The Future of Space Flight

B.W.Wojciechowski, May 2017

Introductory Remarks

Of recent we have been intrigued by news that several rocket building enterprises and government agencies are contemplating a manned expedition to Mars. This can be done, but the problems with and the future of such an effort need to be better understood by the public. Below I will describe what we have achieved, what can be done using available technology, and what has to be done if we want to venture beyond our solar system.

As the twenty-first century dawned our genuine space age had begun. We have achieved space flight and explored much of our solar system, both by photography and by actual landings of men and robots on bodies circling our Sun. We found no aliens on nearby planets, not even little ones, but we made our first baby-steps towards becoming space farers.

Rocket science has made amazing progress as we enter the future and contemplate what to do with the new-found worlds we have accessed. We have arrived at this point after protracted efforts to reach ever more distant places as fast as possible. However, human expeditions into the unknown always entail great risk. This will not change; future space voyages using the technology presently available will be costly and risky. We will no doubt send a manned expedition to Mars in the near future, but it will be difficult to support a robust follow-up using our current technology.

Mankind Takes Flight

Late in the 1800s hot-air balloons became a popular method of leaving the ground and soaring through the air. Although this method of flight had some early military applications it was a mainly used for recreational purposes. It did not immediately become a means of transport because it was not easy to control, and it was costly to operate, considering its relative small payload capacity.

In the early 1900s, soon after the automobile appeared for ground transportation, air transportation was revolutionized by airplanes, which in the span of less than a hundred years has become the preferred mode of long distance travel, over both land and water. The earliest aircraft were dangerous and unwieldy contraptions, limited in range and speed. But progress in aircraft technology proceeded in great strides, improving comfort, safety, range, and payload capacity. We humans became obsessed with efficient time-utilization, and quickly took to the three-dimensional freedom and speed of air travel for both passengers and freight. However, the maximum speed of these flying machines was limited for a long time by the “Sound Barrier.” And we, as always, wanted to go faster.

This barrier presented a technical, environmental and economic roadblock. It was initially thought to be impenetrable, but was eventually overcome by military aircraft where cost and the ecology were not debilitating considerations. The knowledge that the sound barrier could be overcome gave impetus to technological progress in civilian air transport. Unfortunately passenger transport at supersonic speeds continues to be stymied by ecological and economic constraints. Today, at the beginning of the twenty-first century, we are still without supersonic civil aviation, despite the brave but uneconomic attempt by the late twentieth-century Concorde.

Now however, we are poised on the cusp of initiating supersonic commercial rocket flight into Earth orbit. Some think that rocket-powered commercial point-to-point ballistic Earth transport will be realized during this century. But the technology is still in its early stages, and it remains a very uninviting mode of routine transportation.

Isaac Newton: Precursor of Rocket Science.

Rockets are an ancient form of propulsion. In times gone by they were primarily employed in warfare and in entertainment. Rockets are the most ancient form of heat engines, powered by combustible fuels and used to carry payloads. The beginnings of rocket power were simple: a fast-burning material was packed into a tube, with the exhaust of the combustion products being vented through one end of the tube. At the other end, the payload depended on the rocket’s purpose, but the basic principle of propulsion stayed the same.

As time went on, rocket propulsion became better understood, and it came under the purview of advancing science and technology. But even before rocket science really took off, Newton wrote the fundamental equations quantifying the basic principles of the process generating the force used for lifting and accelerating a rocket:

Force = mass x acceleration

F = meae

This is Newton’s second law. It tells us that the force which lifts the rocket from its launch pad is provided by accelerating (ae) the mass of exhaust gases (me) derived from the combustion of fuel and oxidizer in the rocket tanks. In a rocket, the mass of propellants is contained as a relatively small volume of stationary liquids or solids. This mass of propellants is ignited, burns rapidly, and exits from the combustion chamber as a huge volume of hot combustion products accelerated to great velocity.

Once the rocket has lifted from the launch pad by the force described by Newton’s second law, its behavior can be understood in terms of Newton’s third law:

For every action there is an equal and opposite reaction:

action = reaction

or in terms of the quantities involved

meve = mrvr

where the subscripts e and r stand for the gas exhaust of the rocket engines and for the rocket itself, while m and v stand for the mass and velocity of each.

In the case of rocket propulsion the process of expelling a relatively small mass (me) of high speed (ve) exhaust gases out of the back of the rocket is the “action.” It comes from:

Flow of the mass of exhaust gasses times the velocity of the gases

action = meve

The action in turn causes the much-more massive rocket and its payload (mr) to move (vr) in the opposite direction as a “reaction,” which comes in the form of;

Propulsion of the mass of the rocket times the velocity of the rocket

reaction = mrvr

Although the mass of the gases the rocket exhausts (me) per unit of time and the velocity of the exhaust (ve) both stay (fairly) constant, this action causes the decreasing mass of the rocket (mr) (the rocket becomes lighter as propellant is used up) to react by gaining speed (vr). Since the rocket velocity increases (dvr) with time (dt), the rocket accelerates:

(dvr / dt) = ar

This causes the G force on the payload to increase until the rocket fuel is exhausted.

After the propellant is exhausted and combustion stops, Newton’s first law takes hold, and as it predicts, the rocket proceeds at constant speed in a straight line, modified by aerodynamic, gravitational, and other forces that affect its trajectory. As an example, after a rocket stops firing a satellite proceeds at constant speed and in a straight line bent into an orbit by the force of gravity.

Engineering improvements in the application of these basic principles has concentrated on more-efficient exit nozzles, increased combustion temperatures, and enhancement of volume increase due to combustion by using advantageous fuels and higher temperatures and pressures; in general, ways of increasing the volume and the acceleration of the exiting mass of gases in order to increase the force available to propel the rocket. The pursuit of this simple goal has resulted in an enormous increase in the complexity of the rocket engine. It went from a straight tube exhausting hot gases to a truly complex device containing sophisticated materials, complex mechanisms, ingenious designs and a variety of fuels.

Nonetheless, a rocket is a heat engine which obeys Newton’s three laws, unlike some other heat engines which are best understood in terms of the more complicated concepts of thermodynamics; they function according to any of several thermodynamic “cycles,” such as the Otto cycle used in gasoline-burning engines. As a result, there are a number of heat engines which differ greatly in their design. In contrast, there is only one principle of rocket design: the laws of Newton.

The optimum application of Newtonian laws in practice is perhaps simpler than optimizing thermodynamic cycles, but it too involves the efforts of many disciplines. Materials science designs materials used where high temperatures and pressures are present, and where light but strong structural materials are needed to reduce the weight of the rocket body and increase payload capacity. Mechanical engineers design pumps and structures capable of withstanding extreme temperatures and pressures. Chemical engineers and chemists develop optimal fuels for various kinds of rocket motors and so on. Modern rocket design is indeed “rocket science,” bringing design of rocket engines to the very edge of our current technological capabilities. That said, the transport of humans using chemical rockets is still risky and uncomfortable and nowhere near being ready for routine availability to the average traveler. Travel by rocket remains the preserve of well-funded organizations and courageous individuals.

Current Rocket Configuration

Since rockets are normally propelled by exhaust from behind the payload and have no wings, the customary configuration of a large rocket is that of a slim vertical cylinder which minimizes the resistance of air to flight. The same configuration applies to aircraft, but the presence of wings allows airplanes to take off and fly with the cylinder in a horizontal position, greatly facilitating passenger-carrying requirements. In both cases, the resistance of air is minimized by presenting a minimal area to forward motion. Unfortunately rockets lifting freight or human payloads involve considerations that dictate a vertical tubular configuration.

A twenty-first century rocket heading for Earth orbit with a human crew can only deliver a puny 4% of its takeoff weight to its destination. A cargo aircraft, on the other hand, can deliver some 50% of its takeoff weight. The rest of the rocket’s weight consists of fuel as well as structure to contain the fuel and to mount the payload and the engines that bracket the stack. Most of this expensive rocket gadgetry is used only once and falls back to Earth as junk. This makes human flight to orbit very expensive, more so than the economics of routine passenger transport will allow. Work-arounds have been investigated, notably lifting the payload and a smaller rocket by airplane to a launching above much of the atmosphere, but the costs and inconveniences for passenger transport remain prohibitive.

Lowering costs by landing a used rocket in order to recover the engines and fuel tankage is a difficult task. It requires that extra fuel be lifted to near orbital altitude so that the returning rocket can be slowed down to a safe landing. The landing itself requires that a slim column, the returning rocket, land in a vertical configuration without toppling over. It has been done, but it is not easy. More importantly, it still does not make rocket trips look attractive as a means of transport from surface-to-surface-point on Earth. The only foreseeable use of manned rockets is to transport passengers or freight off the Earth into space.

Passenger comfort is a serious problem. Rocket propulsion is extremely noisy and the required acceleration on takeoff and the subsequent deceleration for landing impose significant G forces on the passengers. Moreover, several factors involving fluid dynamics, air currents, combustion and exhaust instabilities can combine to induce a great deal of vibration as the craft takes off. None of this would be an attractive prospect for the general public.

Another problem of rocket transport of passengers is that commercial transport requires large numbers of passengers to be economically feasible. Even if we were to imagine a giant passenger rocket which would accommodate as many as 100 passengers, the question of loading and locating the passengers on board is complicated. The diameter of the rocket may allow no more than ten crowded passengers per level, so ten separate levels with separate entrances up and down the column of the passenger spaces would be required. Onboard services, even washrooms, would be difficult to provide, as any circulation of crew or passengers to gain access to facilities would require waste volume in the passenger compartments at each level.

All in all, the prospect of chemical rockets of the kind we are presently capable of constructing ever becoming a routine means of transport on Earth is dim, as is the transport of significant numbers of people as far as the Moon or Mars. A Herculean technical and financial effort might be able to sustain an outpost of several dozen highly-trained and courageous people on the Moon, but unless and until the inhabitants acquire the means to supply their outpost with essential and often bulky consumables such as food, water, energy and so on from on-site resources, that would be the limit.

But this expense and effort might be justified. I personally believe there is an adequate and accessible supply of water on the Moon so that, at great cost and slowly, a permanent and self-sustaining outpost could be established there. What would justify such an expensive venture? It would provide a stepping stone for more-economical transport for the colonization of Mars, with a low-gravity refueling port. Mars offers a more attractive destination than the Moon for exploitation and for a human settlement of significant size.

Spaceship Design for Human Passengers

To make even nearby space travel for commercial purposes possible, the layout of the craft needs to allow passengers movement and access to facilities such as sleeping accommodation, toilets, recreational spaces, food distribution and so on. Even journeys to the Moon will take much more time than the longest current air routes, and will call for quarters comparable at least to those in first class on modern aircraft. Passenger transports will therefore require much more structural mass per passenger than the minimalist accommodations presently provided for the transport of astronauts to and from the space station.

The ideal configuration of a passenger-carrying rocket would be a disk-shaped structure; this would allow easy boarding into a spacious cabin. The disk would still take off straight up from its horizontal position as it rests at the embarkation point. This would present a huge area for aerodynamic resistance during liftoff and transit through the atmosphere, requiring much more power per passenger than the slim pointed-stick rockets of today. A large disk would also present structural challenges much greater than the columnar configuration of todays manned rockets. The engines, for example, would have to be many, and distributed over the lower surface of the disk in such a way that the propulsion stresses would be distributed over a maximum number of stress points in the lower disk structure.

To make the takeoff tolerable to average travelers, we should not impose more that 2G during takeoff and settle to 1G as soon as possible, to allow easy passenger circulation during the long flight. Unfortunately maintaining 2G during liftoff and 1G thereafter is an inefficient procedure for most rocket engines. A passenger rocket with those capabilities would require a great deal of fuel, even on trips to our relatively-nearby Moon. On arrival, the vessel would have to decelerate for the landing, again keeping G-forces to a tolerable level and burning more fuel. The return flight would require additional fuel for takeoff from the Moon and landing on Earth, making the initial fuel load very large for a return trip using a chemically-powered vessel with no way of refueling enroute or on the Moon.

The fuel problem could be greatly reduced if there is water on the Moon and the rocket could be refueled there for the return flight. However, this would require a complex infrastructure in order to produce the hydrogen/oxygen propellants, store them, and keep them at the required conditions of temperature and pressure, as well as to house the spaceport crew and visitors and their support equipment. Robots should be used as much as possible to minimize the need for human operators.

An electrolysis plant could produce the hydrogen and oxygen to fuel the rockets for the return journey; tankage, pumping facilities and water “mining” would also have to be put in place. A space-adapted nuclear power plant would be needed to provide energy for life support and for the electrolysis plant and other facilities. All this would have to be provided before a bunkering port could be made operational. And all the personnel, materials and equipment would have to be delivered to the Moon using our currently-available rockets.

An expensive and protracted effort using reusable rockets with their less-than-4% payloads would have to be organized to create this facility. Once built, however, such a lunar colony would be capable of producing some of its own essential supplies, as well rocket fuel to create additional space for useful payloads from Earth. More importantly, it would be an ideal port for fueling and launching trips to Mars by much bigger space ships. They would benefit from the Moon’s low G and the lack of atmosphere. We might even consider a space port at one of the Moon/Earth Lagrangian points.

Energy is Critical to Extended Space Travel

Throughout human history, step changes in all of our technological progress have depended on access to ever more energy. Muscle power could only take us so far. Heat engines involving various fuels took us much further and faster. A plethora of fuels starting with wood, coal, various hydrocarbon liquids and gases, and finally nuclear reactions, have supplied the heat required to power heat engines. As energy became available and its distribution became important we mastered electricity which could be generated at remote locations from various sources including heat engines and transferred to distant users.

Heat and solar energy in various forms, usually transferred by electrical wires, have been the mainstays of our energy supplies to date. But it was the energy of heat engines in the form of rockets, not electricity that gained us a foothold in space. This technology gave us a taste for space adventures and we now want to go there in person and en masse. We dream of colonizing the Moon and Mars. We may even want to terraform Mars! All this will take much more energy than that available from chemical fuels and even from the more universally-available nuclear resources, which I expect to be available elsewhere on rocky bodies in the solar system.

But the availability of fuel is not the only problem holding back our space venturing. Chemical rockets, even if powered by nuclear heat or electrical fields, are dependent on Newton’s laws which require that mass be ejected from the vehicle in order to make it move. That means we have to load the vessel with mass that will be ejected on our trip, and with additional mass needed for the return journey. The mass of propellant required by Newtonian devices guarantees that humans will be stuck in the solar system until we come up with a different way of powering space vessels.

Local water sources at destinations in our solar system, combined with nuclear power plants and electrolysis facilities, would increase the possibility of visiting solar planets by providing refueling stations off Earth, but would not remove the constraints of chemical rockets which make interstellar flight impossible.

New Energy Sources?

To get around Newton’s laws and acquire a true space-faring capability, we need a new source of energy and engines designed to use it. The era of heat engines has run its course. Even electric-powered rocket engines require mass to be ejected when used to propel a Newtonian rocket. We need a breakthrough in energy sources and engines if we are to conquer space travel and undertake planetary colonization.

What are the requirements of such an energy source? Here are some

· To make space travel beyond solar planets or establish viable colonies on solar planets we need a source of energy that is readily available throughout space, wherever we may wander; something like the wind which is globally available on Earth and which made possible extensive voyaging by large numbers of people in the days of sailing.

· We must master the use of new energy sources by developing new engine designs. Just as heat engines involving pistons and turbines made heat energy usable, the new energy sources will require devices which can convert the new energy into motion.

· The new energy must be plentiful in space, so that there will be no chance of running out of power wherever we may be. It must also be a more concentrated form of energy than that available from heat sources fueled by consumables.

· The new energy must not require a large infrastructure to produce. Enough of this energy must be readily available for the takeoff of a space ship wherever it may have landed.

· The energy source and the engines using it must be durable and safe for long-term operation in the proximity of human passengers. It must be free of radiation and other hazards and be usable without complex machinery.

· The propulsors used to generate motion will have to be essentially indestructible. Journeys to other solar systems will require propulsion and life-support functions to operate faultlessly throughout the duration of many-year-long journeys.

· The engines must be able to accelerate smoothly from zero to full power. On long trips the power of the engines will have to be constantly and smoothly adjusted in order to maintain a constant environment on board. Step changes familiar to automobile drivers as “gear shifts” will not do.

This kind of energy must be accessed and utilized, if we are to venture beyond the solar system. Looking at the broad aspect of energy issue in our civilization, we can see that we have progressed from operating on muscle power, to water and wind power, to the use of chemical energy sources, to nuclear energy sources. But now we need to access some unknown sub-nuclear or even non-nuclear energy source. In adopting ever more energy-intensive technologies, we have been reaching for energy sources dependent on a successively more sophisticated understanding of nature.

A power source that meets these requirements for space travel could possibly come from the universally-present Quantum Fluctuations, Zero Point Energy, the Casimir Effect, or even the recently discovered Dark Energy which is present throughout the universe in great abundance.

Navigation Using EK Power

Let us assume that the proposed universally available energy is captured and engines using it for propulsion and power generation are constructed. How would we use this new power? We begin with the replacement of chemical rockets by “propulsors” using this new power source which I call the “EK” drive.

The EK drives will have to generate motion by non-Newtonian means. They will have to be more like maglev propulsors, which move the vehicle by accessing energy from an off-board source and propel it on a wave of potential. Since we will not be building rail systems in space, space itself will have to be the rail. The propulsors will have to pull/push the vessel through space by creating a force which will draw or push the craft forward.

As we have already mentioned, a space-faring vessel needs to be as flat and as wide as possible. In space faring, this would afford interior space and vistas for travelers to enjoy and minimize the need to change deck levels. The configuration will favor a flat disk rather than a cylinder. The disk may well have multiple levels, making it a short cylinder whose height would be dictated by its function as a domicile for humans rather than by aerodynamic considerations. For example, living quarters may be on one or more levels, while engines and service areas would be on other decks. The layout would be more like that of a passenger ship rather than a high-rise apartment building.

Such a ship would take off vertically at a low acceleration powered by the propulsors. Passengers would have a brief period of between 1G and 2G during the takeoff from Earth and could enjoy 1G acceleration along the vertical axis of the disk throughout the rest of the journey. Fuel would not be a concern and the constant acceleration would afford familiar 1G gravity as the vessel journeys, always keeping the G vector perpendicular to the deck floors. In principle such constant acceleration would in due course result an unlimited final speed but, as we will see there is a limit. However, near-luminal velocities could be approached and so the duration of journeys to solar bodies and beyond would be greatly reduced.

As the ship approaches its destination it would turn around, using a simple half-orbit maneuver that would not disturb the constant 1G environment, and decelerate at a constant 1G until it lands at the destination spaceport. Since there will be no exhaust ejected by the EK propulsors, space ports could be located close to population centers without creating pollution or noise.

Interstellar ships will be very large, several kilometers in diameter, so that they will in fact never land. Lack of sufficiently-flat landing areas and gravitational stresses on the structure will make this impossible. The interstellar ships will be built in orbit and will orbit their destination planets, while smaller EK-powered tenders will ferry passengers and equipment to the surface.

All energy requirements on board the vessel would be provided by EK-powered devices which would control the environment and provide power for the recycling of waste products in order to minimize the volume of supplies which otherwise would have to be loaded at takeoff. On long voyages, natural gardens and synthetic food producing facilities would be operated by the same unlimited power propelling the vessel.

Relativity Presents the Next Speed Limit

On interstellar voyages lasting many years, the ship would approach luminal velocity after protracted acceleration at 1G. However, somewhere above 80% of the speed of light, relativistic effects would begin to rapidly increase the relativistic mass of the vessel and all it contains according to the equation:

m = m0/((1 — v2/c2))1/2

where m is the relativistic mass, m0 is the rest mass of the vessel, v is the speed of the vessel and c is the speed of light.

At 0.8c the relativistic mass of the vessel would be 1.7 times its rest mass and at 0.9c, 2.3 times. The cost-effectiveness of building a vessel that could stand the stresses of even larger relativistic mass I will leave to future engineers. For now, let us consider 0.8c to be the maximum design speed. It would take about 1 year to reach that speed at constant 1G acceleration. However, the distance travelled during the period of acceleration would be only 0.1 of the distance to the nearest star. Continuing to accelerate at 1G would at some point overstress the structure of the increasingly massive vessel.

This is because the strength of the materials of construction would not increase, while the forces pushing the vessel along to maintain the 1G environment would, and the structure of the vessel would fail if a high enough acceleration continued. Moreover, the travelers themselves would have increased their departure weight by 70% percent. Clearly at some speed the constant 1G acceleration would have to cease or be reduced.

However, termination of acceleration would cause discomfort to the travelers if the sense of gravity were lost and a zero gravity environment established for the greater part of the journey. The period of zero gravity at 0.8c on a trip to Alpha Centauri would last more than 4 years (our time, relativity will shorten this for the crew on board by a factor of 1.7) between the end of forward acceleration to reach 0.8c and the beginning of deceleration as the ship approached the destination. In that time, voyagers would suffer serious problems in the absence of gravity. A means of maintaining 1G or less to accommodate the increased relativistic mass of the travelers has to be devised.

Fortunately there are two simple and effective solutions available. Considering that unlimited energy would continue to be available for propulsion, it would be simple to put the vessel into a spiral trajectory around the vector of its forward motion. This would require only a small force on the structure and generate a centrifugal force on the vessel as it orbits its vector of forward progress at its constant 0.8c speed. If the vessel is oriented with its “floor” to the outside of the spiral, a sense of gravity will continue to be maintained on board without further acceleration and therefore without a further increase in relativistic mass and stress on the ship’s structure.

This maneuver could be maintained as long as needed, keeping the desired constant G force on board for the duration of the journey. Let us call the maneuver the “BW Gyre” or BWG. The radius of the BWG could be made as large or small as required for purposes of navigation around dangerous regions or to explore objects lying along the way but off the direct route, always maintaining the desired G on board. All one would need to do is adjust angular velocity and orbital radius of the BWG to achieve the desired orbit without departing from the direction of forward motion or the selected G environment on board.

The other solution would be to maintain acceleration at a constantly diminishing rate so that the G forces on vessel and passengers remain constant at the original design value. This could in principle allow vessel speed to continue to approach c without deviating from its course while reducing the time of travel due to the increasing speed. The navigators on board would have two options; they could use the BWG to explore more space on their journey or to avoid hazards, or take the direct course at increased speed to reduce the time of the journey.

Conclusion

Despite our inordinate pride in current rocket technology, it will not get us far: we remain constrained by Newton’s laws. We may be able to place a manned outpost on the Moon, but Mars would require an unsustainable effort using present technology. We should rather concentrate on robotic exploration of our solar system. The capabilities of our robots will continue to improve and they will be able to provide us with a detailed picture of our surroundings in nearby space.

Meanwhile, we can dig deeper into quantum mechanics until we find how to capture the vast energy of the very small and/or the yet unknown. Then we will build machines which will take us to the next level of technological development; into our era as a space faring civilization.

For more ideas on space faring, see my Zamora Texts, available on Amazon.com:.

Space Exploration: at http://amzn.to/1nLSisM

Chronicle of Martian Colonization: Terraforming a Planet: at http://amzn.to/1RjnE8f

and: The Region of Luna: at http://amzn.to/1pm6hGX