On Apr/18/2017 at 7:08 AM, Alain Fournier wrote :
On Apr/17/2017 at 9:48 PM, Jeff Findley wrote :
In article , says...

On Apr/17/2017 à 12:07 PM, JF Mezei wrote :
On 2017-04-16 22:46, Fred J. McCall wrote:
Looks like NASA's first two launches of the SLS for their lunar tests
will be delayed by a year or more. That means SpaceX will almost
certainly be there before them.


The announcement of the first flight being manned may have more to do
with the delay than budgets.

That article had a link to a NASA web page which describes its concept
for Mars. That page does not paint Orion/SLS as sending man to Mars.

NASA wants to build ISS-2 in lunar orbit to test the transit ship
there.
So SLS/Orion act as shuttles to/from the vehicle in lunar orbit.

NASA admits Orion isn't big enough to being crews on months long
mission
to Mars and back.

If you will assemble a transit ship in lunar orbit, you might need
something like SLS to bring modules up there.

Assembling in LEO costs less in module launches, but more in fuel to
escape from Earth. Assembling in Lunar orbit costs more in launches of
modules, but less to escape earth/moon orbit. Does the balance tip
heavily on one of those or is it more or less even ?

It is cheaper to do most of your acceleration low in the gravity well.
You can read on the Oberth Effect, for instance:
https://en.wikipedia.org/wiki/Oberth_effect

How does a fly-by maneuver fit into this?

Jeff

I'm not sure of what you mean here. A gravity assist from the Moon is
too small to be important. If the Moon happens to be at the right place
at the right time, sure use it, but it isn't worth delaying the mission
for a lunar gravity assist.

A gravity assist from Earth gives a bigger push, but that would imply
an impossibly long journey for a manned mission. You would have to go
into solar orbit then, maybe two years later, meet Earth again and use
the push to get to Mars. You do save fuel, but it really isn't worth it.

I should add that you can use Earth's gravity to help get to Mars from
lunar orbit if you are using a high impulse rocket such as a chemical
rocket. From Lunar orbit, you lower your perigee to about 100 km above
Earth's surface. Then near perigee, you do your main rocket burn. You
get to Mars with less energy than if you would make a direct burn from
Lunar orbit directly to Mars. This is not quite what is normally called
a fly-by manoeuvre or a gravity assist. But you still improve your
efficiency by the Oberth effect.

If you are using a low impulse propulsion such as an ion-drive. Such
a manoeuvre would be counter productive.


Alain Fournier