The problem with space travel is cost. For frequent travel, the price of reaching low earth orbit from the surface of the earth must drop by a factor of 100.
As a practical and safe matter, this means using air augmented rockets. With such rockets, oxygen is taken from the air for the first part of the trip. Because the rocket does not have to carry all its own oxidizer, the effective specific impulse doubles.
In October 2003, China became the third country to launch people into orbit. It, too, chose the same expensive method for going into space as the US and the Soviet Union. To some extent, this makes sense as it is cheapest method. It is a follow up of the 1930s German experiments in `long range artillery without the gun barrel', and is known to work. Moreover, launching a human into space is difficult.
Unfortunately, the cost of this method is always high. It is expensive to throw away a precision instrument, the rocket, after one use; and the energy densities of chemicals mean that rockets will carry small payloads.
The military do not mind the cost, since their prime goal is to build a device that can destroy an enemy city. One rocket is cheaper than 1000 manned bombers, as were used for city raids in World War II. Over Japan in World War II, the US used flights of 500 to 1000 manned bombers to destroy 62 cities and two flights of one bomber each to destroy two more cities, using nuclear weapons.
Indeed, I suspect that countries that have developed traditional long range rockets want to keep them expensive. The governments think of rockets primarily as a form of nuclear artillery and, secondarily, as a way to loft spy satellites into orbit. They do not want the equivalent of second-hand bombers being purchased by less rich foreign nations. If rocket flights were cheap, many rockets would be built. Eventually, they would be sold. There is no difference between a civilian freight and passenger carrying rocket and a military one. In both situations, the purpose is to carry mass into orbit. The mass could be civilian passengers or a re-entry vehicle with a warhead.
At the moment, space flight is expensive and has few users:
Sadly, the current demand for space flight will not much increase even if the cost to carry a ton into orbit is halved or quartered.
If the cost comes down to a level that people and ordinary businesses can afford, then we will see a huge increase in demand — whole new industries will be invented or existing industries changed. But not until then.
Unfortunately, the major US and foreign companies in the space business have no incentive to reduce costs dramatically: to do so would also reduce their profits dramatically. Not only that, such a cost reduction would require they abandon their current more or less predictable future for one that is full of organizational unknowns.
The companies do have an incentive to keep track of possible cost cutting technologies, in case someone else introduces them. Hence, the various `advanced' research projects you can read about. Also, these projects make for good PR. However, unless the alternative is to lose their current business, the companies have no reason to institute programs that would reduce their current profits and not be predictable by current `good business' criteria.
In addition, as an organization, NASA has no incentive to cut launch costs radically. For one, NASA employees can clearly foresee both their future and that of their organization when the current methods are followed. Moreover, much NASA development is actually done by companies and some think of the agency as a mechanism to provide corporations with disguised welfare. (Scientists, engineers, and such like people think differently; but they don't count bureaucratically. They are useful for creating things that produce good PR, like the Hubble space telescope, and the current unmanned landing on Mars.)
Worse, the US and other spacefaring governments can clearly see the military danger of relatively inexpensive earth to orbit travel: an enemy country could launch several dozen space craft that appear to be normal and civilian. They will cross over the US; that is how orbits work. An enemy military could arrange that all cross the US as the same time, apparently accidentally. If they carry bombs, they could launch them with almost no warning. Large weapons could be detonated in orbit, not giving any warning at all. (It is for this reason that I expect that the US and other countries will insist on an inspection regime.)
As for inexpensive earth to orbit travel: there are two obvious methods:
The initial US research in the 1960s did not do so well (rocket engines crumbled) but eventually tests lasted "until the hydrogen ran out". Given its mass, one kind of nuclear thermal rocket engine produced too low a thrust to lift off the planet; but other kinds had thrust-to-mass ratios of 30 to 1. They could be used in single-stage-to-orbit rockets. These are for tested nuclear rocket engines.
There are advanced designs, too, that provide more impulse and release less radiation into to the atmosphere. These have never been tested.
I have been told that a nuclear rocket development program, making use of a viable current design, would cost no more than $5 - 10 billion US dollars. I don't know whether this is true.
The problem with nuclear thermal rockets is two fold. Firstly, current designs always put some radioactive fission products into the exhaust. The impression I get is that the releases per launch are less than a 1 GW coal-fired electric power station puts into the air (from uranium dust in the coal that goes up the smoke stack). But I don't know.
Secondly, some nuclear thermal rockets will crash. That is inevitable, just as some nuclear submarines have sunk. Launch trajectories can be designed so that not too much damage is done by a crash; but people will worry. Put another way, how confident are you that a Russian or Ukrainian built rocket will be safer than the nuclear power station at Chernobyl?
The way to reduce the number of crashes is to reduce the number of rockets, planet-wide. This raises the price of going into orbit and reduces the military risk. It also means that the great powers have to police every technologically adept country to prevent them from building and using such rockets.
Currently, rockets carry all the oxygen they need with them. An air-augmented chemical rocket operates part of the time as a ram jet, taking in oxygen from the atmosphere. This reduces the mass of oxidizer the rocket must carry.
It is difficult to develop this combination of ram jet and pure rocket, but not impossible. (If I remember rightly, the current jargon calls this kind of rocket a `combined cycle' engine because it can operate as a subsonic ram, a supersonic ram, a hypersonic ram, and as a pure rocket. I am using a name that is more than half century old.)
Another development difficulty is the speed at which fuel burns in a ram jet: when the jet is traveling hypersonically, the fuel has a very short time to burn in the thrust portion of the engine. (I don't say `tube' because some designs use a hypersonic shock wave as one side of the engine.) However, the success of the US X-43A supersonic ram jet in 2004 shows that this problem has been solved.
As with a nuclear rocket development program, I have been told that an air-augmented chemical rocket development program would cost no more than $5 - 10 billion US dollars. I don't know whether this is a valid estimate or simply a number chosen as politically expedient — that is to say, as an amount larger than most private investments, but small enough for any of five or ten rich governments.
Because they are not nuclear, air-augmented chemical rockets could become widespread. They pose the same accident and highjacking danger as airliners. The military danger can be overcome with inspections.
Of course, air-augmented rockets, like current airliners, put water into the stratosphere. Some have argued that this water is or will upset the climate. The US is covered with contrails, which are a visible indicator of such water. And over the past 30 years, people have seen a decrease in the amount of measured sunlight in western Europe. (And maybe elsewhere; I don't know.) I have heard talk that airliners should fly lower, which uses considerably more fuel, so as to put less water in the stratosphere. I don't know the science of this, nor how many air-augmented rocket launches would be acceptable.
As I said I do not believe that private companies have the motivation to build air-augmented rockets, since the competition is obvious and would hurt their return on investment. Nor do I believe that any of the current spacefaring nations have motive either, since such an act would reduce the cost of potential enemies' weapons.
As far as I can see, only two countries have a motive: Taiwan and India. Taiwan gains an immediate military capability against mainland China. India, which does not already have a large program using current, expensive technologies, has an establishment who want the country to become a great power. (Israel has more of a motive to research superconductors for controlled hydrogen fusion, since it does not need inexpensive rockets to fight its neighbors militarily, but it does want to bankrupt them and preserve its national identity.)