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Off into Space

Short periods of microgravity can be produced on the ground or in aircraft. However, in order to produce longer periods and a higher quality of microgravity requires going into space.

Sounding Rockets

parabolic flightThese are real, solid fuel rockets that carry scientific payloads. They vary in size from the 10 m tall ‘miniTEXUS’ that reaches an altitude of 140 km and delivers up to three minutes of microgravity up to the 16.2 m tall MAXUS that reaches an altitude of 705 km and delivers 12.5 minutes of microgravity. In all cases the sounding rockets are launched from Kiruna in Northern Sweden. Once their rocket motors are burnt out they continue to coast upwards. Once free of the last significant traces of atmosphere (around 100 km altitude) the freefall stage has begun and the sounding rocket and the equipment on board experience microgravity. The rocket continues to coast upwards and reaches it apogee (highest altitude) and then falls back to Earth. The microgravity period ends when the rocket starts to encounter significant amounts of atmosphere (again around 100 km). Eventually the rocket together with its scientific payload touches down with the fall softened by a parachute.

The great advantage of sounding rockets is that the period of microgravity is so long and therefore a wider range of experiments can be carried out. Usually a sounding rocket will carry several, totally unrelated, experiments. They may run simultaneously or sequentially depending on the durations of the individual experiments and their needs for common resources such as power and telemetry. Other characteristics of sounding rockets are:

 Hookes Law

Whilst many experiments need never use anything more sophisticated than a sounding rocket it can also be a stepping stone to the most advanced microgravity platforms.

There is a major flaw in the last question and it lies in the statement, “Take g as 9.8 m s-2.” The strength of gravity due to the Earth depends on its mass and, importantly, the distance from its centre of mass. For most questions any change in this distance is unimportant as the change is usually tiny compared to the 6400 km radius of the Earth. However, for a sounding rocket, particularly the high flying MAXUS, the difference cannot be ignored. The value of the acceleration due to gravity is given by:

 Hookes Law

Where g(h) is the acceleration due to gravity at a height h, G is Newton’s Universal gravitational constant, MEarth is that mass of the Earth and R0 is the distance from the centre of the Earth to the surface. Since the strength of gravity is also an acceleration we can rewrite this as:

 Hookes Law

Mathematicians would describe this as a second order, nonlinear differential equation and solving it is as unpleasant as it sounds and you should not be expected even to look at it before you are at university. Fortunately we can use a spreadsheet or similar program to model it numerically. An Excel spreadsheet is attached that allows you to carry out this modeling. Right click the link and select "Save Target As..."

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Sounding Rockets Spreadsheet

When the sheet is first opened the initial altitude is set to 705,000 m and initial velocity to 0 m s-1, the initial conditions for a MAXUS flight at apogee. Δt is set to 1 s. All of these figures can be altered, either manually or by using the ‘goal seek’ facility, all other cells are locked.

In the connected sheets, charts are automatically generated that plot altitude against time and velocity against time.

 Hookes Law

How long would it take a MAXUS sounding rocket to fall from its apogee of 705 km to the end of the microgravity phase at 100 km? How does this relate to the stated microgravity period of twelve and a half minutes? What is the velocity of the sounding rocket at this point?

Examine the two graphs that the spreadsheet generates. What can you say about their shapes compared to what you might expect from a fall with uniform acceleration.

Use the spreadsheet to estimate the microgravity periods for the TEXUS (apogee 260 km) and  miniTEXUS (apogee 140 km).

The rows are set up to be separated by equal periods of time; look carefully at the distance intervals and the amount of the spreadsheet dedicated to different sections of the descent. Can you suggest a better way of dividing up the rows?

Retrievable Capsules

Assuming that you don’t need a human to operate your experiment ‘hands on’ then retrievable capsules are the ‘Rolls-Royce’ of microgravity platforms, this is because it offers the highest standard of microgravity available outside the International Space Station. The standard capsule is the ‘Foton’ (a western rendering of the Russian word for photon). It is based on the design of the capsule that carried the world’s first astronaut, Yuri Gagarin, in to orbit in 1961 but it is still going strong. It is launched into Earth orbit by a Soyuz-U rocket, again a descendant of the early days of space flight.

 Hookes Law

The capsule orbits the Earth at an altitude of approximately 300 km for as long as is needed be the experiments. The actual duration is limited by the energy consumption of the experiments’; this is because the capsule is powered by batteries rather than solar cells. As a result the maximum flight duration is normally around eighteen days. After this point a rocket motor is fired in the direction of travel which causes the capsule to re-enter the Earth’s atmosphere and land near its original launch site of Baikonur, Kazakhstan.

On the negative side the complexity of a full orbital flight does bring some problems:

The capsule has a roughly circular orbit with an altitude of approximately 300 km. By using Newton’s Universal Law of Gravitation calculate the orbital speed. And so the total energy that needs to be given to each kilogram of the capsule in order to place it in orbit?

A particle of matter is under observation in the centre of a piece of equipment that is 1.7 m long (the maximum possible for the Foton capsule). If the standard of microgravity is 10-5 g how long will it take before the particle meets the end wall?

The International Space Station

In November of 1998 a Russian Proton rocket launched a 12.6 m long 19.3 tonne payload in to orbit. This was ‘Zarya‘, the first module to the International Space Station. Two years and several modules later the ISS received its first crew; since then it has been permanently manned. The ISS is much larger now (over 80 m long when the European Automated Transfer Vehicle (the ATV) is docked) and has a permanent crew of six. The crew carry out an extensive scientific research programme using the station’s laboratories, which includes the European ‘Columbus’ Module.

The microgravity quality on the ISS can be excellent although there are periods when accelerations and vibrations from visiting spacecraft, the need to regularly boost the orbit and the disturbances due to the crew themselves can reduce the standard. The great facility that the ISS offers is the opportunity for astronauts to carry out science, hands on. It is of course also the only microgravity platform where research in to human physiology can be carried out over long periods.

 Hookes Law

The specific advantages of the ISS include:

Such benefits do of course have drawbacks, specifically:

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