Dear readers, recently the news is filled with more and more messages about plans to send, in the very near future, astronauts to the moon and Mars and even build there inhabited bases. There are many educational programs and competitions for children in which they are invited to develop a project of a habitable base on Mars (for example, NASA’s “Imagine Mars Project”, or Vivify’s “Design a Mars Colony: STEM Project”)[1], and even some of our readers are seriously concerned about the health issues of astronauts during long space flights. We already wrote about the idea of leaving Earth, which suffers from an environmental disaster, and move, for example, to Mars (“Escaping Earth – Is It Our Next Step?”). But in this article, we are talking about something else – about our technological capabilities of making manned flights in deep space (beyond low-Earth orbit (LEO)).
Over the past 50 years, technological progress has been very intense, but not in all areas of knowledge. Microelectronics, information technology, biotechnology, and medicine have developed very rapidly. On the other hand, for example, in the automotive industry, progress is much less impressive. Modern cars are packed with electronics, but the engines are fundamentally not very different from those that were used 50 years ago. In aircraft manufacturing, the situation is even more interesting. On the one hand, modern electronics, avionics (radars) and software (apropos progress in microelectronics and information technologies) are widely used in modern airplanes, but the engines are the same, the design of the airframes has not changed since the 70s of the last century, and there is even a step back. Civil supersonic aircraft, such as Concord and Tu-144, began commercial flights in the 70s but were retired at the end of the 20th century. The main reason for the retirement: The high cost of flights [2]. A similar story with the fastest ever Lockheed SR-71 Blackbird military aircraft. The first flight was in the late 60s, and retired in the late 90s, also, mainly due to the high cost of operation [3]. No replacement or new generation of ultra-fast passenger and military aircraft has not yet been proposed, so in this sense, progress is negative.
The situation with progress in the field of astronautics is even more confusing. On the one hand, in recent years, thousands of communications satellites, navigation satellites, and other commercial and scientific satellites have been put into LEO. Moon- and Mars-rovers move along the Moon and Mars, and unmanned spacecraft launched to the Jupiter moons and the Sun.
And on the other hand, the farthest manned flight today (to the moon under the Apollo project) was in 1972. The most powerful Saturn V rocket, capable of delivering a payload of 140 tons into LEO, was retired in 1973. The next most powerful launch vehicle, which was able to deliver into LEO 27.5 tons of payload, the Space Shuttle, retired in 2011. To date, there is not a single operational launch system of the “Super Heavy-Lift” class (that is, capable of bringing more than 50 tons of payload into LEO) [4].
And the reason for this is… the engines! Today, like fifty years ago, chemical rocket engines are used to launch satellites into space. They are powerful and relatively reliable, but they have a fundamental limitation – 90% (!) of the starting mass of such rocket is occupied by fuel, about 5% of the mass is engines and the body of the rocket, and only 5% of the mass is the payload. Moreover, most often both the launch vehicle and the spacecraft are expansible. All this leads to the enormous cost of launching a useful payload into space (from $4,000 to $20,000 per kilogram) on the one hand, and the enormous complexity of the launching large masses into orbit (such as an interplanetary spacecraft with a large amount of fuel needed for space travel).
In recent years, a lot of attempts (for a while unsuccessful) have been made to reduce the cost of launching a payload into orbit. For example, attempts to make reusable expensive launch vehicles and spacecraft. In the Space Shuttle program, for example, both the spacecraft (orbiter) and two solid-fuel boosters were reusable. Only the external fuel tank was expansible (orange in the illustration).
However, this did not help to reduce the launch cost (~250M$) due to the high cost of maintenance between the flights (for comparison, launching an expansible Soyuz with approximately the same payload cost half the price). SpaceX founder Elon Musk claims that by utilizing the “innovative” idea of reusable stages of his Falcon 9 launch vehicle and its Falcon Heavy modification, it will be possible to significantly reduce the launch cost, but so far these are just unconfirmed promises (Falcon Heavy launch in 2018 with a virtual load in the form of Tesla Motors electric car cost about $1.2 billion).
To understand what prospects we have, let’s try to figure out what engines we have today, and maybe what we will have tomorrow. Today we use three types of engines for space flights: solid rocket engines (usually used as boosters at launch), liquid-propellant rocket engines (used at all stages), and ion engines.
Each one of these engines has some fundamental limitations. Solid-fuel rockets have great thrust (the force pushing the rocket in the direction of travel), but a very small specific impulse (an indicator of the efficiency of fuel consumption – the greater the specific impulse, the less fuel must be spent to get a certain amount of acceleration). Besides, after the ignition of a solid-fuel rocket, the process cannot be stopped, it runs continuously until all fuel has burned (usually 3-4 minutes for the largest rockets). Liquid-propellant rockets also have a large thrust (but less than the solid-fuel ones), and a small specific impulse (but more than solid-fuel ones). The liquid-fuel engine can be reignited several times.
A low specific impulse leads to the fact that in all chemical rockets (solid and liquid) most of the starting mass (~90%) is the fuel that is necessary to bring the payload (<5%) to low Earth orbit. Ion engines are very efficient (large specific impulse), but they have a very limited thrust – millions of times less than chemical engines. Therefore (and not only) it is impossible to launch a spacecraft into orbit with the help of ion engines, and they are used as auxiliary engines for correcting of satellite’s orbit. Even starting from outer space, such an engine is too weak to deliver a mass of more than several hundred kilograms, for example, to Mars in a reasonable time – such a flight will take hundreds of years.
In addition to the above-discussed rockets, the Internet is full of active discussions on (past 20 years) Solar sails, as an ultimate solution to the fuel problem. Let’s check it out together.
A solar sail is a huge sail made of a very lightweight and light-reflecting material that can be deployed in outer space so that it propels a spaceship, as an ordinary sailing ship does, but instead of wind, it uses sunlight (very intense in open space). The idea, of course, is not very new – it was first proposed by Johannes Kepler in 1610. But, in general, the idea is brilliant – you do not need fuel (the specific impulse is practically infinite), but what should be the size of such a sail to give significant thrust? It turns out that to give enough acceleration to a ship several hundred kilograms (the size of a small automatic satellite), the solar sail should be a square with a side of several hundred kilometers! And how to deliver such a huge sail into orbit? Not to mention the fact that such a sail in outer space receives constant acceleration from the solar wind, and, potentially, can gain tremendous speed. But how to slow down to enter an orbit, for example, of Mars? In the case of rocket engines, the answer is simple: halfway – with constant acceleration, and the second half of the way – with constant deceleration (the question is where to get so much fuel?). But in the case of the solar sail – braking is not possible. To date, the only demonstrator of this principle (the IKARUS project, Interplanetary Kite-craft Accelerated by Radiation Of the Sun, launched by the Japan Space Agency in 2010) was 40m x 40m, and after 5 years of flying to Venus was able to increase its speed by about 400m/s) a little more than the speed of sound – a tiny speed in the scale of space travel). Despite the technologically perfect construction of this sail – the thickness of a sail made of polyimide (PI) is only ~7.5µm (for comparison, the thickness of a human hair is ~70µm) with built-in thin-film liquid crystals and solar panels – its propulsion characteristics are more than modest – the thrust of 0.012 [5] Newtons (with up to 12,000,000 Newtons for a rocket engine).
There are also proposals and ideas regarding how to increase the thrust of solar sails, as well as to adapt them to distant (interstellar) flights (the farther from the Sun, the lower the pressure of sunlight on the sail). To achieve this, it was proposed, for example, to use heavy-duty LASERs or microwave emitters located on Earth, and to push forward the solar sail with their concentrated radiation. As an example, in 2016, the “Breakthrough Starshot” project was launched, in which it is planned to launch a small satellite (several kilograms) to Proxima Centauri (the closest star to the Sun). The main problem is that in order to accelerate a solar sail with a tiny satellite, one will need a phased array of tens of thousands of LASERs (all LASERs emits radiation with the same phase), with a total power of 100 Gigawatts [6] (!). For comparison, a powerful nuclear power plant produces about 5 Gigawatts. That is, it will need 20 powerful nuclear power plants to accelerate such a sail!
Thus, while using technologies that are available today, it is theoretically possible to send astronauts to the Moon or Mars, but it’s very difficult (de facto today there are no operational super-heavy launch vehicles), and very expensive – estimated cost of manned Mars mission is at least $500 billion [7]. By the way, the economical aspect of space exploration is very important, and it is the main factor in the cancelation of space projects. This was the official reason for the cancelation of the Apollo project in 1972, after four successful manned flights to the moon, and the reason for the cancelation of the tremendous “Space Exploration Initiative” [8] project in 1993, in which it was planned to establishing an inhabited base on the moon and manned landing mission on Mars (estimated cost of the project was about 500 billion dollars for 30-50 years, and it was rated as too expensive even for a global international project). In 2009, the “Constellation Program” was canceled, with the goal of sending astronauts to the Moon and Mars, also because of its high cost. In 2017, a similar (not yet canceled) “Artemis Program” started. Since this program uses the same technologies as in previous programs (small modifications of the rocket engines of the Apollo and Space Shuttle projects) [9], its cost is likely be similar to the cost of previous programs (hundreds of billions of dollars), which in the past was the reason for the cancellation of all Lunar and Mars programs. And those are just a few examples of canceled Mars programs over the past 50 years.
It is interesting (and very sad) to note that all developments of significantly more efficient engines, which could make manned space flights beyond low Earth orbit more realistic, were canceled many years ago due to the high cost and long development time.
For example, the “Orion Project” (Nuclear pulse propulsion) was canceled in 1965. The idea was to throw a small nuclear charge behind the ship, and the explosions of these nuclear bombs will push the ship forward very effectively. Such an engine possessed tremendous thrust and a very large specific impulse. It is noteworthy that all the technologies for the implementation of this project already existed at the time, and already in the 60s and the 70s we were able to create a spacecraft capable of flying to Mars and back in 175 days (!) with eight cosmonauts and 100 tons of cargo [10].
The NERVA (Nuclear thermal rocket engine) project was canceled in 1973 after more than 20 years of development. The idea is to pass hydrogen through a nuclear reactor, which heats it to a very high temperature and creates thrust. The thrust of such an engine is slightly larger than that of a chemical engine, but the specific impulse (efficiency) is three times bigger. NERVA demonstrated that the nuclear rocket engine is fully operational and suitable for space exploration, and at the end of 1968, SNPO (Space Nuclear Propulsion Office) confirmed that the latest NERVA modification, NRX/XE, meets the requirements for a manned mission to Mars. Although NERVA engines were built and tested as much as possible and were considered ready for installation on a spacecraft, most of the US space program was canceled by the administration of President Nixon.
Taking into account all the above, a question arises: Why there is so much excitement in the media and social networks about an upcoming men’s flight to the Moon and Mars? At the current state of space technology, in order to send astronauts to Mars, we (mankind) must abandon wars (for example, the annual defense budget of the United States alone is ~600 billion dollars). But to my great regret, we, I think, are not ready for this step yet.
[1] https://mars.nasa.gov/imagine/students/
[2] https://www.thetravelinsider.info/2003/0411.htm
[3] Graham, Richard (7 July 1996). SR-71 Revealed: The Inside Story. Zenith Press. ISBN 978-0760301227.
[4] “Seeking a Human Spaceflight Program Worthy of a Great Nation” (PDF). Review of U.S. Human Spaceflight
[5] http://www.jaxa.jp/press/2010/07/20100709_ikaros_j.html
[6] “Breakthrough Initiatives”. breakthroughinitiatives.org. Retrieved 25 December 2017.
[7] Taylor, Fredric (2010). The Scientific Exploration of Mars. Cambridge: Cambridge University Press. p. 306. ISBN 978-0-521-82956-4.
[8] Steve Dick. “Summary of Space Exploration Initiative”. NASA.
[9] Chris Bergin (October 4, 2011). “SLS trades lean towards opening with four RS-25s on the core stage”. NASASpaceFlight.com. Retrieved January 26, 2012.
[10] G.R. Schmidt; J.A. Bunornetti; P.J. Morton. Nuclear Pulse Propulsion – Orion and Beyond (PDF). 36th AIAA / ASME / SAE / ASEE Joint Propulsion Conference & Exhibit, Huntsville, Alabama, 16–19 July 2000. AlAA 2000-3856
Featured image by 272447 from Pixabay.