Space exploration to date relies heavily on rockets. Although most rockets have been driven by chemical propellants, there are many other types of rocket, such as ion engines and nuclear rockets. In this chapter we look at the general principles of a rocket, regardless of the mechanism used to generate thrust. We derive the important rocket equation and examine its consequences for space travel.

Chapter 2.4 gives an overview of user charges. User charges are out of pocket (OOP) payments made at the point of use for health services. Nearly all countries have some user charges, most commonly for medicines. They are intended to raise revenue and also to reduce the use of unnecessary health care services and products. Key learning includes that

• User charges can generate revenues but have many unintended negative consequences – creating barriers to access, contributing to inequities and increasing the risk of financial hardship for households.

• Health systems can reduce unnecessary or wasteful use of care without user charges by

• – Strengthening prescribing and referral systems to make sure care is appropriate

• – Offering more information to steer patients and providers towards more cost-effective care.

• Supply-side mechanisms that guide providers’ behaviour are more equitable and effective than demand-side mechanisms like user charges and have fewer negative impacts on patients – especially those with chronic or severe conditions or the economically disadvantaged.

• User charges are a suboptimal policy but – if they are to be used – health systems can mitigate the harm they cause and protect health care users through mechanisms such as exemptions, reduced copayments, income-related copayment caps, and to a lesser extent, price control and regulation.

The special theory of relativity rests on just two axioms: the identity of the laws of physics in all inertial reference frames, and the invariance of the speed of light. After briefly reviewing the basics of special relativity, including the Lorentz transformations and time dilation, we look at the effects of relativity during an accelerated journey. Time dilation has implications even for a spacecraft moving at low velocities. We examine the relativistic rocket equation and then rockets that are propelled by photons or a combination of matter and photons. General relativistic effects are not considered. In a later chapter we look at the impact of relativistic motion on optical phenomena.

Chapter 2 develops the conceptual foundations for studying hype. Synthesising insights from Science and Technology Studies, Organisation and Management Theory, Cultural Entrepreneurship, and Economic Sociology, it positions hype as distinct yet overlapping with adjacent concepts such as bubbles, fashions, and organising visions. The chapter advances seven tenets for analysing hype, emphasising its collective rather than individual origins, its cyclical promise–requirement dynamics, and its shifting role across the innovation lifecycle. Hype is treated as both performative — enacting futures — and reflexive — subject to recalibration as evidence emerges. Special attention is given to accountability mechanisms, which are increasingly shaping expectations of whether ventures can ‘live up to the hype’. This framework reorients scholarly debate from moralising accounts of deception to the situated practices and infrastructures that sustain hype. By formalising these tenets, the chapter provides a conceptual platform for the empirical chapters that follow.

Chapter 1.3 considers voluntary health insurance (VHI). VHI is paid for privately by or on behalf of individuals and normally covers care in addition to the publicly financed benefits package. Premiums are not typically based on the policyholder’s income but may well vary depending on their risk of ill health. Key learning includes that

• Despite prepayment and risk pooling, VHI has limitations and does not align well with progress towards universal health coverage because:

• – Risk pools in VHI schemes are typically much smaller than pools established through statutory schemes which means there are fewer people to share risk

• – Inequities are created because of the cost of premiums, which may not be affordable or accessible to everyone including those most in need.

• VHI has wider equity implications because it offers those who can afford to pay faster access or greater choice of services (supplementary insurance) or coverage of excluded services or user charges for statutory care (complementary insurance)

• Governments seeking to use VHI to expand coverage typically have to make significant interventions, including through tax subsidies to make premiums more affordable, but this creates market distortions and is inefficient.

• Policy-makers can secure better value for money by improving access to publicly financed health care than by promoting VHI.

The general principle of a rocket is to expel propellant in one direction, thereby forcing the rocket to move in the opposite via conservation of momentum. Most rockets built so far are propelled by burning a fuel to produce a gas jet. In this chapter we see how a convergent–divergent nozzle is used to accelerate the gas to a large velocity by maximizing the conversion of its thermal energy (random motion) into kinetic energy (linear motion). We will examine the characteristics of liquid and solid fuels, and look at a few examples of real rockets that use them.