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Aviation History
1957
1957 - 1522.PDF
612 FLICHI A NEW REALM OF FLIGHT ... : ; „. Satellite Principles -^ TO send a satellite around the Earth, so that it moves steadily* in an orbit until traces of gas and meteoric dust cause it to fall and burn up in the high atmosphere, it is necessary to supplythe satellite with potential and kinetic energy (height as well as velocity) and to set it on its desired path. This can only be doneby a rocket-driven vehicle fitted with accurate guidance, because any air that would sustain ordinary engines is left far behindsoon after the moment of launch, and because small errors in direction have a very large effect on the shape of the orbit thatis achieved. The rocket system used for the Russian launching vehicle issaid to be composed of three stages. These can be separated in flight, so that the later stages do not have to accelerate the massof the empty earlier stages, and so can rise to great heights. A possible form of three-stage launching vehicle is shown in Fig. 1:the first stage is probably the lowest stage of an intercontinental ballistic missile. After a vertical take-off, this will get the vehicleup to a height of about 50 miles and a velocity of, say, 4,000 m.p.h. (see Fig. 2). During this pan of the flight, the control system(which may be in the second stage) will tilt the vehicle so that it begins to point in the general direction of the orbit. When the propellants axe consumed, the separation devicebetween the first and second stages is actuated, so that the first stage is free to fall back to earth: it may land, say, 300 miles fromthe launching point. After separation, the second stage will be fired, and this will get the remaining parts of the vehicle to analtitude of 200 miles and a speed of 10,000 m.p.h. There may then be a long coasting period, during which the vehicle risesalmost to orbit height, and in which auxiliary "vernier" rockets in the second stage place the vehicle on the precise path to givethe proper final orbit: this will be done with the aid of the com- puters in the guidance compartment. When the vehicle is finallyon course, the second stage is separated (extremely gently so as not to disturb the third stage) and will fall to earth in its turn,from an altitude of about 500 miles. The velocity of re-entry of the second stage will be so high that it may bum up. The third-stage rocket, with the satellite on the front, will thenbe fired, so placing the satellite securely in its orbit at an altitude of 500-600 miles with a velocity of 18,000 m.p.h. Final separa- Fig. 1, Possible arrangement of a three-stage launching vehicle (the diameter is slightly exaggerated ta show internal features). 3rd STAGE n-A—c 2nd STAGE Overall height 180200ft, total weight 80-100 tons. Diameter of Stage 1 about 15ft. Stage 1 would have at least two liquid-propellont motors (gimbal-mounted for guidance) with a total thrust of say 250,000-300,000 Ib for 2 min. Stage 2 might have a single (gimballed) motor giving 30,000 Ib for 60 sec. Stage 3 (no guidance system) might employ either a solid or a liquid (nitric acid/hydrazine) propellant giving 3,000 Ib for 10 sec. The skin of the first stage (perhaps of stain- less-steel foil) would be so thin that the rocket would not stand without propellants in the tanks to stabilize it. U-U •tot STAGE A Anti-heating nose-cone, blown off at 20-30 miles B Satellite Cl Separation device (spring or explosive) C2 Ditto C3 Ditto (latch or explosive) D Guidance for entire vehicle (gyros, occelerometers, integrators; ampli- fiers for gimbal tacks of 1st and 2nd stage motors) E Helium bottles to pressurize tanks F Propellant tanks (perhaps liquid oxygen/hydrazine, or fluorine/hvdra- zine) G Auxiliary rocket motors for guidance after burn-out of main motor H Second-stage gimballed motor J Propellant tanks (perhaps liquid oxygen/kerosine or gasoline) K Anti-slosh baffles L Propel lant-pumps (turbines driven by auxiliary gas-generators from main propellant supply) M First-stage gimballed motors tion of the satellite from the third stage will be done by a singlespring or small explosive device, controlled by clockwork and started from the second stage before it was separated: a relativevelocity of only a few feet per second is required, since the satellite and third-stage rocket will gradually drift apart. If thenose cone has not been separated earlier, it will be jettisoned now. The total take-off weight of the launching vehicle is likelyto approach one thousand times that of the satellite itself, that is, about 80 tons for a satellite of 184 Ib.The Russians stated that the orbit in which the satellite is travelling is inclined to the equator at 65 deg; that is, at itsfurthest passage north and south of the equator, the satellite is over latitudes 65 deg north and south. (Fig. 3.) The plane ofthe orbit remains nearly stationary in space (in fact it rotates slowly, because of the effects of the Earth's oblateness), and thismeans that, since the satellite makes one revolution about the Earth in about li hours, the Earth rotates under the satelliteorbit during each revolution by the angle which it always moves in this space of time, namely 360 deg x li/24, or 22£ deg. To INITIATE 3RD STAGE SPIN SEPARATE 2ND STAGE THRO STAGE IGNITION THIRD Sl| BURNOUT SEPARATE SECOND STAGE BURNOUT VELOCITY 25,000 FT/SEC\ ALTITUDE 200-400 MIES-1 RANGE 1500 MILES TINE 10 MIN. AFTER LAUNCHING FIRST STAGE BURNOUT AND SEPARATION IGHJ Fig. 2. Launching of satellite by three-stage vehicle. Data refers to U.S. Vanguard project; Russian launching trajectory was similar to that illustrated. (From M. Rosen's paper at the Cranfield symposium. observers on the Earth, therefore, it seems as though every orbit is twisted about the Earth by about 22J deg. As the Earth makes one complete rotation, any place on itssurface (between latitudes 65 deg N. and 65 deg S.) will be beneath the orbit of the satellite on two occasions. The satellitewill pass over a given position—or fairly near it—twice each day, once from a northerly direction and (when the Earth hascompleted half a revolution) once from a southerly one. The satellite can be seen from practically all the inhabited pans ofthe Earth, excluding only the central polar regions. Projecting the satellite into an orbit of this type is more difficultthan either making it revolve above the equator all the time, or launching it a small angle to the equator (as is to be done with theU.S. Vanguard satellite), for the high-angle orbit does not permit much advantage to be taken of die Earth's surface velocity—about 1,000 m.pJi. at the equator. A satellite launched eastward at the equator would need only 17,000 m.p.h. extra velocity,instead of 18,000 m.p.h. for a completely polar orbit, so that the Russians have chosen an orbit which is by no means the easiestto achieve. It might be remarked that any satellite must be launched into an orbit which is a "great circle," since the centreof gravity of the Earth must lie in the same plane as the satellite orbit. The satellite having been placed in the orbit, with adequateheight and speed, there remains the problem of knowing what shape the orbit will be. This depends on the angle of elevationof the satellite's path at the moment of separation, as well as its speed, and only by chance will the orbit be a circle about theEarth with the satellite travelling at constant height. The orbit will almost certainly be an ellipse for, even if the elevation angleat separation from the third stage were zero (that is, if the path of the satellite were parallel to the Earth's surface immediatelybelow), the satellite would move towards or away from the Earth as it travelled round the orbit unless its velocity and heightwere so precisely matched that the Earth's gravitational attrac- tion (which depends inversely on the square of the distance of th esatellite from the Earth's centre) was exactly equal to the centr - fugal force tending to make the satellite fly off tangentially. Such an elliptical motion is stable, since as the satellite fal:it converts potential energy into kinetic energy, speeds up arc) increases the outward centrifugal force at a greater rate thinthe inward gravitational force increases. The orbit will have a point of nearest approach to the Earth (perigee), at which thsatellite velocity will be greatest, and a point of greatest distanc away (apogee), at which the velocity will be least. An elliptical orbit which has a considerable difference betwee ;the perigree and apogee altitudes is not in itself inferior to A
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