Why is the White Knight required?

SS1 gets dropped from 47,000 ft.

From there it has enough gas to get to... 115km or so, which is to say
around 100km higher.

This suggests then the White knight trip is not used for a bit of
extra altitude. So what is it for?

To reduce under carriage requirements for horizontal TO?
To avoid some kind of problem with vert TO?
To avoid high MACH low altitude aerodynamic requirements, if full
rocket is used from sea leavel, and a slow initial climb would require
too much fuel?


Please enlighten me!


Gunn

"Chris Gunn" wrote in message
...

SS1 gets dropped from 47,000 ft.

From there it has enough gas to get to... 115km or so, which is to say
around 100km higher.

This suggests then the White knight trip is not used for a bit of
extra altitude. So what is it for?

To reduce under carriage requirements for horizontal TO?

Partly, but a "trolley" could be used for that.

To avoid some kind of problem with vert TO?
To avoid high MACH low altitude aerodynamic requirements, if full
rocket is used from sea leavel, and a slow initial climb would require
too much fuel?

Partly.

Mostly because then you can optimize the engine and nozzle for higher
altitude and get more out of it.




Please enlighten me!


Gunn

Chris Gunn wrote:
SS1 gets dropped from 47,000 ft.

From there it has enough gas to get to... 115km or so, which is to say
around 100km higher.

This suggests then the White knight trip is not used for a bit of
extra altitude. So what is it for?

No, it mostly is used for 'a bit' of extra altitude.

Altitude helps in three main ways:

a) the rocket nozzle only ever sees low pressure, so it can be optimised
for that, which makes it more efficient

b) the vehicle starts off nearer to 100km, which means that the rocket
doesn't have to be quite as big (there's a subtle distinction between
how big it *has* to be, and how big it actually is- there's always some
spare capacity to deal with variations in the atmosphere and the equipment)

c) Atmospheric drag depends on atmospheric density, which halves every
18000 ft or so, so starting higher up gives much less drag

Of the three c) is probably the most important, a) second and b) the least

Atmospheric drag is very important for small vehicles; it's more or less
the first 30km that is the hardest due to the high atmospheric drag, the
next 70km is relatively trivial.

Big vehicles don't care so much; for example the Shuttle essentially
only slows down to avoid ripping the wings off(!), the fuel used isn't
the limit. It's basically because a vehicle twice as big in general
presents 4 times the cross-sectional area to the atmosphere, but has 8
times the fuel to deal with it.

if full
rocket is used from sea leavel, and a slow initial climb would require
too much fuel?

Yes. Getting to high altitude on rocket power alone in a small vehicle
consists of holding yourself up on a column of fire at a roughly
constant mach 0.85 till you reach approximately 30-35km, and only then
using full thrust. Standing on a column of fire like that wastes lots of
fuel.

Because SS1 starts higher up, I think it barely even reaches mach 0.85
by 30km, and so doesn't need to worry so much and can reach higher
altitude with a smaller vehicle.

Please enlighten me!

Hope you have attained enlightment grasshopper.

Gunn

In article ,
Ian Woollard wrote:

Because SS1 starts higher up, I think it barely even reaches mach 0.85
by 30km, and so doesn't need to worry so much and can reach higher
altitude with a smaller vehicle.

SS1 reached Mach 1.3 on its first powered flight, which ran the engine
for less than 15 seconds, and peaked at an altitude of 68,000 ft (call
it 20 km).


It's probably doing about Mach 2 by 30 km -- and don't forget engine
cutout is at over Mach 3 at only about 50 km.

--
Bruce | 41.1670S | \ spoken | -+-
Hoult | 174.8263E | /\ here. | ----------O----------