Difference between sub-orbital and orbital spaceflights
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There sometimes appears to be confusion among the general public about the difference between sub-orbital and orbital spaceflights. This article is an attempt to clarify this issue. It also elaborates on the technical implications of the differences between orbital and sub-orbital spaceflights.
A spaceflight is a flight into or through space. The craft which undertakes a spaceflight is called a spacecraft.
The general public often thinks of orbital spaceflights as spaceflights and of sub-orbital spaceflights as "something less than actual spaceflights". This is not accurate; both orbital and sub-orbital spaceflights are true spaceflights.
Strictly speaking, the term orbit means any trajectory in general. In common usage the term orbit refers to a closed trajectory around the Earth (or another central body). The term sub-orbital refers to a trajectory which intersects the central body before a complete orbit is achieved. An orbital spaceflight is one which completes an orbit fully around the central body.
For a flight from Earth to be a spaceflight, the spacecraft has to ascend from Earth and at the very least go past the edge of space. The edge of space is, for the purpose of space flight, often accepted to lie at a height of 100 km (62 miles) above mean sea level. Any flight that goes higher than that is by definition a spaceflight. Where the Earth's atmosphere ends space begins but the atmosphere fades out gradually so the precise boundary is difficult to ascertain - hence the need for an arbitrary altitude for the edge of space.
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Angular velocity
An orbital spaceflight is achieved when the spacecraft travels around the Earth in space at sufficient lateral velocity (or equivalently, enough angular velocity) for the centrifugal force to cancel out the pull of Earth's gravity. Lateral velocity is the speed of something around an object and it is this which is the critical factor. Although the angular velocity required is a function of the height of the orbit, orbital spaceflight is possible at any altitude beyond the edge of space.
A body which does not have sufficient angular velocity cannot orbit the Earth. The actual speed of a sub-orbital spacecraft could exceed that of an orbital one and the height that a sub-orbital spacecraft attains may even exceed that of an orbital one, but the critical difference between the two - the achieving of a closed orbit - depends crucially on the angular velocity. Travelling straight up will never result in a closed orbit — doing so faster than escape velocity will have the obvious effect and a closed orbit is still not attained.
Difference in the real world
That said, typical sub-orbital craft need go only just past the accepted edge of space at 100 km (62.5 miles) for the flight to be a spaceflight. At this arbitrary boundary there is still too much atmosphere present for a long term stable low Earth orbit (LEO). In order to be stable for more than just a few weeks or months the satellite or spacecraft is placed in orbit at an altitude where drag from the atmosphere truly is negligible. A stable LEO is usually at least 350 km up.
But again, the difference in height should not be overemphasized: Whether the altitude is 100 km or 350 km the distance from the centre of the Earth is only different by less than four percent.
The difference between the lowest speeds required for orbital and sub-orbital space flights is substantial: a spacecraft must reach about 29,000 km/h (18,000 mph) to attain orbit. This compares to the relatively modest 4,000-4,800 km/h (2,500-3,000 mph) typically attained for sub-orbital crafts.
The important difference in energy requirements between a sub-orbital spaceflight such as that required for the Ansari X Prize and for an orbital spaceflight is that no lateral or angular velocity is required for the sub-orbital flight. The energy required for the necessary lateral velocity of orbital space flight is over 30 times the energy required to get to 100 km altitude; see Energy Calculations below.
In terms of the semi-major axes <math>a</math> of the elliptic orbits: the total specific orbital energy is Template:Il</math>|70}} where <math>\mu\,</math> is the standard gravitational parameter. Being at rest at the surface of the Earth corresponds to <math>a=R/2</math> (with <math>R</math> the radius of the Earth). Reaching a height of 100 km means an increase of <math>a</math> of 50 km, while a LEO requires an increase of <math>a</math> of more than 3000 km. See also low-energy trajectories.
A vertical sub-orbital flight with the same energy as a LEO would reach a height of ca. 7000 km above the surface.
Atmospheric reentry a much bigger challenge with orbital flights
Because of that speed difference, atmospheric reentry is much more difficult for orbital flights than it is for 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 plunging downwards too quickly. To date (as of 2004), the problem of deceleration from orbital speeds has mainly been solved through using atmospheric drag itself to slow down. On an orbital space flight initial deceleration is provided by the retrofiring of the craft's rocket engines. Aerobraking in turn has so far mainly been achieved through orienting the returning space craft to fly at a high drag attitude coupled with ultra strong heat shields on the space craft, to protect against the high temperatures generated by atmospheric compression and friction caused by passing through the atmosphere at supersonic speeds. The thermal energy is dissipated mainly as infrared radiation. 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 dependant 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.
Energy calculations
Lifting a craft to 100 km altitude requires pushing against the force of gravity over that distance. For the sake of calculation, we'll assume the force of gravity is 9.8 N/kg (about 1 Gee) on the surface, which is 6370 km from the Earth's center. Gravity will decrease with the inverse square of the distance, which starts at 6370 km and finishes at <math>6370 + 100 = 6470</math> km, where gravity will be 9.5 N/kg. Over such a small interval, we will introduce little error if we assume the gravity is a constant at an average of 9.65 N/kg.
Given that work equals force times distance
- <math>E = fd\;</math>
and force equals mass times acceleration
- <math>f = ma\;</math>
we have
- <math>E/m = ad\;</math>
We substitute <math>a = 9.65</math> N/kg and <math>d = 100 \times 10^3</math> m, giving:
- <math>E/m = 965\;</math> kJ/kg
This corresponds to a required delta-v of ca. 1.4 km/s, or in the case of an air launch, 1.3 km/s from that launch.
In contrast, imparting Low Earth orbit velocity of 7800 m/s from rest requires kinetic energy of <math>1/2mv^2</math>, which is 30420 kJ/kg, which is over 31.5 times as much energy. (Starting from the Earth's surface, which is not at rest, requires slightly less energy.)
These calculations only deal with the energy imparted to the craft itself; it ignores the energy required to lift the rocket fuel and propellant. The rocket equation indicates the actual energy requirements as follows:
- <math>E = \frac{1}{2}m\left(e^{\Delta v / v_e}-1\right)v_e^2</math>
The energy requirements depend on <math>v_e</math>, the effective exhaust velocity of the propellant. A good propellant combination like hydrogen and liquid oxygen can achieve <math>v_e</math> of about 4400 m/s. With <math>\Delta v = 7800</math> m/s, we have:
- <math>E/m = \frac{1}{2}\left(e^{7800 / 4400}-1\right)4400^2 \approx 47200</math> kJ/kg
A less efficient propellant — that of the Space Shuttle Solid Rocket Booster — has <math>v_e = 2570</math>, giving <math>E/m</math> roughly 65400 kJ/kg, with the energy requirements increasing exponentially as the propellant efficiency decreases.
Note, however, that this same effect also affects the sub-orbital flight. Also, real flights of both kinds experience atmospheric drag and gravity drag. For an orbital launch, these require an additional delta-v of typically 1.5–2 km/s (see delta-v budget).
Summary
- Sub-orbital spaceflights flights are spaceflights just as orbital flights are.
- Both go beyond the atmosphere and past the edge of space.
- A sub-orbital flight may reach a higher height than an orbital one, but in practice generally does not.
- The most important requirement for an orbital flight over a sub-orbital one is horizontal speed.
- The shock wave produced by high speed atmospheric reentry generates lots of heat from which the spacecraft must be protected.