Manned Spacecraft Design Principles

Pasquale M. Sforza

• Publisher: Butterworth-Heinemann
• Year: 2015
• ISBN:
  9780124199767
• Topic: Technology & Engineering
• Subtopic: Aeronautic & Astronautic Engineering
• Index: Technology & Engineering

Manned Spacecraft Design Principles presents readers with a brief, to-the-point primer that includes a detailed introduction to the information required at the preliminary design stage of a manned space transportation system.

In the process of developing the preliminary design, the book covers content not often discussed in a standard aerospace curriculum, including atmospheric entry dynamics, space launch dynamics, hypersonic flow fields, hypersonic heat transfer, and skin friction, along with the economic aspects of space flight.

Key concepts relating to human factors and crew support systems are also included, providing users with a comprehensive guide on how to make informed choices from an array of competing options. The text can be used in conjunction with Pasquale Sforza's, Commercial Aircraft Design Principles to form a complete course in Aircraft/Spacecraft Design.

• Presents a brief, to-the-point primer that includes a detailed introduction to the information required at the preliminary design stage of a manned space transportation system
• Involves the reader in the preliminary design of a modern manned spacecraft and associated launch vehicle
• Includes key concepts relating to human factors and crew support systems
• Contains standard, empirical, and classical methods in support of the design process
• Culminates in the preparation of a professional quality design report

Chapter 1

Manned Spaceflight

Abstract

The space race was precipitated by the convergence of reduced weight nuclear weapons and increased payload capability of ballistic missiles. The geopolitical implications of putting satellites into orbit were great and the next step of safely sending humans back and forth to space bespoke a technical mastery that fired national pride and power. A brief description of the development of manned spaceflight starting from Vostok launched by the USSR in 1961 and Mercury launched in reply by the USA in 1962. National competition ultimately became planetary cooperation with the operation the International Space Station. The outlook for the future of human spaceflight now ranges from interplanetary missions of discovery to vacation cruises for space tourists.

Keywords

Manned spaceflight history; the Karman line; escape velocity; weightlessness; space tourism

1.1 Where Space Begins

In the early days of space travel the renowned aerodynamicist Theodore von Karman suggested a useful definition for the edge of space: the altitude at which an airplane flying in a straight path at the orbital speed can no longer sustain its weight using only aerodynamic lift. This equilibrium condition on lift may be written in terms of the airplane’s maximum lift coefficient C_L,max as follows:

$L=\frac{1}{2}{C}_{L,\max }\rho (z)S{V}^2=mg$ (1.1)

(1.1)

From Eqn (1.1) the density must then be

$\rho (z)=\frac{2}{C_{L,\max }{V}^2}\left(\frac{mg}{S}\right)$

Using the orbital velocity as V=7900 m/s, as discussed subsequently, and a nominal wing loading of mg/S=3000 N/m² (62.7 lb/ft²), similar to that of the Space Shuttle Orbiter or the X-15 hypersonic research aircraft, the density (in kg/m³) becomes

$\rho (z)=\frac{9.61\times {10}^{-5}}{C_{L,\max }}$

Earth’s atmosphere is discussed in Chapter 3 and standard atmospheric data shows that in the altitude range 50 km<z<70 km the density, in kg/m³, is in the range 3×10⁻⁴>ρ>8×10⁻⁵. Therefore for a maximum lift coefficient in a reasonable range of about 0.3<C_L,max<1 the corresponding altitude is between about 50 km and 70 km. Because the definition is somewhat arbitrary, the altitude of the edge of space is usually rounded off to z_e=100 km and is often called the “Karman line.” The Federation Aeronautique Internationale (FAI) uses the Karman line to define the official boundary between aeronautics and astronautics activities. The US Air Force (USAF) definition of an astronaut is a person who has flown more than 50 miles (approximately 80 km) high while NASA uses the FAI’s 100 km figure. The mean radius of the earth is R_E=6371 km so the edge of space is quite near (z_e/R_E=0.015) and the gravitational acceleration in “near space” is essentially equivalent to that on the surface itself, as presumed in carrying out this analysis.

1.2 Staying in Space

To stay at this edge of space we can no longer fly in a straight path like an airplane but instead we must follow a curved path. For simplicity we consider a circular path of radius r=R_E+100 km, such that the centrifugal force mV²/r balances the vehicle weight mg. The balance struck in this circular path we call “weightlessness”: the net radial acceleration is zero so although the mass is fixed and the earth’s gravity is still essentially the same as on its surface, the net force on the payload is zero. This scenario gave rise to the term “zero-g” which is not strictly correct; rather it is zero-net-acceleration. A circular orbit is used for clarity here but a more general treatment of orbits and trajectories is presented in Chapter 5.

We see then that this means the net radial acceleration V²/r=g. For this equilibrium condition to exist, the velocity of the vehicle V=(gr)^1/2 where r=R_E+z and g=g(z). The earth’s mean radius R_E=6371 km and at the earth’s surface r=R_E and similarly g=g_E so that V=V_E~7,900 m/s and this is called the circular velocity. Strictly speaking, it is the velocity required to maintain a circular orbit at the surface of the earth. For the edge of space we chose the altitude z_e=100 km arbitrarily and see that the exact altitude is not important in the calculation when z/R_E

1. As will be shown in subsequent chapters, there is only a few percent variation in V between the surface and an altitude of 100 km.

Although the density at the edge of space is low, it isn’t zero. The aerodynamic drag on a body can slow it down sufficiently to cause its orbit to decay sending the body on an entry trajectory down to the surface. The departure of a spacecraft from orbit is initiated by a retarding thrust force which forces the spacecraft into a high speed descent toward the earth. The details of the atmospheric entry process and its effects on astronauts are treated in detail in Chapter 6. Experience shows that at altitudes below about 150 km the drag on an unpowered orbiting body is great enough to cause the orbit to decay. Typical orbits for manned spacecraft, like that of the International Space Station (ISS), are situated at an altitude of about 400 km and are called low earth orbit (LEO). Even at that altitude a large spacecraft like the ISS experiences orbital d...