The initial projects of the Bifrost space program are likely to focus on concepts seeking to achieve a basic unmanned access to space for research purposes with the most minimalist systems. This is also likely to be a predominant philosophy among designs from the Open Space Project of the Open Source Everything Project, where Bifrost is likely get its start, early in the course of the Foundation phase. The SkyScraper concept is an expression of this minimalist spacecraft idea, consisting of a series of lighter than air assisted launch systems that are intended to radically simplify the construction and deployment of a spacecraft by softening the rigors of launch conditions.
Battling the Atmosphere[]
A conventional rocket launched at sea level must use tremendous propulsive power to overcome gravity and accelerate to an orbital velocity in a short period of time. And because its engines must operate in such a broad range of atmospheric pressure as it rapidly climbs, their efficiency is greatly reduced because their engineering, which is typically only optimal at a single pressure, must compromise on a spectrum of pressure. This has been a particular challenge for the development of single-stage-to-orbit systems. The use of such tremendous power over such a short acceleration period subjects the structure of a vehicle to great stress and vibration, thus demanding structures of very high strength employing the best strength-to-weight performance possible. This, coupled to the use of streamlining, tends to lead to heavier and more complex and difficult to fabricate structures than might be necessary if a vehicle had the option of a much longer lower-g acceleration period.
Tethered Aerial Release Developed In Style[]
Scientists have long understood that if one could launch rockets at very high altitude they would enjoy a benefit in propulsion efficiency. Many experiments with the use of balloon-lofted rockets have been performed to demonstrate this. But this benefit has generally been considered too small to justify the development of systems of any scale to support a high altitude launch. But what has not been seriously considered by spacecraft designers is the potential effect high altitude launch has on the design of systems as a whole and the benefits one can potentially gain by reducing the robustness of structures and eliminating their need for streamlining. To put it simply, rockets launched at sea level tend to be limited in structure to monocoque hull systems which are difficult to fabricate, difficult to adapt, and limited to propulsion systems that are complex, heavy, and large. At stratospheric altitudes where conditions approach those in space itself, one can potentially use modular open space frames with much reduced mass that can be assembled on demand and employ large assortments of modular components easily reconfigured for different needs while using propulsion that is lighter (both by engine type and the option to use pneumatic fuel tanking), simpler, and lower in power by virtue of better efficiency, lower overall system mass, and a favoring of Isp over thrust volume. While this may not be the ideal type of system for every application, nowhere is it written in stone that launch systems must be one-size-fits-all. But for the spectrum of uses it may suit, it would be cheap and easy to develop for organizations with limited industrial resources and engineering manpower – such as universities and space advocacy groups. You may not be able to build your own ISS with such systems, but you could potentially still do quite a lot.
The SkyScraper project may explore many different systems but the vehicle most likely to evolve is a simple structure based on the use of a single ‘core truss’ to which a radial array of lighter-than-air lift cells are attached on the outside and to which most of the active components of the system attach on the inside. The core truss – a structural feature we will see repeat frequently in other areas of TMP2 – could be a conventional rigid truss using alloy, carbon fiber, or pneumatically rigidized struts or it may be an even lighter (but more limited in possible attachment points) tensegrity truss based on a combination of compression struts and carbon fiber cable. Lift cells would be super-pressure gas cells using hydrogen and would be made of aluminized elastomeric. They would be either simple cylinders, wedges, or cylindrical rings, the latter being used where a vehicle would retain its lift cells into orbit where they might serve double-duty as solar collectors or mounts for ‘flex-cell’ photovoltaics.
Several possible configurations are likely to be explored, depending on whether the design attempts to retain the initial acceleration from LTA lift or initiates flight from a horizontal static position at altitude and whether it retains its lift cells or primary structure into orbit or discards them for recovery and reuse. Let's briefly examine how these may vary by looking at these modes of flight in turn.
SSTO[]
this would be the simplest form, with the whole structure and its lift cells carried to orbit. Would suit either a horizontal or vertical release mode, though this type of system would always employ a more-or-less horizontal flight trajectory for most of its long launch period, initial vertical lift velocity perhaps employed as a means to initialize more exotic hybrid forms of propulsion such as SCRAM jets employing hydrogen discharge gas from the lift cells. This form would likely employ toroidal lift cells and be most useful for the simplest of research vehicles and for vehicles that are employing the lift cells for another purpose on-orbit, such as a solar dynamic collector or photovoltaic surface. Such cells are generally an obstruction on-orbit where payloads must communicate through the sides of the core truss for access to the space environment. This mode also offers the potential of a vehicle that is reusable whole by virtue of using its lift cells as a pneumatic re-entry shield. However, these vehicles would tend to be so cheap and minimalist in structure to begin with that recovery is not as important as it would be for other systems.
DSTO/Lift Cell Discard[]
A dual stage vehicle based simply on discarding its lift cells as the first ‘stage’ once the vessel has achieved sufficient velocity. Cells could detach from the core truss rather like booster rockets leaving the whole truss intact –which would favor designs that place propulsion at the ‘aft’ end of the truss– or a two-part structure could be employed where an ‘upper stage’ section detaches from a ‘lower stage’ hosting the lift cells which could optionally be recovered for reuse –this form favoring the more exotic propulsion approach of rotary boom mounted centrifugally pumped aerospike thrusters ‘towing’ the vehicle from a forward position.
MSTO[]
Essentially the same at the DSTO mode except that the core truss would be arrayed into multiple stacked segments much like traditional multi-stage rockets with the first stage discarding radial lift cells and then the subsequent stages separating from aft to fore. Would be most useful for the larger scale systems and systems intended to achieve GEO.
As with the Aquarian Airship, the SkyScraper system would see radical improvements in its performance with the advent of new nanomaterials, potentially allowing for the creation of a vacuum lift cell systems based on tensegrity membrane cells. This could also be possible with existing materials relying on a transition to vacuum lift at high altitude in order to increase the launch altitude as much as possible. However, the gain here may be only on the order of a 10-20% increase in maximum altitude which may not be sufficient to justify the increased mass and fabrication complexity of the hybrid cells using current materials – which must employ an array of internal compression struts and reinforced attachment nodes.
SkyScraper is potentially infinitely scalable in theory, but in practice would most likely be limited to fairly small payload systems owing to the unwieldiness of very large deployable LTA structures. But with such a gentle and safe mode of initial launch by ground release and the ability to quickly assemble structures on demand at a launch site it may prove very popular as a research launch platform.
Peer Topics[]
- Mountain Waverider
- UltraLight SSTO
- MODroc
- Exocet
- Wizard
- LightCraft
- Marine Mass Launcher - MML
- Bifrost Space Elevator
- Bifrost Support Systems
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