Acid Sky (31 page)

The
Langley
is named after Samuel Pierpont Langley (1834–1906), as mentioned in the story. The callsign ‘Houseboat’ for the
Langley’s
air wing refers to Samuel Langley’s experiments with launching an aircraft from a houseboat in the Potomac River near Quantico, Virginia. Langley endured the scorn of the press following a conspicuous failure, a matter of days before the Wright Brothers’ first successful manned flight in 1903. His design finally flew in 1914, following modifications made by Glenn Curtiss.

Flying in high-speed winds

The high-altitude prevailing winds on Venus move at speeds up to 360 kilometres per hour at the equator. An aircraft released at this altitude, with no propulsion system, would simply glide towards the ground, while moving with the wind. Because the aircraft is not tethered to the ground like a kite, but is free to be carried along with the wind, it experiences almost no relative airflow due to the wind itself, and can only generate airflow over its wings by moving forward independent of the wind.

On the
Langley
, the thrust for forward motion is provided by its large nuclear-powered turbofans. The
Langley
can fly with or against the wind; it makes no difference except to the ground speed, which affects the length of the day as experienced by those on board.

For an aircraft coming in to land on the carrier, the aircraft must also be moving through the air to generate lift. If the velocity of the incoming aircraft is slightly greater than that of the carrier, then the aircraft will gradually catch up with the carrier and can land on its deck. In practice, a higher closing speed than this is described in the story, to ensure sufficient control in the airflow over the deck.

The carrier’s flight deck does not have to be as long as that found on a terrestrial aircraft carrier, as the relative landing speed is not as great, and the length of runway required is much reduced. The arresting gear, similarly, does not have to be as large since the kinetic energy of the incoming aircraft relative to the carrier is much lower.

Once the aircraft has come to a halt on the deck, however, it is part of the carrier, and will experience the headwind of the carrier’s forward motion relative to the air. This will tend the blow the aircraft backward along the flight deck unless the motion is counteracted by brakes, forward thrust or being clamped to the deck.

For takeoff, the relative headwind is an advantage, as it provides an initial airspeed that is sufficiently high to generate lift. An aircraft waiting to takeoff must disrupt the lift over its wings with spoilers or risk an inadvertent takeoff. For takeoff, all that is required is for the spoilers to be withdrawn and sufficient thrust applied, and the aircraft will take off and climb away.

Flying in Venus’s atmosphere

At first sight, using a gas turbine engine in an atmosphere of carbon dioxide (CO
2
) would seem impossible; there is no oxygen to support combustion of the fuel. Only a small proportion of the air entering a turbojet engine, however, is actually used for combustion, the remainder serves as a
working fluid
that is compressed, heated in its compressed state, and then allowed to expand and do useful work by rotating the turbine. In a turbofan engine, an even greater proportion of air is accelerated by a large fan around the central turbojet core, and this
bypass air
takes no part in the combustion cycle.

The only requirement for oxygen for combustion comes from the (relatively) small amount of air needed for fuel combustion, which heats the compressed air in the very core of the engine. Only about 20% of the air on Earth is oxygen in any case – the remainder is nitrogen, which takes only a small part in the combustion process.

The aircraft (and spaceplane) described in the story carry liquid oxygen as well as fuel, and their engines have combustors that are fed directly by a mixture of fuel and oxygen, and can operate even when surrounded by carbon dioxide. A similar principle allows an oxy-acetylene cutting torch to operate underwater.

A practical implementation of such an engine would be far more involved than this explanation might suggest. The basic principles of subsonic flight, however, would be the same in a CO
2
atmosphere at the same temperature and pressure as on Earth.

The green flash

A green flash is seen on Earth under certain conditions when the Sun just disappears below the horizon; it is caused by green light being preferentially refracted over the horizon and into the observer’s line of sight. In the story, the flash is depicted as being more pronounced, due to the greater apparent size of the Sun.

The spaceplane

The Olympus spaceplane used on Venus is a specialised variant of the spaceplane first depicted in
Below Mercury
, and is optimised for operations in the atmosphere of Venus. There are no landing jets, as they are unnecessary in the dense atmosphere. An arresting hook is fitted, and the landing gear is strengthened to cope with the greater stresses of carrier landings.

Internally, the passenger seating is increased from six to eight, as there are no ejection seats installed; there are no survivable escape scenarios in the Venusian atmosphere. The liquid oxygen tank is also enlarged (and the liquid propane tank reduced correspondingly) to cope with the different proportions of propellants needed during the ascent through the Venusian atmosphere.

The engine intakes utilise diverterless, variable-geometry intakes. These close completely for spaceflight and re-entry, open progressively for supersonic flight, and open wide for subsonic flight. Inside the inlets, movable internal ramps slow down the entering hypersonic air to subsonic speeds, with precoolers and liquid oxygen injection to control the enormous temperature rise.

The spaceplane can carry up to five tonnes of freight down from Venusian orbit to the carriers. The ‘down’ direction is most important for freight, as this is the resupply direction. For ascents, the spaceplane has almost no freight capacity when carrying a full complement of passengers, as the ascent direction is the most prohibitive in terms of load carried.

The materials from which the spaceplane is made are also different due to the need for enhanced corrosion resistance; no magnesium alloys are used, and other alloys are chosen carefully to ensure resistance to the atmosphere while retaining adequate strength and heat resistance.

Orbital mechanics

The most efficient trajectory between two planets orbiting the Sun (or, two moons orbiting a primary) lies on a half-ellipse between the two bodies, referred to as a
Hohmann transfer trajectory
. While other routes are possible, they always involve more fuel, more time, or both. Whatever type of transfer trajectory is used, it is important that these transfer trajectories are started when the two bodies are in the correct relative positions, i.e. that the target planet has moved in its orbit so that it coincides with the arrival of the ship by the time it gets there. There is a range of dates either side of the optimum transfer opportunity when a less efficient transfer may be made, but the fuel penalty rises prohibitively the further away the transfer is from the ideal date. Outside of a certain range – the
launch window
– the transfer simply cannot be achieved within the limits of the launch vehicle. For Earth and Mars, these launch windows are centred 780 days apart and this is too restrictive for a regular transfer operation, which would be needed to support permanent bases on other planets.

Venus is an
inferior
planet to Earth, meaning that it orbits closer to the Sun. Mars is a
superior
planet and orbits further away. It is counter-intuitive that voyages between Mars and Earth can go past Venus, but the explanation lies in the slow movement of Mars compared to the inferior planets.

Because Venus orbits closer to the Sun, its orbital period is shorter than Earth’s, and launch windows between Venus and Mars occur more frequently, every 334 days. Depending on the situation of the planets, it can be faster to travel from Earth to Venus, then on to Mars, than to wait for a direct transfer window to Mars. The same is true in reverse, for voyages from Mars back to Earth.

In the story, the orientation of the planets, the journey times, and the rising and setting of the Sun, Earth and Mercury are as they would be in December 2141. The orbital transfer trajectories, journey times and other astronomical events described in the story are consistent with the technologies described, and the performance data as shown in the drawings.

Units and conventions

The nautical mile (NM) and the knot (kt) are used commonly in aviation as the units of distance and speed, respectively. The NM is convenient for navigation as it represents one second of arc on Earth’s surface. On other planets, however, the terrestrial NM would be less appropriate, and the kilometre (km or ‘klick’), metre per second (m/s) and kilometre per hour (kph) are used instead, as a consistent system of measurement that would work in all situations. One metre per second is approximately two knots.

I have described the US Astronautics Corps as loosely following US Air Force customs and practices when dealing with aviation, and the US Marine Corps and Navy when on board the carrier. The carrier has a forward, an aft, and port and starboard quarters. It is steered by a helmsman, but from a control room, as the carrier does not have a bridge in the conventional sense.

There are inconsistencies, however, as Lieutenant Gray notes in the story. The carrier has a left wing (aviation) on its port quarter (nautical).

MANNED DEEP SPACE VESSELS
IN USAC INVENTORY, 2141

Deep Space Tug (STN)
–
Chicago-, Boston-, San Diego-
and
Omaha-
class space tugs. The
Chicago
-class was the first tug type to be constructed.
Chicago
(STN-01) and
Detroit
(STN-02) were decommissioned in 2138 and 2140 respectively.

Deep Space Explorer (DXN)
–
Columbus-
and
Magellan-
class science exploration vessels based on the
Chicago-
and
Philadelphia-
class space tug designs.

Deep Space Interceptor (DIN)
–
Philadelphia-
class dedicated asteroid movers based on the
San Diego-
class space tug designs, with increased propellant and thrust capability.

Venusian Carrier (VCN)
–
Wright-
class airborne carriers, designed to operate in the lower atmosphere of Venus.

GLOSSARY

Albedo
(Bond albedo) – in astronomy, the fraction of solar radiation reflected back into space. Venus, for instance, has a very high albedo compared to Earth due to its reflective clouds.

Alkane
– a family of hydrocarbon compounds where each of the constituent atoms are linked together by single bonds. Methane, propane and butane are all alkanes.

Aphelion
– in astronomy, furthest distance of an orbiting body from the Sun.

Arresting hook
– a long hook that hangs from under the tail of an aircraft, designed to catch the
arresting wire
.

Callisto
– fourth Galilean moon of Jupiter.

Comlink
– cell-based, handheld telecommunications device.

Cryogenic
– super-cold; generally any temperature below about –100 °C or 173 K.

EGT
– Exhaust Gas Temperature, an important operational parameter for jet engines.

EICAS
– Engine Indicating and Crew Alert System. A dedicated display that shows key engine parameters and alerts the crew to the situation of other key systems.

Elevon
– a single control surface combining the functions of an elevator (for pitch control) and an aileron (for roll).

EVA
– Extra-Vehicular Activity. Any activity conducted outside the shelter of a pressurised vehicle or habitation.

Fischer-Tropsch (process)
– a series of chemical reactions that convert carbon monoxide and hydrogen into various liquid hydrocarbons.

FSAA
– Federal Space and Aviation Administration, a twenty-second century evolution of the FAA.

Gee (g)
– an acceleration equal to that exerted by gravity at Earth’s surface, approximately 9.8 m/s
2
.