Space Transport and Engineering Methods/Methods/1
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[edit] structures
D.1 Structural Methods
[edit] static structures
D.1a Static Structures
Static structures have parts which are mostly fixed in relation to each other, although the structure as a whole may move with respect to the ground. Large structures are primarily governed in their design by the ratio of strength to density, or specific strength. Other important properties in certain cases include stiffness, temperature dependence of properties, and resistance to decay from the surrounding environment.
Methods of movement on the structure include:
- (i) Standard elevator:
(refer to standard engineering references for design details)
- (ii) Inchworm type winch:
A small motor driven trolley pulls a length of cable behind it as it climbs up the structure. It then hooks the cable to a fixed point on the structure. The cargo elevator remains attached to the next lower point on the structure during this time. The elevator then uses an on-board winch to reel itself up from one attachment point to the next. This type of winch is useful where continuous attachment track or full length elevator cable would be too heavy. Requires independant power for winch.
- (iii) Fluid transfer in pipes:
For example, Dr. Dana Andrews has suggested pumping gas generated on the Lunar surface up to the Lunar L2 point. A column of Oxygen at .1 atmosphere at L2, and a temperature of 1000 K (a solar heated pipe can be used to keep the gas hot) would have a pressure of 2310 atm (234 MPa) at the bottom. Another approach is to have pumping stations spaced along the tower.
[edit] large towers
1 Large Towers
Alternate Names:
Type: C.1b/B.2a (Potential Energy via Mechanical Traction)
Description:
Use of advanced aerospace materials makes possible the construction of towers that are many kilometers tall. Such towers can be used as a high altitude platform, as a launch platform for a propulsive vehicle, or a support structure for an accelerator system. Structural design is a major issue.
If a tall structure is being considered, the weight of the tower structure becomes the driving issue, because it can end up being many times the weight of the 'payload' the tower is supporting. If the 'payload' is at the top of the tower, the structure just underneath only has to support the payload's weight. The next piece of structure below that must support the payload plus the top bit of structure, so it has to be a little bit beefier (have a larger cross sectional area). Going down the structure, it has to get stronger and stronger to support the greater weight above.
To put some numbers to the problem, let us take a plain carbon steel structure (the type of steel used for ordinary building construction). It has an allowable load of 125 MPa. To make the problem simple, assume we are holding up a 1275 kg payload on top of the tower, which under one gravity has a weight of 12,500 N. Therefore we need one square centimeter of cross sectional area of steel to hold up the weight. Steel has a density of 7800 kg per cubic meter. The top meter of the tower has a volume of 0.01x0.01x1.0= 0.0001 cubic meter. This has a mass of 0.78kg. So the structure 1 meter down from the top has to support a mass of 1275.78 kg, i.e. the payload plus the top meter of steel. The load has increased by 0.06%, so the cross sectional area also increases by 0.06%. The area increases in a compound interest fashion at the rate of 0.06% per meter as you go down the tower. Over the course of 1 km in height, the increase is by a factor of 1.8433.
We define the scale height of a structure as the length over which the cross sectional area increases by a factor of e (2.71828...). In the case we have been using it is 1635 meters. The scale height can be found by dividing the allowable load of the material by the density times the local acceleration (one gravity in the case of the Earth): h(scale) = load / (density x acceleration) = 125 MPa / (7800 kg/m^3 x 9.80665 m/s^2) = 1635 meters
So a tower 4.9 km tall would have an area at the bottom e cubed (20.08) times the area at the top, and the weight of steel would be e^3 - 1, or 19.08 times the payload weight.
Now, plain carbon steel is not a very good material to use if you want a really big tower. Let us look at advanced carbon composites, such as is used in modern aircraft and spacecraft. One specific formulation (Amoco T300/ERL1906 if you must know) has a compressive strength of 1930 MPa (280,000 psi). We derate this by half to get the allowable load. This is the same as is done for the steel, where you only use 50% of the strength to give you a safety margin. So we have 965 MPa (140,000 psi) as an allowable load. The density is 1827 kg/m3 (0.066 lb/cu in.) Dividing we have a scale height of 53,878 meters (176,800 feet, or 33.5 miles) If you build several scale heights tall, you can see in theory you could build structures hundreds of kilometers tall.
In a real structure the payload probably won't all be at the top. For the bottom 20 kilometers or so wind loads, ice build-up, and other environmental effects have to be accounted for. Above this height, atomic oxygen can attack your carbon/epoxy structural material, so a protective layer is needed. This adds weight so there will be some reduction in how high you can build. But you still can build many times taller than anything built so far.
These types of towers can be built 'from the top down' in order to avoid construction work in a vacuum. In this process, the top section of the tower is assembled at ground level. Jacks raise the section up by one section length. The next section down is then installed underneath. The process is repeated for the whole tower height, so all the construction work takes place near ground level. Special anchoring provisions are required to stabilize the tower while being built in this fashion.
Status:
The tallest existing structure is a TV antenna which is 655m (2150 ft ) tall. Some engineering/ architectural studies on very large towers have been done. No attempts to build anything over 1000 meters tall are known. This concept should be within current technology for structural materials, although it may require an advance in construction techniques.
[edit] unguyed mast
1a Unguyed Mast
In an unguyed mast, the base of the tower needs to be 1/10 to 1/20 of the tower height to provide avoid buckling. In the lower part of the tower, wind loads will require the base to spread at a greater slope than the upper part, which only depends on buckling for its necessary base width. This approach assumes that most of the loads on the tower act vertically, as in an elevator riding up and down the tower height.
1b Guyed Mast
If the loads are substantially sideways the tower mast may be stabilized by a set of guy wires that spread out at a 30-45 degree angle.
1c Series of Towers
A very long, tall structure, such as a 300 km long electromagnetic accelerator, may use a series of towers as supports.
References:
[edit] tethers
2 Tethers
Alternate Names: Beanstalks, Jacob's Ladder, Space Bridge, Geosynchronous Towers, space elevator
Type:
Description:
Tensile members in orbit store and transfer momentum to vehicles. The tethers may be gravity-gradient stabilized or rotating end-over-end. A ground-to- geosynchronous cable is not feasible with structural materials available in 2007. Tethers, of which a geosynchronous cable is a special case, obey an exponential mass-ratio-to-payload-weight relation similar to that for chemical rockets. It is possible, with existing materials, to build tethers which will provide several km/s of delta v. In a launch system application, an orbiting tether can be set rotating so that the lower end travels slower than orbital velocity. A launch vehicle could rendezvous with the tether, drop a payload, then release. Since only the payload remains in orbit, the propulsion system on the tether only has to provide momentum to add to the payload; the launch vehicle never has to take itself to orbital velocity. In this case the tether acts as a 'momentum bank', lending velocity to the launch vehicle temporarily while the payload is unloaded.
Tethers are the generalization of the 'beanstalk' or geosynchronous tower concept. In the original concept, a cable is placed so that it hangs vertically over the equator, and is in a 24 hour orbit. It thus appears to hang vertically over one spot on the Earth. The task of reaching Earth orbit then reduces to a very long (35,000 km) elevator ride. Unfortunately for the original idea, tensile strengths approaching 2 million pounds per square inch (12.5 GPa) are required for reasonable designs.
Tethers generalize on the original concept by (1) allowing any length, (2) allowing any orbital period, (3) allowing any swinging or rotating states, and, (4) allowing multiple tethers to be connected in various geometries.
One simple case would be a tether vertically oriented in earth orbit, spanning the altitudes from 300km to 2000km. A cargo could be carried on an elevator over this altitude range. While it is not as elegant as the geosynchronous case, it is constructable with existing materials.
Material strength to density ratio is the critical criterion for designing tethers. To build a minimum mass tether, one wishes to taper it's cross section by a factor of e per scale length. The scale length is the length at which under one gravity, the weight of a constant section cable equals the tensile strength (i.e. just breaks). While the gravitational field around a planet is non-uniform, the 'depth' of the gravity well is equal to the surface gravity times the radius of the planet. The following table shows the taper factors derived for each gravity well given materials available at different times:
Taper Factors Required For Various Gravity Wells and Technology Levels |
Mars' 1289 7.8E5 160 58 28 17
| Gravity Depth | ---------------- Time Period -------------- | ||||
| Well (g-km) | 1960s | 1970s | 1980s | 1990s | 2000s |
| ---------- ------ ----- ----- ----- ----- ----- | |||||
| Moon's 287 | 21 | 3.1 | 2.5 | 2.1 | 1.9 |
| 1/2 Earth's 3190 | 3.8E14 | 2.7E5 | 2.3E4 | 4000 | 1060 |
| Earth's 6375 | 1.4E29 | 7.2E10 | 5.1E8 | 1.5E7 | 1.1E6 |
| Material Fiber- | Kevlar | Carbon | Carbon | Adv.
glass || Carbon |
|
|
Tensile Str. (MPa) || 2410 || 3625 || 5650 || 6895 || 8273 |
|||||
| Density (kg/m^3) | 2580 | 1450 | 1810 | 1827 | 1840 |
| Scale length (km@1g) | 95 | 255 | 318 | 385 | 458 |
Status:
Variations:
2a Orbital Hanging Tether
2b Orbital Rotating Tether
2c Terrestrial Tether
One vehicle pulls another without direct mechanical attachment. Allows modification of one vehicle without reconfiguration of joined pair. Allows one type of vehicle to pull another. Reduces loads on lead vehicle by lift-to- drag ratio.
References:
[D1] Ebisch, K. E. "Skyhook: Another Space Construction Project", American Journal of Physics, v 50 no 5 pp 467-69, 1982. [D2] Carroll, J. A. "Tether Space Propulsion", AIAA paper 86-1389, 1986. [D3] Penzo, P.A. and Mayer., H.L. "Tethers and Asteroids for Artificial Gravity Assist in the Solar System" Journal of Spacecraft and Rockets, Jan- Feb 1986. (Details how a spacecraft with a kevlar tether of the same mass can change its velocity by up to slightly less than 1 km/sec. if it is travelling under that velocity wrt a suitable asteroid.) [D4] Baracat, William A., Applications of Tethers in Space: Workshop Proceedings Vols 1 and 2. (Proceedings of a workshop held in Venice, Italy, Octover 15-17, 1985) NASA Conference Publication 2422, 1986. [D5] Anderson, J. L. "Tether Technology - Conference Summary", American Institute of Astronautics and Aeronautics paper 88-0533, 1988. [D6] Penzo, Paul A. and Ammann, Paul W. Tethers in Space Handbook, 2nd Edition, NASA Office of Advanced Program Development, 1989. (NTIS N92-19248/3)
3 Aerostat
Alternate Names: High altitude balloon
Type:
Description:
One approach to minimizing drag and gravity losses is to carry a vehicle aloft with a high altitude balloon. Research balloons have carried ton-class payloads in the range of 15-30 km high, which is above the bulk of the atmosphere.
Status:
Variations:
References:
4 Low-Density Tunnel
4a Light Gas Tunnel
Alternate Names:
Type:
Description:
One or more light gas balloons are strung along the path of a vehicle or projectile. The gas has a lower density than air. The formula for drag is 0.5*C(d)*Rho*A*v^2, where Rho is the density. Thus the lower density will lower drag.
Status:
Variations:
References:
4b Evacuated Tunnel
Alternate Names:
Type:
Description: An evacuated tunnel is supported up through the atmosphere (as by one or more towers). A launch system such as an electromagnetic accelerator fires a projectile up through the tunnel. Drag losses are minimized within the tunnel, and are low in the remaining part of the atmosphere which must be traversed. If the top end requires some means of keeping air from flowing in and filling the tunnel - such as a hatch that remains closed until the accelerator is about to fire.
Status:
Variations:
References:
D.1b Dynamic Structures
Static structures rely on the strength of materials to hold themselves up. Dynamic structures rely on the forces generated by rapidly moving parts to hold up the structure. The advantage of this approach is it can support structures beyond the limits of material strengths. The disadvantage is that if the machinery that controls the moving parts fails, the structure falls apart.
5 Fountain/Mass Driver
Alternate Names:
Type:
Description:
An electromagnetic accelerator provides a stream of masses moving up vertically. A series of coils decelerates the masses as they go up, then accelerates them back down again, at a few gravities. When they reach bottom, the accelerator slows them down and throws them back up again, at hundreds of gravities. Thus the accelerator is many times shorther than the fountain height. The reaction of the coils to the acceleration of the fountain of masses provides a lifting force that can support a structure. The lifting force is distributed along where the coils are located. This can be along the length of a tower, or concentrated at the top, with the stream of masses in free-flight most of the way.
Status:
Variations:
References:
6 Launch Loop
Alternate Names:
Type:
Description:
A strip or sections of a strip are maintained at super-orbital velocities. They are constrained by magnetic forces to support a structure, while being prevented from leaving orbit. A vehicle rides the strip, using magnetic braking against the strip's motion to accelerate. Several concepts using super-orbital velocity structures have been proposed. One is known as the 'launch loop'. In this concept a segmented metal ribbon is accelerated to more than orbital velocity at low Earth orbit. The ribbon is restrained from rising to higher apogees by a series of cables suspended from magnetically levitated hardware supported by the ribbons. The ribbon is guided to ground level in an evacuated tube, and turned 180 degrees using magnets on the ground. A vehicle going to orbit rides an elevator to a station where the cable moves horizontally at altitude. The vehicle accelerates using magnetic drag against the ribbon, then releases when it achieves orbital velocity.
Status:
Variations:
References:
7 Multi-Stage Tethers
Alternate Names:
Type:
Description:
A multi-stage tether has more than one tether, with the tethers in relative motion. For example, a vertically hanging tether in Earth orbit can have a rotating tether at it's lower end. The advantage of such an arrangement is to lower the mass ratio of tether to payload compared to a single tether. The mass ratio of a rotating tether is approximately proportional to exp(tip velocity squared). If two tethers each supply half the tip velocity, then the ratio becomes exp(2(tip velocity/2)squared), which is a smaller total mass ratio.
Another feature of a multi-stage tether is that the tip velocity vector of the two stages add. Since one rotates with respect to the other, the sum of the vectors changes over time. Given suitable choices of tip velocities and angular rates, one can receive and send payloads with arbitrary speed and direction up to the sum of the two vectors.
Status:
Variations:
References:
