Orbital spaceflight
From Wikipedia, the free encyclopedia
An orbital spaceflight (or orbital flight) is a spaceflight where the spacecraft enters into orbit, usually meaning low Earth orbit. To do this, it must have an altitude higher than 100 km (which is the conventional boundary of space), and have a horizontal speed higher than orbital speed, which is usually about 7-8 km/s depending on the altitude. Another way of saying this, is the outward centrifugal force on the spacecraft due to its angular velocity is greater than or equal to the inward force due to gravity. Note that with this definition, an orbital spaceflight need not complete a full orbit.
The expression "orbital spaceflight" is mostly used to distinguish from sub-orbital spaceflights, which are flights where the spacecraft reaches space but does not have a high enough angular velocity to enter into orbit. The required speed to achieve orbit with known methods is about 9.3 km/s (18,000 mph), while some sub-orbital spacecraft can reach space without travelling faster than 1.1 km/s to 1.3 km/s (2,500 mph to 3,000 mph).
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[edit] Methods
Orbital spaceflight from Earth is normally achieved by large rockets that are capable of accelerating from rest to about 10 km/s. This figure allows for atmospheric drag (approximately 300 m/s with the ballistic coefficient of a 20 m long dense fuelled vehicle), gravity losses (depending on burn time and details of the trajectory and launch vehicle), gaining altitude, and the horizontal acceleration needed to reach orbital speed (~7.8km/s depending on altitude).
The only proven technique involves launching nearly vertically for a few kilometers and then progressively flattening the trajectory out at an altitude of 170+ km and accelerating on a horizontal trajectory (with the rocket angled upwards to fight gravity and maintain altitude) for a 5-8 minute burn until orbital velocity is achieved. Frequently 2-3 stages are needed to achieve sufficient delta-v.
Project HARP was a failed attempt, and a ram accelerator is another design, to launch an object into orbit with a gun, possibly with additional propulsion by a rocket.
[edit] Stability
Any object in orbit at an altitude of less than roughly 200 km is considered unstable due to the amount friction caused by the atmosphere. For a satellite to be in a stable orbit (i.e. sustainable for more than a few months), 350 km is a more standard altitude for low Earth orbit.
[edit] Orbits
There are three main 'bands' of orbit: low Earth orbit, intermediate circular orbit and geostationary orbit.
[edit] Re-entry
Due to the high speeds of orbital spaceflight, atmospheric reentry is much more difficult compared to sub-orbital flights. Note however, that such considerations only apply to orbital flights where the vehicle needs to return to Earth intact. If the vehicle is, say, a satellite that is ultimately expendable, then there naturally is no need to worry about reentry.
Returning craft though (including all potentially manned craft), have to find a way of slowing down as much as possible while still in higher atmospheric layers and avoid hitting the ground ("lithobraking") or burning up. The problem of deceleration from orbital speeds is solved through using atmospheric drag (aerobraking) to lose nearly all of the speed. On an orbital space flight initial deceleration is provided by the retrofiring of the craft's rocket engines, perturbing the orbit (by lowering perigee down into the atmosphere) onto a suborbital trajectory.
Aerobraking is achieved by orienting the returning space craft to fly so as to present the heat shields forwards towards the atmosphere so as to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at hypersonic speeds. The thermal energy is dissipated mainly by compression heating the air in a shockwave ahead of the vehicle using a blunt heat shield shape, with the aim of minimising the heat entering the vehicle. Sub-orbital space flights, being at a much lower speed, do not generate anywhere near as much heat upon re-entry.
This has allowed maverick aircraft designer Burt Rutan recently (July 2004) to demonstrate an alternative or complementary approach to heat shield dependent reentry with the suborbital SpaceShipOne. It may be possible that the concepts utilized in SpaceShipOne's design can be applied to orbital space craft design and result in intrinsic stability of the vehicle through reentry (as opposed to the active stability used on the Space Shuttle.) Currently however, the need for an ultra high-performance and ultra reliable heat shield is a major difference between crafts designed for orbital flights (as opposed to sub-orbital ones), demonstrated in the Mercury program wherein the orbital flights used a typical ablative heat shield while the sub-orbital flights relied simply on a large metal heat-sink.
