Category Archives: Rockets

Rockets

Too many people use the word efficiency when they actually mean efficacy. According to Wikipedia, efficiency is:

Efficiency in general, describes the extent to which time, effort or cost is well used for the intended task or purpose.

Whilst efficacy:

Efficacy is the capacity to produce an effect.

From this we can conclude that efficiency is a relative measure, and that efficacy is an absolute measure. The conflation of efficiency and efficacy is in particular frustrating (on-line) discussions regarding space colonization.

Quite obviously, space colonization requires the use of rockets, both to launch people and equipment from Earth, and to transport resources throughout the Solar System. However, it’s clear to anyone who studies rocketry even shallowly, that there is a wide variety of rocket types. And each different type of rocket has each own advantages and disadvantages.

Before we continue, we need to devote a few words to non-rocket space launch. Though there are many proposals for non-space launch systems, none of those have been tested in practice. The proponents of those systems often claim that their proposals will reduce the costs of space launch. This might be true in the long run, but at the short-term we need to take into account the cost of research and development of such systems.

And since it is uncertain whether these proposals will actually work, or when they will be available, it will be hard to find people who want to invest in such launch systems. On the other hand rockets are proven technology, which enables us to start with space colonization quite soon. This also makes it more likely for people to invest in space colonization.

In rocketry efficiency is indicated by the specific impulse I of a rocket, and the thrust T of the rocket. Basically thrust is the product of the specific impulse and the amount of mass exhausted by the rocket. By a given exhaust mass, a higher specific impulse would imply a higher thrust. Hence we should pick the rocket with the highest specific impulse from the catalogue.

Well, if we chose the rocket, we would pick an ion thruster. But is this also the best choice? Not entirely, though ion thrusters have a high specific impulse, they simultaneously have a very low flow of exhaust mass. So low actually, that their overall thrust is low. Because of this, ion thrusters aren’t used for launching spacecrafts from Earth, and is their use limited to deep space. Additionally because of their low thrust, ion thrusters are quite slow.

Although ion thrusters are usually advertised as very efficient, we have to realize that this claim is based on the specific impulse of this type of rockets. However, there are other ways of looking at the efficiency of a rocket. Especially, we should look at the ratio between the energy consumed by the ion thrusters, and the addition of kinetic energy.

Ion thrusters use electricity, generated either by Solar arrays or radio-isotope batteries, to ionize a propellent (usually xenon). The ionization of atoms takes a lot of energy, energy which does not increase the velocity of the spacecraft and hence kinetic energy. Consequently this would decrease the overall efficiency of an ion thruster.

The popularity of xenon as a propellent for ion thrusters, is due to the fact that xenon has a relatively low ionization energy (and hence increasing overall efficiency). However xenon is a rare noble gas, that is present in trace amounts in the Earth’s atmosphere. In order to obtain 1 liter of xenon one has to process 11.5 million liters of air. Hence it’s not hard to imagine that xenon is quite expensive.

For purely scientific space missions these disadvantages of ion thrusters are not that important. A scientific space mission might take several years, if not decades. And scientific mission do not have to return any profit. The humanization of space, however, demands enormous investments, which are only possible if there is a prospect of profit.

What alternative do we have for ion thrusters? An interesting possibility are so-called thermal rockets. Thermal rockets differ from chemical rockets, in that in the former a propellent is heated by an “external” source of energy, instead of a chemical reaction. There are several types of thermal rockets, the most important ones being nuclear and solar thermal rockets.

Nuclear thermal rockets use nuclear reactors to heat the propellent. Given the complexity of reactor technology, and additionally political concerns arising from the launch of nuclear fuels from Earth, we rule out this type of rocket for space colonization in the near future.

Solar thermal rockets (STRs), however, are much simpler. They use solar power to heat up the propellent, and instead of a reactor they need mirrors to concentrate solar heat upon the reactor. For ground launch, however, STRs will not work, so we still need another, most likely chemical rockets, to launch a STR from the ground. But once in space the STR will take over, and bring the spacecraft either to the Near Earth Asteroids or the Sun-Earth Lagrange points.

The main benefit of thermal rockets is that they can be “refueled” with propellent, also in space. Near Earth Objects contain substantial amounts of water which could be used as a propellent. Recall that spacecraft do not need to use their rockets all the time, only to reach the desired speed and to slow down at their destination.

Compared to ion thrusters thermal rockets have a smaller specific impulse, but they have a greater thrust. Consequently they have much shorter travel times than ion thrusters Further thermal rockets do not need to waste energy to ionize their propellent. And finally they don’t require expensive substances as xenon, instead they can use water or ammonia instead.

Why rockets are necessary for Space Colonization

This post was originally posted on blogspot.com on May 11, 2012

In this posting I will argue why we should stick with chemical rockets for our program of space colonization.

Many proponents of space colonization are in favour of several types of non rocket space launch. Most commonly proposed are space elevators and projectile launchers, their advocates favor these because they believe that is launch methods will lower the costs of launching objects into orbit (and beyond). At this moment it costs us$10,000 to us$25,000  per kilogram to launch something into space (Wikipedia, see here) and its argued that the minimum energy required for space launch is much less than with rockets and so it would, in theory, to lower the costs of space launch (the Wikipedia article don’t show any explanation for this claim, so I am somewhat skeptical about it).

Although multiple launch schemes are proposed in the past, however no one, save for chemical rockets, has actually proven to be successful. Yes, there have been a multiple test of several proposed systems, but none of them has ever launched anything in space. Chemical rockets are proven technology, quite easy to build. This will lower our development costs, this is very important if we want to start with space colonization as soon as possible. Although the several proposals for non rocket space launch will work in theory, and they may lower launch costs, we cannot know when these methods are ready to use. Here we can compare this with the research on nuclear fusion power, which has started at same time as nuclear fission, but today there are still no commercial fusion power plants (at this moment they are not expected before the 2050s).

If, say, research will show that rail guns are the most efficient method of space launch, but it will take fifty years before this will be operational, then we should not go for it. Why? Because no one will invest in such project if there is no guarantee that it will work, while in the meantime it will not generate any revenue. Unlike in the case of fusion power research, which is funded by governments, we cannot afford to waste our time with developing speculative technology. Many space advocacy groups have lost their credibility because they were not able to come with realistic plans to get started with space development within the next ten to fifteen years. With realistic plans I mean plans that not rely on governmental funding and have clear ideas about generating revenue for repaying investors. In an upcoming post I will present a plan, which I believe will satisfy my criteria. Instead many “space advocacy” groups are wasting their time and their money with lobbying, not very successful if we look at the miserable state of the US space program. Also in an upcoming post I will argue why space tourism will not helpful for space colonization, this because many space advocates believe that space tourism will act as a catalyst for space development.

Since at this moment chemical rockets are the only proven method of space launch and we do not have the time for develop more speculative technologies, we should stick with chemical rockets. Even if could reduce launch costs, we have to take into account development cost and since we cannot predict the actual amount of time and money spent in research, we cannot afford to take this risk. Not that I am against research, but we should devote most of our funds and time for solving the real issues of space colonization, not for reinventing the wheel over and over again. This is actually what NASA is doing, while the Russians are using the same launch system for decades, with several improvements over time.

Rockets are maybe not the most efficient launch system, but at least they work.