On spaceflight

Spaceflight is important because it may decouple the fate of life from the fate of Earth.

This article answers three questions:

Other reading

I have not found great literature on spaceflight technology and economics. Some reports:

Supply of spaceflight

Cost structure: Why does it cost $4,000/kg to launch stuff into orbit?

Let’s learn from today’s lowest cost rocket, the Falcon 9 by SpaceX.

$62M is the list price of a Falcon 9 launch.1,2

At max payload, a $62M launch is:

The cost of a launch is dominated by the cost of the rocket:

The rocket dominates, so let’s look at that. Rocket costs are driven by labor and equipment. Raw materials are less than 1% of the final cost. One way to gauge the effort needed to make a product is by its cost per kilogram. A Falcon 9 is comparable to an airplane:

How low can costs go?

Payload / $ can be factored into: (payload / launch) * (launches / rocket) * (rocket / $). These 3 factors suggest 3 routes to reducing cost:

Although all 3 frontiers are being pushed, they oppose one another. Efficient rockets are more expensive to build. Reusable rockets carry less payload per launch.

As of 2018, SpaceX’s lower costs have come primarily from cheaper manufacturing, not from greater efficiency or reusability (yet).

Of course, even approaching the marginal cost of spaceflight requires a decent volume of launches over which fixed costs can be spread. Otherwise the “tyranny of space transportation costs (page 9)” can spiral out of control.

What drives the marginal cost of spaceflight?

Physical limits: How low can the cost go?

Conservation of energy and conservation of momentum limit what is possible.

Conservation of energy

At minimum,3 sending 1 kg to orbit requires ~33 MJ:

33 MJ isn’t much - it’s a quarter gallon of gas or a dollar of electricity.

Energy also goes into two losses:

Because these losses cannot be minimized together, the only way to approach the 33 MJ limit is eliminate one so the other can be minimized:

None are even close to feasible with today’s technology and interest rates. So from here on, I will focus on rockets.

A rocket’s energy is stored as chemical potential energy between fuel and oxygen. The engine converts chemical potential energy into kinetic energy. An efficient rocket maximizes the fraction of energy that ends up in the payload’s motion, and minimizes the energy that goes to:

A Falcon 9 carries 15,600 kg of payload using ~500,000 kg of RP-1 fuel and liquid oxygen. LOx and RP-1 release ~6 MJ/kg. That works out to ~200 MJ of fuel per kilogram of payload, only about ~6x the theoretical minimum.

Conservation of momentum

Because momentum is conserved, the only way to accelerate is to push off of something else. Rockets get around by pushing off of their exhausted fuel.

Delta-v is the budget that says how much pushing your rocket can do. Like a fuel gauge, it tells you which destinations you can and cannot reach.

The rocket equation has three variables:

Delta-v is convenient to work with for two reasons:

This fantastic map shows how much delta-v is needed to reach destinations in the solar system:



Historical progress

The number of orbital launches per year has been pretty steady at around 50-150, declining from the 1960s through 2000s, and then rising again in the 2010s

Propulsion choices

Demand for space travel beyond Earth orbit


  1. Source: SpaceX website in April 2020 

  2. Note: Although $62M is the list price, customers may pay more or less. On the high side: the US Air Force has repeatedly paid ~$90M for launches of its GPS satellites (the other bidder, ULA, charges twice that). Heavier payloads requiring an expendable booster are certainly priced millions higher than $62M, but I could not find details. On the low side: Flying on a reused booster discounts the price by ~20% to ~$50M. Modest discounts are also offered for contracting multiple launches. Outsiders have estimated the marginal cost to be $37M$48M

  3. “Minimum” subject to the constraints of not changing the Earth’s shape, mass, or atmosphere.