs to be worked out in detail.

First, a list of technical requirements is derived from the scientific requirements. For instance, the scientists may want to have a robotic rover driving over the surface of Mars to investigate the rocks there. A technical requirement following from this is that a system needs to be developed to land the rover softly on Mars.

Moreover, the fact that the rover will be on Mars results in technical requirements about the minimum and maximum temperatures the rover must be able to survive while on the surface. From previous Marslanders and orbiters we now know that the coldest temperature on the surface is about -140 degrees Celsius (-220 degrees Fahrenheit) at the polar caps, while it gets no warmer than 20 degrees Celsius (68 degrees Fahrenheit) anywhere else. As this is too cold for the electronic equipment on board a rover, a heating system is required.

Sometimes the technical requirements cannot be captured into such sharply defined parameters. For instance, when ESA's Huygens probe was designed to investigate the murky atmosphere of Saturn's moon Titan, little was known about the temperatures and pressures a spacecraft would encounter. The probe had therefore to be designed to operate in a relatively wide range of possible atmospheric environments - which was a bit like designing an all-terrain car that has to be able to drive well on both asphalt roads and muddy sand paths.

Moreover, if the probe would make it all the way to the surface alive, would it then hit a super-cold icy surface as hard as steel, or plunge into a bitterly cold lake of liquid hydrocarbon gas? Although Huygens was not specifically intended as a lander, scientific measurements from the surface would provide a valuable bonus to the mission. Huygens, therefore, should not only be able to land on hard, solid ground but should also be able to float, and in both cases be able to gather data.

Starting from the scientific and technical requirements, the spacecraft's mass, the amount of electrical power needed to run all the scientific and supporting equipment, and the size of the probe needed to fit all the hardware and propellant have to be calculated. Moreover, experts need to precisely list the required types and numbers of sensors, solar cells, antennas, tanks, computers, etc.

Typically the main issues the scientists and engineers need to tackle are how to make the probe as light and as small as possible, while, at the same time, achieving all the mission objectives. Robotic interplanetary spacecraft typically have a launch mass somewhere between 300 and 3,000 kilograms (660 and 6,600 pounds), while their size may vary from that of a washing machine to the dimensions of a large shipping container.

In general, the smaller the spacecraft the less cosdy it is to develop and build. Moreover, the lower the mass and volume, the smaller and thus cheaper the rocket required to launch it. For each kilogram/pound you may save on your interplanetary space probe, you may save some 60 kilograms/ pounds of propellant and 7 kilograms/pounds of launcher hardware.

If you save enough on your spacecraft mass, this means you can switch to a smaller and cheaper launcher. As the launch price typically comprises 30 percent or more of the total mission cost, such savings are very important. Making your space probe small and light enough to fit inside a smaller rocket instead of a larger one easily saves you several tens of millions of dollars.

Detailed questions to be answered during the spacecraft design work are, for instance:

• How many propellant tanks should we fit, and what volume should they hold to carry all the propellant?

• How many thrusters (small rocket motors) are enough to stabilize the spacecraft?

• Where do we place the antennas so that at least one is positioned just right at any time to exchange radio signals with Earth?

• How much surface area do we require for the solar arrays to make sure that, even if the efficiency of the solar cells degrades over time, we always have sufficient power to run all the onboard systems?

Many times the scientists and engineers have to cope with conflicting requirements. For instance, to remain cool a spacecraft orbiting Mercury should stay in the planet's shadow as much as possible. If it didn't, its electronics would get fried very quickly. On the other hand, the spacecraft also needs sufficient sunlight to get enough electrical power from its solar arrays. The engineer designing the thermal control subsystem of the spacecraft wants to hide it behind Mercury all the time, while the engineer responsible for the electrical power supply would like it to be exposed to full sunlight as much as possible.

Keeping the spacecraft in the shadow at all times means that it will receive no energy and will die when the battery eventually empties. Facing the Sun continuously requires very complex thermal protection with thick layers of insulation and large radiators. This would make the probe too heavy and too large. Clearly, a compromise has to be found between the preferences of the thermal control engineer and the electrical power engineer.

In addition, the scientists may have very strong requirements for the amount and direction of light they need for their cameras, and that must also be taken into account.

The designing of space probes is not a garage project in which you just scribble a rough sketch on a napkin and start building almost immediately, working out the details as you go. This process only works in some game shows, where contenders have to quickly build machines from whatever they can find in the scrap yard, making adjustments as they progress and fixing problems or even modifying their design during the contest.

Spacecraft are much too complicated for this. Let's say that you have started to build your robotic Jupiter explorer and later find that your battery is too small. You will have start again and incorporate a bigger one, which is of course also heavier. As a result, your spacecraft increases in mass, and therefore needs not only more propellant for its orbit maneuvers but also larger tanks. This makes the whole thing even heavier and larger, requiring the spacecraft's basic structure to gain weight. Also, you will need more thermal isolation material to cover the now larger spacecraft. Such snowball effects can quickly get out of hand.

Furthermore, unlike most other machines, interplanetary probes cannot be repaired after launch. Broken equipment cannot be replaced, so spare units have to be actually built into the spacecraft. This is called "built-in redundancy." If something breaks along the way, such a backup system may take over from the failing equipment, but otherwise there is not much you can do about it. The spacecraft is far out, and gone forever. Your only link with it is via radio communication.

Spacecraft therefore need to be carefully designed on paper (or in computers actually) before anything can be built, bought or constructed. Mistakes made early in the project can come back to haunt you years later, during the detailed development, construction, testing or even when the spacecraft is already flying through space.

Designing spacecraft is not a linear process where you start with some requirements and then simply follow a list of things to calculate and decide on. Instead, space probe engineers work through so-called iterative "design loops."

For instance, when you want to land something on Mars you will normally use parachutes. However, since the atmosphere of Mars is very thin, these may not brake your lander enough to prevent it from smashing into the rocky surface. You will need something else to slow the spacecraft down just before it reaches the ground.

There are two basic possibilities. One is to use a system of braking rocket thrusters to slow the lander down and softly place it in the red Martian dust in a fully controlled way. The benefits of this system are that you can land really carefully and that you always touch down with the right side up. The downside is that you will need a sophisticated control system, instrumentation to very precisely measure the altitude, and a lot of

FIGURE 2.1 The Viking lander used a heatshield, parachutes and rocket engines to land softly on the surface of Mars. [NASA]

rocket propellant. The two Viking landers that NASA sent to Mars in the mid-1970s were based on this system.

The other method is to enclose your spacecraft inside a cluster of airbags (similar to those used in cars) that inflate immediately before hitting the surface, and just let your lander fall, bump and roll until it stops. This results in a much simpler, smaller and lighter system than the use of braking rockets. However, even with the baggy protection the landing is relatively hard; the lander has to endure something that can be compared to pushing your computer from a chair onto the floor here on Earth. Because you lander has been bouncing over the surface, it is shaken quite a bit, and if your spacecraft is heavy the airbags have to be enormous. Moreover, you can never be sure which side is up when the thing finally stops, so the lander has be able to right itself mechanically if it is upside down.

The famous 1997 NASA Mars Pathfinder used this system for the first time, with success. The lander hit the Martian surface with a total velocity of about 65 kilometers (40 miles) per hour; approximately 45 kilometers (28 miles) per hour in the vertical and 45 kilometers (28 miles) per hour in the horizontal direction. It then bounced some 15 meters (50 feet) into the air, bounced another 15 times and rolled over and over before finally coming to rest about 1 kilometer (0.6 mile) away from the initial impact

FIGURE 2.8 Airbags are used to protect a lander when it hits the surface of a planet. The landing is however much rougher than with a rocket brake system [NASA]

site. It had been rolling and bouncing for some 2.5 minutes. More recently, the two NASA Mars Exploration Rovers Spirit and Opportunity were safely landed with airbags.

To make a trade-off on which of these possibilities to use, you could make two preliminary designs: one with a rocket system like Viking and one with airbags like Pathfinder. You could then evaluate the complexity and estimate the masses, sizes and costs for each option and compare them with the original list of requirements for the maximum launch mass, maximum cost and maximum landing speed. The one that best fits the requirements is the winner of the little Darwinian competition.

However, often the trade-offs are not as straightforward as in this example. In his Akin's Laws of Spacecraft Design, professor David Akin of the University of Maryland comments: "There is never a single right solution. There are always multiple wrong ones, though."

After all major trade-offs have been made, you can design the chosen concept in more detail. When you have checked the outcomes with the requirements for maximum mass, maximum landing speed, etc., you have finished the first design loop of your new space project. If the concept does not entirely fit the requirements, you will have to do another design loop involving changing your design, recalculating everything and comparing the new numbers with the requirements. If you did it right, your updated design will fit the requirements better than the earlier concept.

Sometimes there is no way to nicely adhere to all the requirements. Your concept may fit the mass and volume demands but be much too expensive. Or it may be too complex to build within the specified timeframe. It has then to be determined whether the requirements themselves need to be changed, or whether the project must be branded "not feasible."

No design is complete without a development schedule plan that shows when to start the development and production of a part of the spacecraft, and when that part needs to be finished. It's no use testing the spacecraft's telecommunication subsystem if the transmitter is not ready yet; it would be like trying to test drive a car while the engine is still being assembled in the garage. Just like an orchestra, where each musician has to be at the same page of the music book and every instrument has to start at the right time, the development and production phases of all parts of a spacecraft need to be harmonized.

The entire development of an interplanetary robotic probe takes about three years minimum, but large and complex spacecraft may take as long as eight years to be ready for launch. These long development times mean that when they are launched, space probes often contain equipment that is already obsolete. When you take into account the projected very long travel times, spacecraft are often fairly antiquated by the time they reach their destination.

When the Huygens probe entered the atmosphere of Saturn's moon, Titan, in January 2005, it had been on its way for over seven years, while its development actually started in the late 1980s. That meant that the technology descending through that strange Titan atmosphere actually dated from the time we were running Microsoft Windows 2.0 on 386 computers!

While designing the spacecraft for minimum mass, volume and complexity, the risk of failures always needs to be taken into account. Is one onboard computer enough? What if it breaks? Maybe you need to put in a backup, but would that make the probe too heavy?

What is the chance that one of the two foldable solar arrays does not deploy properly? Do you design the spacecraft so that it can operate with only one solar array, and use the second one only as a backup? Or do the scientists accept that if one solar array does not work, they will need to operate the scientific instruments only part of the time and thus obtain less valuable information?

The level of risk is evaluated by looking at the combination of impact and likeliness of something going wrong. The impact of aliens hi-jacking your space probe on its way to Mars would, for instance, be very high, because if that happens the mission would be completely lost. However, the probability of such an abduction is extremely small. The risk, as function of the impact and likeliness, is negligible because the high impact is nearly nullified by the extremely low probability of occurrence.

In contrast, the chance that a piece of electric wire will get damaged during the construction of the spacecraft is rather high, but the impact is very low. You just keep some inexpensive spare wire at hand to replace it.

Risks with a relatively high impact and likeliness need to be dealt with by, for example, having some reserve money included in the development budget to buy replacements for faulty equipment.

Relatively high risks connected to equipment failure during the mission can be diminished by the earlier mentioned built-in redundancy - that is, having spares on board that can take over when needed. For instance, two radio transponders (combined transmitters and receivers) are usually included, because without the means of communication any spacecraft is rendered useless. Since this is now more or less standard practice, the two transponders often come integrated as one single box (this is called "internal redundancy").

Another way to lessen the risk of losing your mission is to design so-

called functional redundancy into the spacecraft, by ensuring that other types of equipment can take over the job of failing devices. For instance, when sensors that orient a spacecraft by pointing it at a certain star (so-called star trackers, see Chapter 3) break down, their function may be taken over by other types of sensors that aim the probe with the help of the Sun.

There are also risks that are relatively high, but cannot really be dealt with. For instance, the chance that your space probe is lost due to a launch failure is not negligible, even after more than half a century of experience in launches. The impact would be extremely high, because when it happens the spacecraft will surely be destroyed. However, there is not much you can do about it; building a copy of your spacecraft to keep as a spare is much too expensive. It is a risk that cannot be neglected but just has to be accepted. The only thing you could do is to financially insure the project against a failed launch.

Increasingly important in any space project is the planning of a solid financial budget. Has sufficient money been allocated to do the whole mission? What are the risks of over-expenditure? Estimates of the costs for development, production, launch and operations of the probe have to be made, and have to be compared with the actual budget available.

An average space probe, mostly based on existing and somewhat modified equipment, may cost something in the order of 100 million dollars for the basic equipment and another 50 million for the scientific instruments on board. The launch with a Soyuz rocket would cost at least 40 million dollars, and another 20 million dollars would be needed to track and control the spacecraft during flight. That's a total of 210 million dollars. A complex mission, requiring lots of new developments and carrying an extensive array of sophisticated instruments, can easily cost over a billion dollars (see Table 2.1).

Spacecraft are usually unique; they need to be specifically designed for a certain mission and therefore normally only one is built of any type. Even equipment that can be used on more than one space probe, such as standardized solar arrays or antennas, is only built in small numbers. As a result, space probes do not benefit from the cost-reducing economies of mass production, such as we see with computers and cars.

Moreover, the fact that no one can repair interplanetary spacecraft once they are launched, and that they have to operate for long periods in the hostility of space, means that only components of very high quality can be used. That may, for instance, mean that special electronic components need to be used that are especially resistant to disturbances by the higher radiation levels in space. Sometimes several tens of units of the same component need to be bought and tested, so that the best single unit can

TABLE 2.1 Comparisons of the costs for interplanetary space missions and other items Cost [$ x 1,000] So with one billion $ you can ...

McLaren Fl LM sport car 1,300

Ferrari Enzo sport car 640

Fl6 fighter plane 22,000

Boeing 747 airliner 230,000

B-2 Spirit stealth bomber 2,200,000

Virginia class submarine 2,600,000

Space Shuttle mission 500,000

Soyuz space tourist trip 20,000

to the International Space

Station buy 770 McLaren F1 LM sport cars buy 1,560 Ferrari Enzo sport cars buy 46 F16 fighter planes buy 4 Boeing 747 airliners buy half of a B-2 Spirit buy 40 percent of a Virginia class submarine launch two Space Shuttle missions make 50 trips to the International Space Station as space tourist on board a Russian Soyuz spacecraft

NASA/ESA Cassini- 3,400,000

Huygens mission

Development, launch and 804,000 operation of the NASA Mars Excursion Rovers Development, launch and 250,000 operation of ESA's Mars Express orbiter

Development, launch and 65,000 operation of NASA's Lunar Prospector

A bottle of beer or lemonade 0.001

pay 30 percent of the entire NASA/ ESA Cassini-Huygens mission to Jupiter develop, launch and operate two identical NASA MER-type rovers develop, launch and operate four different ESA Mars Express-type orbiters develop, launch and operate 15 different NASA Lunar Prospectortype orbiters buy every US citizen four bottles of beer or lemonade be identified. Only that one unit selected from the whole group may then be used in the actual spacecraft.

However, sometimes costs can be reduced by building spacecraft out of left-over spare equipment and test models of other missions. When ESA's four Cluster spacecraft were all lost during the failed launch of the first Ariane 5 rocket, one nearly complete spacecraft test model and some spare equipment were used in building the replacement satellites of Cluster II. NASA's Magellan Venus orbiter even incorporated all kinds of equipment left over from several previous projects that had totally different mission objectives.

Space projects are notorious for their initial underestimated costs (often done to insure that funding for new missions would be approved by politicians) and consequent overspending at a later stage. Few spacecraft ever get finished under budget; most need additional funding as the development progresses and the project is found to be increasingly expensive.

However, more and more space probes are "built to cost," meaning that the maximum financial budget is set before the designing of the spacecraft is started. If the cost estimates for the first design indicate that it would probably cost more to develop than is allowed, the probe needs to be redesigned until it fits the budget. Often it means that the scientific return of the mission is lowered, for instance, by deleting instruments from the spacecraft to make it lighter, simpler, smaller and thus cheaper.

There is an important difference in how projects are funded between NASA and ESA. NASA projects have to battle for money through the federal budget each fiscal year. This means that even though a project may have been getting sufficient funding for half a decade, there is no guarantee that there will be money to complete it in the following year.

For ESA missions, the budget for the whole project is more or less secure once it has been given the go-ahead; the agency does not have to convince politicians of the need for further funding every year. However, if the initially allocated budget is found to be insufficient, political approval for additional money has to be obtained, of course. Most space scientists and project managers believe that ESA's way is better, at least for their nerves.

Compared to how robotic explorers uniquely enrich our knowledge and understanding of the Universe, the financial investments required are actually quite small. The missions of the two Voyager spacecraft that showed Jupiter, Saturn, Uranus and Neptune and its moons in unprecedented detail have cost each US taxpayer less than a simple lunch. And for the cost of a couple of beers per year the European citizens have been able to explore the Sun, the Moon, Venus, Mars, Saturn's moon Titan and Halley's comet.

Today, NASA's total budget represents less than 1 percent of the total federal expenditures. Even at its peak, in 1966, the space agency's budget was only 5.5 percent of all the money spent by the US government. In Europe the ESA member states spend even less than the USA on space exploration.

In the end, the design reports, blue prints and computer simulations must together show a design that is technically coherent, able to fulfill its scientific objectives with a reasonable amount of certainty, and can be developed and built in a reasonable time and for a reasonable budget.

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