Balloons and airships are humanity's oldest aircraft technology. Today, they are also one of the cheapest and easiest to engineer compared to other flying vehicles, especially for the realm of the upper atmosphere. Throughout the Twentieth Century, they helped to explore extreme altitudes and hoist aloft a number of instruments that explored the realm beyond Earth's envelope of air.
Recently, with advent of advanced but cheap GPS and communications equipment, high-altitude balloons have been used by amateurs to achieve Near Space altitudes (100,000+ feet or about 25 miles up, the altitude between the lower layers of the atmosphere and the official edge of space) for relatively a little amount of money. In the future, balloons may present a more affordable means to access space.
Rockoon is a mash up of the words "rocket" and "balloon." In common modern usage, they are a solid-fuel sounding rocket carried aloft by a balloon that were first used by atmospheric researchers in the US in 1949. However, a number of space interests are currently looking into different kinds of balloon/rocket hybrids as a means of greatly reducing the cost and complication of a space launch.
In practical terms, the balloon (or array of them in some cases) acts as the first 'stage' of the rocket, carrying the vehicle out of the lower atmosphere. When the balloons reach maximum altitude, they disengage and the rocket automatically fires, carrying the payload the rest of the way, whether it be on a suborbital or full orbital trajectory.
This arrangement can save on propellant and make the launch considerably cheaper than normal. The thicker layers of air nearer the ground will induce a great deal of a drag on a rocket no matter how aerodynamically it may be designed. Fighting against all this atmospheric drag while trying to build up orbital speed can eat up a large portion of a rocket's fuel. Using a balloon to lift it above most of the thick layers of atmosphere can eliminate much of this problem, reducing propellant needs by as much as 25%, depending on launch altitude. And without having to carry so much fuel, the rocket can also be made smaller, adding to the project's savings.
It should be noted that just getting enough altitude is not good enough for orbital insertion or even most sub-orbital trajectories. Those depend a lot more on the rocket's velocity, and the balloon does not provide much assistance in terms of speed. Still, the reduction of required propellant and subsequent rocket mass does help reduce mission costs and makes rockoons a very attractive option for many interests that cannot afford a more conventional launch.
But there are still severe restrictions on how much weight even large balloons can carry aloft, and that in turn limits the kind of missions rockoons can undertake. Orbital insertions will probably only be possible with so-called nanosatellites massing less than 10 kilograms or so. With the current and anticipated near-future level of miniaturization technology, this would not be a barrier to a great deal of satellite applications.
Its also possible to carry heavier loads for strict sub-orbital flights which require short or non-existent lateral ranges. At least one manned project, the Da Vinci project, proposed sending actual astronauts up with a rockoon system to the edge of space on a suborbital trajectory as part of the Ansari X-Prize competition. Though its systems, particularly its hybrid rocket, was extensively tested, it was never flown. However, the concept seemed very feasible and could be revived in the future.
The original rockoons had the rocket launch directly up, right through the relatively flimsy canopy of the lifting balloon. However, more recent designs have the rocket firing from a launch tube canted at a slight angle so it misses the balloons. In this way, the balloons can be brought back to the ground and reused, further helping to save on costs.
One main operational drawbacks to using rockoons is that the initial lifting phase up to launch altitude can take many hours, even days.
ORBITAL ASCENDER AIRSHIP
Tech Level: 12
|Image copyright JPAerospace.|
The Orbital Ascender is a design being developed by the firm JPAerospace as the final stage in an orbital insertion system. It is perhaps one of the most unusual concepts for orbital flight yet discussed on this site; basically a dirigible flown directly into space.
The ascender system uses three distinct stages. One, a normal airship that operates between the ground and a stratostation, carrying cargo and crews. Two, is the stratostation itself, a very large stationary airship designed to act very much like a way station twenty five miles or so up. A more detailed article on stratostations is linked to at the bottom of the page.
The third stage is the Orbital Ascender, a V-shaped airship designed to only operate at extreme altitudes and in space. To save on weight and allow it to eventually build up to orbital speeds, it would be built too light to operate effectively in the turbulent lower atmosphere. In fact, it would have to be constructed at the stratostation itself, and would never actually be able to come closer to the ground than 25 miles.
In order to operate at such altitudes with miniscule air pressures, the gas envelope of the airship would need to be enormous to keep the airship's total density lower than that of the surrounding air. In order to achieve this, the Orbital Ascender is expected to have a volume seven times that of the Hindenburg. However, even in the upper fringes of the atmosphere, helium, designed to be the primary lifting agency, would still be lighter than a similar volume of the surrounding air.
The airship is also expected to have a very unusual design, basically shaped like an enormous 'V.' This way, traveling with the nose up, it would take advantage of aerodynamic forces in the same way that supersonic airfoils do, generating additional lift through forward motion.
According to JPAerospace, the airship's engines are currently planned to be hybrid chemical/electric rockets. Future versions may use advanced lightweight ion engines. The airship would accelerate slowly but steadily over three days or more, gradually gaining the speed and altitude needed to obtain orbit. To return to the stratostation, the airship would reverse the process, decelerating and descending slowly over a period of days.
The airship could be covered over in thin solar cells for supplemental power. At the Tech Level these aircraft may realistically come into common use (Tech Level 12, or circa 25 years from now), paper-thin and light solar cells should be available that would not add enough weight to significantly affect the performance of the vehicle. Given the enormous surface area of the Ascender craft's gas envelopes, these cells would be able to provide a good deal of the vehicle's power needs, even for its hybrid electrical engine.
Though the source material is fairly sparse on details, it is easy to infer some possible problems with this approach. The enormous gas envelope would be very vulnerable to potential impacts and flight stresses, and even minor leaks could compromise its buoyancy at the lower altitudes of its flight range.
There is also the issue of initial assembly. The material of the Ascender's gas envelope would be so flimsy, even using advanced building materials such as graphene, that it could only be practically accomplished at the stratostation twenty-five miles up. However, no one has any experience in constructing objects at such altitude, much less something so immense and relatively fragile. Though the transorbital airship would be able to pay for itself through repeated reuse, its initial construction cost using experimental techniques (including the stratostation itself) might give a number of potential investors pause.
Also, because of its long time ascending and descending, and it relatively very thin hull, any crew it might carry would be exposed to high-altitude radiation conditions for much longer than most orbital or suborbital flights, up to six days (three days ascending and three days descending, minimum.) If the orbital ascender is manned, the crew quarters would likely be as well shielded as their weight restrictions would allow, but may still be less than some would find comfortable.
Exact cargo capacity of the orbital ascender airship is never given in any of the sources. However, in order to be economically practical, it would likely be able to carry at least several hundred kilograms, the size of a small satellite or one or two passengers bound for an orbital station.
If the airship-stratostation-ascender system ever came fully online, it would represent one of the cheapest means of accessing orbit available in terms of resources consumed per launch. Advanced versions could even bill themselves as tourist destinations in their own right, allowing passengers to take week-long cruise to space and back, with the best possible scenery available the whole time. Very advanced versions could even use hydrogen (with no chance of combustion, given how thin oxygen would be at its operating altitudes) or vacuum cells to generate more efficient lift.
http://www.astronautix.com/lvs/rockoon.htmhttp://academy.grc.nasa.gov/y2008/group-project/proposal-for-a-balloon-assisted-launch-system http://en.wikipedia.org/wiki/High-altitude_balloon http://en.wikipedia.org/wiki/Rockoon http://www.davinciproject.com/technical/index.php
Transorbital Airshipshttp://en.wikipedia.org/wiki/Orbital_airship http://orbitalvector.com/Aircraft/Stratostations/STRATOSTATIONS.htm