[extropy-chat] Huygens: First visitor to Titan

Eugen Leitl eugen at leitl.org
Sun Jan 16 18:52:21 UTC 2005


On Sun, Jan 16, 2005 at 07:26:29PM +0100, Eugen Leitl wrote:

> Last one is a 404, but you're absolutely accurate. People don't understand
> what 32 MJ/kg difference (between 100 km orbit, and 100 km of what
> SpaceShipOne did, zero velocity at 100 km height) means.
> 
> http://en.wikipedia.org/wiki/Outer_space

Sorry, that Earl wasn't quite the real thing. Here:

http://en.wikipedia.org/wiki/Difference_between_orbital_and_suborbital_spaceflights

Difference between sub-orbital and orbital spaceflights
From Wikipedia, the free encyclopedia.
(Redirected from Difference between orbital and suborbital spaceflights)

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.

The term orbit can be used in two ways: it can mean a trajectory in general,
or it can mean a closed trajectory. The terms sub-orbital and orbital
spaceflights refer to the latter: 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.
Contents [showhide]
1 Angular velocity
2 Difference in the real world

2.1 Atmospheric reentry a much bigger challenge with orbital flights
3 Summary
4 See also
[edit]

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 an orbit - depends crucially on the angular velocity.
Travelling straight up will never result in an orbit, doing so faster than
escape velocity will have the obvious effect and orbit is still not attained.
[edit]

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 18,000 mph to
attain orbit. This compares to the relatively modest 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 X Prize and for an orbital
spaceflight is that no lateral or angular velocity is required for the
sub-orbital flight. The energy required to get to 100 km or even 350 km
altitude is dwarfed by the energy required for the necessary lateral velocity
of orbital space flight.

In terms of energy: accelerating a spacecraft to orbital speed requires about
31 times as much net energy as just lifting it to a height of 100 km
(together 32 times), see computation.

But this is the energy which must be imparted to the orbiting mass: For a
rocket the fuel and oxygen (and their tanks) must be accelerated as well and
so the energy requirement is actually much more than the factor of 32
identified. (See the rocket equation article for a more detailed treatment).

In terms of the semi-major axes a of the elliptic orbits: the total specific
orbital energy is \epsilon=-{\mu\over{2a}} where \mu\, is the standard
gravitational parameter. Being at rest at the surface of the Earth
corresponds to a = R / 2 (with R the radius of the Earth). Reaching a height
of 100 km means an increase of a of 50 km, while a LEO requires an increase
of a 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.
[edit]

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
aerobraking, ie. using the 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 a markedly reduced need for a massive heat
shield. 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).
[edit]

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 can reach a higher height than an orbital one.
    * The most important requirement for an orbital flight over a sub-orbital
    * one is speed.
    * The shock wave produced by high speed atmospheric reentry generates
    * lots of heat from which the spacecraft must be protected.

[edit]

See also

    * Boundary to space
    * Low Earth orbit
    * Atmospheric reentry
    * Aerobraking


Retrieved from
"http://en.wikipedia.org/wiki/Difference_between_sub-orbital_and_orbital_spaceflights"

Categories: Space | Human spaceflight

> 
> ...
> 
> Space does not equal orbit
> 
> A common misunderstanding about the boundary to space is that orbit occurs by
> reaching this altitude. Orbit, however, requires orbital speed and can
> theoretically occur at any altitude. Atmospheric drag precludes an orbit that
> is too low.
> 
> Minimal altitudes for a stable orbit begin at around 350 km (220 miles) above
> mean sea level, so to actually perform an orbital spaceflight, a spacecraft
> would need to go higher and (more importantly) faster than what would be
> required for a sub-orbital spaceflight.
> 
> Reaching orbit requires tremendous speed. A craft has not reached orbit until
> it is circling Earth so quickly that the upward centrifugal "force" cancels
> the downward gravitational force on the craft. Having climbed up out of the
> atmosphere, a craft entering orbit must then turn sideways and continue
> firing its rockets to reach the necessary speed; for low Earth orbit, the
> speed is about 7.9km per second (18,000 mph). Thus, achieving the necessary
> altitude is only the first step in reaching orbit.
> 
> The energy required to reach velocity for low earth orbit (32 MJ/kg) is about
> twenty times the energy to reach the corresponding altitude (10 kJ/km/kg).
> [edit]
>  
> > Which is all moot, since "per minute" is a very silly
> > figure of merit for science.
> 
> Given that the amount of available .gov funds is very limited 
> (and shrinking) questions of ROI is very relevant.
> 
> -- 
> Eugen* Leitl <a href="http://leitl.org">leitl</a>
> ______________________________________________________________
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-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a>
______________________________________________________________
ICBM: 48.07078, 11.61144            http://www.leitl.org
8B29F6BE: 099D 78BA 2FD3 B014 B08A  7779 75B0 2443 8B29 F6BE
http://moleculardevices.org         http://nanomachines.net
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