Part 2: Space Transport Methods
The Transport Challenge
An ideal transport system has an infinite life and minimal operating cost when delivering a desired payload. Real systems never reach that goal, and the designer's job is to approach it as closely as possible given the level of technology and other program limits.
The Historical Problem
The difficult job of the mid-to-late 20th century rocket designer was to find the best compromise between high cost and small payload when going to Earth orbit. This compromise was forced by three factors conspired to produce a small payload fraction, and therefore high relative cost:
- The depth of the Earth's gravity well, which determines the velocity needed to reach orbit,
- The energy contained in chemical rocket propellants, which determines how much of them you need, and
- The strength and other properties of the materials used to build the rocket, which determines how much the hardware itself weighs.
It happens that the combination of these three factors conspire to produce a small, or even negative payload in an ideal one piece, long life rocket. The total launch mass of such a rocket consists of 3 main parts: Propellant, Vehicle, and Payload. Payload is what is left after accounting for the first two, and in 20th century designs that remainder could be negative, or at best a few percent. For example, a single stage LO2/LH2 vehicle with a mission velocity of 9000 m/s and an exhaust velocity of 4500 m/s might be 86.5% fuel, 10-15% vehicle hardware depending on operating life, and thus -1.5 to +3.5% payload. The lower hardware weight was associated with more flimsy single use construction, and the higher weight with thicker long life design. The large ratio of rocket to payload weight led to high launch cost measured in $/kg.
Various compromises had to be made from the ideal concept of a vehicle you could just refuel and launch again like an airplane. One was to make the hardware last just a single flight, thus allowing lighter structures than ones built to last many flights. Another is to drop parts of the vehicle during flight (staging). As fuel is used up less thrust is required to maintain acceleration, so fewer or smaller engines. Tanks also get emptied, so you can drop the excess engines and tanks once you don't need them any more. The remainder of the vehicle starts anew to accelerate towards orbit, but has the benefit of the velocity gained with the previous stage, and less vehicle weight. Low service life and staging are are expensive - you have to replace or re-assemble the hardware, but were necessary given the state of technology. The designer had to find the best balance between high cost due to small payload, or high cost due to discarding or rebuilding a complete rocket.
The Modern Solution
To reach a better result, the modern approach is to break one or more of the last century's limiting factors. For example:
- You can build large structures that span part of the Earth's gravity well. This reduces the velocity the vehicle needs to reach. Even a small reduction shifts the near-zero payload into positive territory. Past systems have implicitly assumed chemical rockets to do the whole job of reaching orbit, because it was the only type of propulsion considered useful for that job. There is no physical law that demands using one type of propulsion for the entire job, and using multiple methods often gets better total performance.
- There are now a multitude of alternatives to traditional chemical rockets. This Part 2 of the book attempts to list all the known ones, of which there are 83 listed so far, not including variations. Many of them have better performance, thus reducing the fuel required and increasing the net payload.
- Since the late 20th century materials with improved properties have emerged, which allows for longer service life or lighter weight.
Although having more options is more complex, the modern designer should consider the full range of available propulsion methods, and apply them where they function best. The potential gain in going from short life, low payload transport to long life, high payload designs more than justifies the extra work.
Beyond Low Orbit
Low Earth Orbit, or LEO is the altitude range above significant atmosphere and below the Radiation Belts, or about 200 to 1,000 km altitude. The above design challenge was to get from the ground into this orbit range. Travel beyond LEO has not been as constrained to chemical rockets as initial launch. That is because for vertical launch you need to be able to accelerate the whole vehicle at more than one gravity (9.8 m/s2) in order to take off, and maintain a relatively high thrust to prevent hitting the ground again while still below orbital speed. One of the key features of chemical rockets is their very high thrust to mass ratio. This made them attractive despite their low absolute efficiency.
Once in a stable orbit, lower thrust levels can be used to travel further, since you are not in danger of immediate re-entry. Thus alternatives to chemical rockets have been examined for those missions, and some of them even put into use in past decades. There are now more alternatives for going beyond LEO, which give better performance or wider design choices. The larger change, however, is in the first step of getting to LEO, which was entirely by one relatively low performance method until recently.
Transport Method Tables
As a starting point for designers, the following table lists the 83 main space transport methods known to the book's authors. They are detailed in the following pages and are organized into logical categories by type. The order listed here and later in the book is by similarity of type, and not by feasibility or development status. Those factors are considered later. Many of these methods also have variations in the concept or application. From this list, the designer can then narrow the choices to the relevant options for a particular project, and eventually the final selection. By starting with all of them, you can be sure no viable option was missed.
This second table shows the same transport methods only by their numbers, since including the names makes the table too large. They are sorted in two dimensions by (1) the energy source and (2) the method that force is applied. This is an aid to organizing and thinking about them. Some of the theoretical methods are not shown on this table since we do not have definite designs for implementing them. Some of the methods indicated could span more than one box if all the possible variations are considered, but we have assigned them to single boxes in this version. Empty boxes can stimulate thought about whether there are possible transport methods which are still unknown, or whether that particular combination truly lacks possible use.
These tables represent the state of knowledge in mid-2012. There are likely some additional methods not yet known to the authors, and new ones will likely be devised in the future. If a serious project is contemplated, designers should survey the relevant specialists and literature to include the latest concepts.