THE PROMISE OF SPACE
Arthur C. Clarke (1968)

Part III
AROUND THE MOON

Chapter 15
THE VEHICLE


    At the foot of the artificial mountain known as the Vehicle Assembly Building there is a briefing room, one large wall of which is completely covered with an immensely elaborate chart. It must be the most complex specimen of scientific graffiti in the world, and it details all the thousands of separate operations that must be carried out, in the correct order and at the correct time, so that three men may make the round trip to the Moon. The planning behind that chart represents an investment of some tens of billions of dollars. We are a long way indeed from the backyard spaceships built, with a little help from their beautiful daughters, by the eccentric professors of early science fiction.
    Yet there is no need to go into this overwhelming degree of complexity to understand the Apollo mission, which breaks down into a series of consecutive, logical steps. Most of the complication arises from the need to anticipate, and to overcome, problems and emergencies that may occur during the two-week, half-million-mile voyage. The whole operation has been planned so that, at any time, the mission may be called off (aborted) and the men brought safely back to Earth. The road to the Moon is like a highway from which many side turnings brach off at intervals, most of them leading back to the startng point. We will ignore all these detours (a few are, literally, dead ends) and concentrate only on the main highway--the Nominal (i.e. desired) Mission.
    Let us start by looking at the payload which has to be dispatched toward the moon. It consists of two separate vehicles, each a complete little spaceship in itself--the Command Module (CM) and the Lunar Module (LM). The Command Module is conical in shape, bearing a family resemblance to the Gemini and Mercury capsules; and like them, it is fitted with a saucer-shaped heat shield for protection as it re-enters the atmosphere--at 25,000 mph--on its return from the Moon. For most of the mission it is the home of the three-man crew, and it is the only part of the huge Apollo-Saturn 5 vehicle which survives the round voyage.
    The Command Module has no propulsion system of its own, though it is fitted with small control jets so that it can position itself at the correct angle when it begins re-entry. The rocket engine that will send it homeward from the moon, with its propellants, is housed in a separate Service Module (SM)--a large cylinder upon which the Command Module sits snuggly, like the nosecap on an artillery shell. The Service Module also contains electrical power supplies and part of the life-support system; its task is completed when it has brought the Command Module back to the edge of the Earth's atmosphere, and it is then jettisoned.
    If only a lunar circumnavigation were intended--without landing--these two modules would suffice for the whole mission. (In fact, if it were not for food and air requirements, this combination would allow even a trip around Mars or Venus.) For the landing, the Lunar Module is carried, tucked away in an adapter section immediately beneath the Service Module. One may liken its function, and indeed its initial location, to a dinghy towed behind a cabin cruiser. Thus the complete Apollo spacecraft consists of three units: Command Module, Service Module, Lunar Module. Their combined weight comes to almost fifty tons.
    To launch fifty tons on an escape trajectory toward the Moon requires a truly enormous rocket. (It is worth remembering how huge the Atlas once seemed -- when it boosted the 1+1/2 tons of the Mercury capsule to only 70 per cent of escape velocity.) The Vehicle designed for the task is the Saturn 5, latest of the evolutionary line V-2 (Redstone, Jupiter, Saturn 1). Standing 280 feet high (without its Apollo spacecraft payload, which adds another 80, to give a total of 360 feet), the Saturn vehicle weighs 3,000 tons. This is almost all fuel and oxidizer; the empty weight of the huge structure is little more than 200 tons.
    It is all too easy to become numbed by statistics when contemplating Saturn 5, but here is a modest figure that is nevertheless highly impressive. The vehicle carries more than thirteen times its empty weight in propellants, despite the fact that two of these--liquid oxygen and liquid hydrogen--require special insulation--because of their extremely low temperatures. And to make matters worse, hydrogen also demands very large storage tanks in proportion to its weight; it is the lightest liquid known, with only one fourteenth of the density of water.
    To lift this 3,000 tons of dead weight off the pad, APOLLO 13the first stage uses five rocket engines (hence the designation 5), each of a million and a half pounds' thrust, giving a total thrust of 7,500,000 pounds, or 3,750 tons. The margin to produce this lift is rather small, and the vehicle will therefore rise quite slowly until it has lightened itself by burning fuel.
    It does this at the unbelievable rate of fifteen tons per second, and this introduces another awesome statistic. The pumps necessary to drive such quantities of fuel and oxidizer into the giant combustion chambers require turbines generating a total of 300,000 hp to drive them; this is twice the engine power of the largest ocean liner. There are few other facts which demonstrate so conclusively the new order of magnitudes involved in space transportation. The giant engines that propel the floating cities of the North Atlantic could not even run the fuel pumps of the Saturn 5.
    The nomenclature of the Apollo booster is somewhat confusing as it is derived from earlier vehicles in the Saturn program. There are three stages, and it would be convenient if they were labeled S-1, S-2 and S-3, or even A, B, and C. But for once the well-known Germanic sense of order has been defeated (the whole Saturn program is managed by NASA's Marshall Space Flight Center, directed by Dr. von Braun), and the final configuration has turned out to be: S-IC, S-II, and S-IVB. We shall just have to live with it
    The first (lowest) stage is the S-IC, with its five enormous 1,500,000-pound-thrust engines. SATURN APOLLO VEHICLE It is by far the largest element of the whole assembly, containing 2,200 tons of propellants--a brand of kerosene known as RP-I, specially processe for rockets--and liquid oxigen (lox). This is not the most powerful combination known, by a wide margin, but it is much the cheapest--three cents per pound--so it is economic good sense to use it for the first and largest stage of a launch vehicle.
    The second (S-II) stage burns high-energy liquid hydrogen and lox and has a total propellant capacity of 460 tons, but because liquid hydrogen is a dozen times as bulky as kerosene, it is not very much smaller than the S-IC stage. Like that stage, it has five engines, though much smaller ones giving a total thrust of 500 tons. Thus it would be unable to lift itself off the ground under its own power, and can function only under orbital conditions.
    The third (S-IVB) stage is also liquid hydrogen--lox fueled; it carries 115 tons of propellants and is powered by a single 100-ton-thrust engine. It is topped by a section carrying the electronics for guiding and controlling the whole launch vehicle; and on top of that is the final payload--the Apollo spacecraft itslef, which is the only thing left when when the spent components of the gigantic Saturn 5 have dropped back into the sea or joined the rest of the debris now orbiting earth.
    At this point it may be as well to take an inventory and to list the vital statistics of the complete vehicle and payload. It must be realized that Table 5 could easily be expanded into a whole shelf of thick volumes, since it summarizes the activities of more than 20,000 companies (including many of the largest in the world) and hundreds of thousands of individuals.
    Table 5 gives the cold facts of the Saturn 5 vehicle; no one would have believed most of them a few years ago. For when those five F-1 engines ignite, the mass of a fully loaded destroyer will climb straight up into the sky.
   

TABLE 5

APOLLO SPACECRAFT
 
CODE
NAME
LENGTH
FEET
DIAMETER
FEET
EMPTY WEIGHT POUNDS
PROPELLANT WEIGHT POUNDS
FULL WEIGHT POUNDS
THRUST POUNDS
PROPELLANT
FUNCTION
MAIN CONTRACTOR
1
LES
Launch Escape System
34
2
- - -
- - -
6,600
150,000
Solid
Removal of CM at launch emergency
Lockheed
2
CM
Command Module
12
13
- - -
- - -
11,000
- - -
- - -
Transporting 3 men to lunar orbit; return to earth
North American
3
SM
Service Module
24
13
10,000
23,000
33,000
22,000
Self-igniting storage liquids
Inserting CM into lunar orbit; returning to Earth
North American
4
LM
Lunar Module
20
22
 
 
 
10,500 (descent) 3,500 (ascent)
Self-igniting storage liquids
Lunar landing and return to CM with 2 men
Grumman
SATURN 5 VEHICLE
5
IU
Instrument Unit
3
22
3,500
- - -
- - -
- - -
- - -
Guidance and control of launch vehicle
I.B.M.
6
S-IVB
Saturn Third Stage
60
22
22,000
230,000
252,000
200,000
LH2 LOX
Injection into lunar trajectory at 25,000 mph; one J-2 engine (rocketdyne)
Douglas
7
S-II
Saturn Second Stage
82
33
80,000
930,000
1,100,000
1,000,000
LH2 LOX
Boost from 5 to 15,000 mph at 115 miles altitude; five J-2 engines (rocketdyne)
North American
8
S-IC
Saturn First Stage
139
33
300,000
4,400,000
4,700,000
7,500,000
Kerosene Lox
Liftoff to 5,000 mph at 40 miles altitude; five F-1 engines (rocketdyne)
Boeing

Link: http://history.nasa.gov/SP-4206/contents.htm