Use ever greater, thinner and faster wings, to glide through ever thinner upper layers of earth's atmosphere. Wile using low thrust engines, for a very long period (days? months?). Up to an altitude/speed, that a simple low thrust engine can send you anywhere in the solar system.
You might be surprised to learn that there is some air at very high altitudes. The atmosphere gets thinner and thinner, but it never disappears abruptly. There's not a clear distinction between atmosphere and vacuum. This residual atmosphere generates drag for the ISS and other objects in orbit. At satellite orbital altitude, air is way to thin for normal wings, but if we imagine very thin wings(light), of very big size and traveling at very great speed, there's a reasonable chance to get enough usable lift (L=(1/2)*ρ*v²*A*Cl).
Rockets, aren't really good commercially. We use airplanes, and not rockets to move around. Rockets are interesting for the military. With rockets we can go really fast, but why would we want to be that in a hurry? Only the military want to be that fast.
The usual way of alternative non rocket schemes, is through the use of high velocities, in some way, in order to attain this satellising speed. Even if the scheme manages to be reasonable from an energy point of view, the required engineering becomes pharaonic. I believe my scheme is much more reasonable than the other alternatives we have seen.
It might seem absurd to use wings, in what is essentially vacuum. What's important, its the right combination of speed, weight and size (and shape). Intuitively the thinner the air, you need to go faster and/or have bigger wings.
The wings can be ultra thin, with minimal structural elements. Maybe something like aluminium foil wings, or thinner. The craft could look like a bat. Technically, it shouldn't be that different then a solar sail. Inflatable parts holding the membranes (gas pressure can be small, basically vacuum out side)
The engines can be jets type, meaning they absorb particles from the outside, in order to generate the trust. They can be rocket type, meaning they just hold all the propellent they will use. Energy could come from photovoltaics, beamed microwaves from the ground, etc. The jet engines, could have larger diameters, greater compression rates, electromagnetic, etc. The rocket types could be some form of electric engines (ionic, plasma, etc). We can imagine hybrids, both on board and outside propellants, supplementing one an other depending on the technical situation. What they will all have in common is that they will generate relatively low trust. At least enough to overcome the drag, and build up speed slowly.
New engines must be designed, for the very special conditions of air density and low trust requirements they will work in. All current engines, either work in normal air, either they are just rockets.
With a relatively normal airplane, we can get up to a certain altitude. There we deploy bigger and thinner wings (more fragile), able to generate enough lift, and survive the wing conditions of the local environment. We use specially designed engines for the thinner air.
When the craft reaches its altitude limit, it redeploys other wings, even bigger and thinner and more fragile, and uses other engines. These wings are too fragile for lower altitudes, but good enough for the new altitudes. The engines are adapted to the much lower air density.
When the second lifting apparatus reaches its limit too, we redeploy a third, and as many other necessary to be able to gain more and more hight.
We can take our time in building up kinetic energy. Hopefully, photovoltaics will generate enough power for the lift off. It might take weeks to reach satellising speed, but it will be with a fully reusable craft.
The craft will have comparable complexity as that of a normal airplane. The craft will never be exposed to sudden changes of the outside environment, materials should be more tolerant and need less frequent maintenance.
The technology requirements seam to be reasonable. It should be doable at a fraction of the cost of rocket technology. Intermediates could be used as a form of very low orbit satellites
The same scheme could be used for landing too. Currently, space ships re-enter the atmosphere at very high speed, needing thermal shielding if it wants to survive. With wings the re-entry can be spread over very long time at a controlled manner, dissipating the excess energy slowly. It will may, or may not use reverse thrust, or some form of drag generating apparatus. All that in a controlled manner.
At high altitude, the wings could be used as solar sails, at least supplementing for trust of the engines. The pressure from solar radiation should be also be taken in too account, near the end of the "launch".
Prevailing "winds" could be helpful. Up lifting flow of particles could push an craft higher and faster then what it is possible just by its own capabilities. Either this will be used as normal use, or use it for saving money.
Can we build the right combination of weight, strength and size?
Never done these things before. Uncharted engineering territory. They are the lift and drag equations, but i don't think they were ever tested for these kind of conditions.
L/D lift/drag force
ρ air density
v true airspeed
A platform area
CL/Cd lift/drag coefficient
not certain they remain valid for ultra thin atmosphere. If we believe the equations, the lower air density can be made up for by higher speed and surface area. The coefficients are determined on a case by case basis by direct experimental measurements.
Very simplistic numeric example: If ρ becomes ρ*1/1000, then if v becomes 10v and A becomes 10A. We should have comparable lift to a normal aircraft. 10v and 10A seam doable.
The problem with these equations, is that the proportions of "ρ*v²*A" don't necessarily stay true in our conditions.
The trick is, to maximize the lift, while minimizing the drag
An appropriate "wind" tunnel would be nice. Probably, it would need to be rather big, in order to be able to measure something. It shouldn't be that expensive. Maybe use cold plasma flow for simplicity.
From bellow: Gradually build gliders that go higher and higher. With second series of deployable wings etc.
From above: Send a craft on rockets at high altitude/speed, use his gliding ability to perform the end of the satellisation.
The two projects will meet somewhere in the middle.