Something wrong here

This is a quote from the NASA site about Cassini

"During the burn, the spacecraft will change its velocity by 626 meters per
second (1,400 miles per hour). Relative to Saturn, at burn start the
spacecraft speed is 24.26 kilometers per second (54,270 miles per hour) and
at the end of the burn the speed is 30.53 kilometers per second (68,293
miles per hour). Mission managers expect periodic interruptions of the
Doppler signal as Cassini passes behind the rings. "


Ummm..this indicates the burn is going speed things up. Am I missing
something?

Hi Mike,

Mike Thomas wrote:

This is a quote from the NASA site about Cassini

"During the burn, the spacecraft will change its velocity by 626 meters per
second (1,400 miles per hour). Relative to Saturn, at burn start the
spacecraft speed is 24.26 kilometers per second (54,270 miles per hour) and
at the end of the burn the speed is 30.53 kilometers per second (68,293
miles per hour). Mission managers expect periodic interruptions of the
Doppler signal as Cassini passes behind the rings. "

Ummm..this indicates the burn is going speed things up. Am I missing
something?

The closer Cassini gets to Saturn, the faster it "falls".

From the diagram of the orbit insertion it looks that the burn is
timed to start sometime before the closest point to Saturn, and to
complete around the closest point, when the speed is highest.

In other words, without the burn, those 30.53 km/s would be more,
so relatively speaking, this is slowing down the things.

Regards,

- Alex

Hi Mike,

Mike Thomas wrote:

This is a quote from the NASA site about Cassini

"During the burn, the spacecraft will change its velocity by 626 meters per
second (1,400 miles per hour). Relative to Saturn, at burn start the
spacecraft speed is 24.26 kilometers per second (54,270 miles per hour) and
at the end of the burn the speed is 30.53 kilometers per second (68,293
miles per hour). Mission managers expect periodic interruptions of the
Doppler signal as Cassini passes behind the rings. "

Ummm..this indicates the burn is going speed things up. Am I missing
something?

The closer Cassini gets to Saturn, the faster it "falls".

From the diagram of the orbit insertion it looks that the burn is
timed to start sometime before the closest point to Saturn, and to
complete around the closest point, when the speed is highest.

In other words, without the burn, those 30.53 km/s would be more,
so relatively speaking, this is slowing down the things.

Regards,

- Alex

"Mike Thomas" wrote:

MT Ummm..this indicates the burn is going speed things up. Am I
MT missing something?

Saturn is still accelerating Cassini even during the burn (e.g. the
spacecraft's speed would be over 31 km/sec without the burn).

"Mike Thomas" wrote:

MT Ummm..this indicates the burn is going speed things up. Am I
MT missing something?

Saturn is still accelerating Cassini even during the burn (e.g. the
spacecraft's speed would be over 31 km/sec without the burn).

Mike Thomas wrote
This is a quote from the NASA site about Cassini

"During the burn, the spacecraft will change its velocity by 626 meters per
second (1,400 miles per hour). Relative to Saturn, at burn start the
spacecraft speed is 24.26 kilometers per second (54,270 miles per hour) and
at the end of the burn the speed is 30.53 kilometers per second (68,293
miles per hour). Mission managers expect periodic interruptions of the
Doppler signal as Cassini passes behind the rings. "


Ummm..this indicates the burn is going speed things up. Am I missing
something?


If its' trajectory is taking it to an impact or excessively elliptical
orbit, a burn to accelerate is likely. AFAIUI, they need an orbit that
is reasonably stable, at the correct mean distance.
Bear in mind that Cassini is effectively falling toward Saturn out of a
transfer trajectory, and could have too much or too little velocity for
the ideal orbit after travelling so far in free fall.
(Or they may just have the numbers round the wrong way g)

Denis
--
DT
Replace nospam with the antithesis of hills

Mike Thomas wrote
This is a quote from the NASA site about Cassini

"During the burn, the spacecraft will change its velocity by 626 meters per
second (1,400 miles per hour). Relative to Saturn, at burn start the
spacecraft speed is 24.26 kilometers per second (54,270 miles per hour) and
at the end of the burn the speed is 30.53 kilometers per second (68,293
miles per hour). Mission managers expect periodic interruptions of the
Doppler signal as Cassini passes behind the rings. "


Ummm..this indicates the burn is going speed things up. Am I missing
something?


If its' trajectory is taking it to an impact or excessively elliptical
orbit, a burn to accelerate is likely. AFAIUI, they need an orbit that
is reasonably stable, at the correct mean distance.
Bear in mind that Cassini is effectively falling toward Saturn out of a
transfer trajectory, and could have too much or too little velocity for
the ideal orbit after travelling so far in free fall.
(Or they may just have the numbers round the wrong way g)

Denis
--
DT
Replace nospam with the antithesis of hills

MT Ummm..this indicates the burn is going speed things up. Am I
MT missing something?

Saturn is still accelerating Cassini even during the burn (e.g. the
spacecraft's speed would be over 31 km/sec without the burn).


God that's hauling ass! That is what they mean by orbital energy I guess.
This is what actually speeds up a craft when it sling shots around a planet.
I guess it hits at some tangent to the orbital curve and picks up speed.

MT Ummm..this indicates the burn is going speed things up. Am I
MT missing something?

Saturn is still accelerating Cassini even during the burn (e.g. the
spacecraft's speed would be over 31 km/sec without the burn).


God that's hauling ass! That is what they mean by orbital energy I guess.
This is what actually speeds up a craft when it sling shots around a planet.
I guess it hits at some tangent to the orbital curve and picks up speed.

"Mike Thomas" wrote in message
news:uHGEc.38991$_5.7814@clgrps13...


MT Ummm..this indicates the burn is going speed things up. Am I
MT missing something?

Saturn is still accelerating Cassini even during the burn (e.g. the
spacecraft's speed would be over 31 km/sec without the burn).


God that's hauling ass! That is what they mean by orbital energy I guess.
This is what actually speeds up a craft when it sling shots around a
planet.
I guess it hits at some tangent to the orbital curve and picks up speed.


If the craft comes up behind the planet at X km/s (relative to the planet),
by symmetry it'll leave in front of the planet also at X km/s (relative to
the planet).

However (and here's the clever bit), if the planet is moving at Y km/s, the
craft will be moving at X+Y km/s relative to the Sun.
Since Saturn's average orbital speed is 9.5 km/s, if Cassini approached it
at 34km/s compared to the Sun, it would leave at 43km/s compared to the Sun.

You'll probably have read that Cassini did 2 fly-bys of Venus and one of the
Earth on its way to Saturn; each time it picked up a significant chunk of
speed - in this respect the inner planets are much better than the outer
ones, since Venus' average speed is 35km/s, compared to the relatively
sluggish pace of Saturn.

Hope this is clear.