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SLS launches likely delayed



 
 
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  #11  
Old April 18th 17, 02:48 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default SLS launches likely delayed

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
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #12  
Old April 18th 17, 12:06 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default SLS launches likely delayed

JF Mezei wrote:

On 2017-04-17 21:45, Jeff Findley wrote:

Those are all tests of individual components. The first "all up" test
will be EM-1.


Once the thing has been assembled and perhaps at the pad, don't they
spend mucho time doing integration tests and simulations, test engine
firings etc?


No, they do none of that. The 'take the vehicle to launch ready' that
SpaceX does is very unusual. Usually the first time the vehicle gets
fueled is when you're going to light it off and fly.


Once it gets to EM-1 with an actual launch day/time announced and it
gets to that date, doesn't that mean NASA is confident it has worked out
the bugs and that the thing won't blow up in its face?


They certainly hope they have, but there is only one way to be sure
that the all up vehicle works and that is to fly it.


With modern computer simulations, doesn't that allow NASA to perform a
lot of validation before first first flight?


You can only simulate the stuff you know about.

snip trip to the weeds


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
  #13  
Old April 18th 17, 12:08 PM posted to sci.space.policy
Alain Fournier[_3_]
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Posts: 548
Default SLS launches likely delayed

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.


Alain

  #14  
Old April 18th 17, 11:17 PM posted to sci.space.policy
Alain Fournier[_3_]
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Posts: 548
Default SLS launches likely delayed

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

  #15  
Old April 19th 17, 02:07 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default SLS launches likely delayed

In article , says...

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.


You're the one who brought up the Oberth effect. It's only applicable
if you're doing a gravity-assist fly-by maneuver. Otherwise, your
statement "It is cheaper to do most of your acceleration low in the
gravity well." makes no sense. From the page you cite:

In astronautics, a powered flyby, or Oberth maneuver, is a maneuver
in which a rocket falls into a gravitational well, and then
accelerates when its fall reaches maximum speed.

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.


Okay, then I really don't know why you brought it up.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #16  
Old April 19th 17, 02:12 AM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 2,307
Default SLS launches likely delayed

In article , says...
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.


Actually that is an example of a gravity assist fly-by maneuver. The
Oberth effect is just the technical name for it.

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


Agreed. From the simulations we did in our Orbital Mechanics class
(eons ago), you want the highest thrust you can get which also results
in a short duration when you're nearest the planet you're flying by.
NASA's proposed electric propulsion wouldn't work worth a crap.

The disadvantage of doing this maneuver near earth is that you have to
pass through the van-Allen radiation belts twice in order to perform the
maneuver. Not a good thing to do when NASA is going to try to do
everything possible to reduce radiation exposure to the astronauts on a
Mars mission.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #18  
Old April 19th 17, 12:23 PM posted to sci.space.policy
Alain Fournier[_3_]
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Posts: 548
Default SLS launches likely delayed

On Apr/19/2017 at 6:02 AM, Jeff Findley wrote :
In article m,
says...

Reality check question:

If you spend X energy from low Earth altitude/orbit to Moon orbit, does
Oberth effect claim you will get a value of energy greater than X
falling back from Moon to low earth altitude?


If you don't leave the earth/moon system, I don't think there is a "free
lunch" to be had here. In order for the Oberth effect to be present,
you have to do a parabolic fly-by maneuver. As in your velocity is
higher than escape going in and much higher than escape when going out.
The extra velocity you gain is "stolen" from the planet which you fly-
by. No fly-by, no Oberth effect.


No that is not what the Obert effect is. The Obert effect is due to
greater efficiency of a rocket burn deep in a gravity well than higher
in the gravity well. Suppose you are in an elliptic orbit with let's
say perigee at 200 km and apogee at 40,000 km. While you go up from
200 km to 40,000 km you lose speed. Then when you go down from 40,000
km to 200 km you gain speed. If you do a rocket burn at 200 km, you
gain yet more speed. After that burn, when you go back up, you will lose
less speed between 200 km and 40,000 km than on previous orbits because
you are going faster and therefore, you reach 40,000 km in less time
so gravity has less time to slow you down. So when you reach 40,000 km
you have the speed you had on previous orbits plus the additional
speed of your rocket burn at 200 km plus the additional speed due to
the Oberth effect, that is the additional speed due to you slowing down
less while going up. If you had done your rocket burn at 40,000 km
you would only get your speed plus your delta-v due to the rocket
burn.

You get that even if the planet was a rogue planet not around a star.
The fly-by gravity assist is a different thing. If you don't do a rocket
burn low in the gravity field of a lonely planet you don't really get
an extra push from going into the gravity field of that planet. You
come back out with the same speed you went in, just in another
direction. If the planet is around a star, you again get out with the
same speed relative to the planet, just in another direction. But that
can mean a greater speed relative to the star. For example, if you
had zero speed relative to the star, you had a large speed relative
to the planet. Now changing the direction of that large speed gives
you a large speed relative to the star. So you go from no speed relative
to the star to a large speed relative to the star with no rocket burn.

There is also another component to gravity assist, in that you actually
change very slightly the orbit of the planet, either stealing energy
from the planet or giving it some energy. That energy goes into the
spacecraft. This doesn't need any rocket burn at all. The Oberth effect
does require an acceleration deep in the gravity well.


Alain Fournier

  #19  
Old April 19th 17, 07:07 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default SLS launches likely delayed

Alain Fournier wrote:

On Apr/19/2017 at 6:02 AM, Jeff Findley wrote :
In article m,
says...

Reality check question:

If you spend X energy from low Earth altitude/orbit to Moon orbit, does
Oberth effect claim you will get a value of energy greater than X
falling back from Moon to low earth altitude?


If you don't leave the earth/moon system, I don't think there is a "free
lunch" to be had here. In order for the Oberth effect to be present,
you have to do a parabolic fly-by maneuver. As in your velocity is
higher than escape going in and much higher than escape when going out.
The extra velocity you gain is "stolen" from the planet which you fly-
by. No fly-by, no Oberth effect.


No that is not what the Obert effect is. The Obert effect is due to
greater efficiency of a rocket burn deep in a gravity well than higher
in the gravity well. Suppose you are in an elliptic orbit with let's
say perigee at 200 km and apogee at 40,000 km. While you go up from
200 km to 40,000 km you lose speed. Then when you go down from 40,000
km to 200 km you gain speed. If you do a rocket burn at 200 km, you
gain yet more speed. After that burn, when you go back up, you will lose
less speed between 200 km and 40,000 km than on previous orbits because
you are going faster and therefore, you reach 40,000 km in less time
so gravity has less time to slow you down. So when you reach 40,000 km
you have the speed you had on previous orbits plus the additional
speed of your rocket burn at 200 km plus the additional speed due to
the Oberth effect, that is the additional speed due to you slowing down
less while going up. If you had done your rocket burn at 40,000 km
you would only get your speed plus your delta-v due to the rocket
burn.

You get that even if the planet was a rogue planet not around a star.
The fly-by gravity assist is a different thing. If you don't do a rocket
burn low in the gravity field of a lonely planet you don't really get
an extra push from going into the gravity field of that planet. You
come back out with the same speed you went in, just in another
direction. If the planet is around a star, you again get out with the
same speed relative to the planet, just in another direction. But that
can mean a greater speed relative to the star. For example, if you
had zero speed relative to the star, you had a large speed relative
to the planet. Now changing the direction of that large speed gives
you a large speed relative to the star. So you go from no speed relative
to the star to a large speed relative to the star with no rocket burn.

There is also another component to gravity assist, in that you actually
change very slightly the orbit of the planet, either stealing energy
from the planet or giving it some energy. That energy goes into the
spacecraft. This doesn't need any rocket burn at all. The Oberth effect
does require an acceleration deep in the gravity well.


Perhaps I'm all screwed up here (hey, I'm retired and it's early
morning), but I'm missing some things in this discussion.

1) 'Gravity slingshot' isn't just about changing the direction of your
velocity vector, is it? In a lot of cases these are designed to
'steal' orbital velocity from the planet by 'falling' from the back
side of the planet's velocity vector so you get 'dragged' along as you
fall inward, aren't they? So you get increased velocity 'free'.

2) Oberth Effect really only applies for orbits in the same plane,
doesn't it? If you want to do something like an orbital plane change,
those are actually 'easier' and 'more efficient' if you are higher up
(the opposite of Oberth Effect) and they will actually raise the
apoapsis of the orbit and do the plane change burn at maximum distance
from the planet and then recircularize.

Am I wrong?


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw
  #20  
Old April 20th 17, 12:30 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 548
Default SLS launches likely delayed

Le Apr/19/2017 à 2:07 PM, Fred J. McCall a écrit :
Alain Fournier wrote:

On Apr/19/2017 at 6:02 AM, Jeff Findley wrote :
In article m,
says...

Reality check question:

If you spend X energy from low Earth altitude/orbit to Moon orbit, does
Oberth effect claim you will get a value of energy greater than X
falling back from Moon to low earth altitude?

If you don't leave the earth/moon system, I don't think there is a "free
lunch" to be had here. In order for the Oberth effect to be present,
you have to do a parabolic fly-by maneuver. As in your velocity is
higher than escape going in and much higher than escape when going out.
The extra velocity you gain is "stolen" from the planet which you fly-
by. No fly-by, no Oberth effect.


No that is not what the Obert effect is. The Obert effect is due to
greater efficiency of a rocket burn deep in a gravity well than higher
in the gravity well. Suppose you are in an elliptic orbit with let's
say perigee at 200 km and apogee at 40,000 km. While you go up from
200 km to 40,000 km you lose speed. Then when you go down from 40,000
km to 200 km you gain speed. If you do a rocket burn at 200 km, you
gain yet more speed. After that burn, when you go back up, you will lose
less speed between 200 km and 40,000 km than on previous orbits because
you are going faster and therefore, you reach 40,000 km in less time
so gravity has less time to slow you down. So when you reach 40,000 km
you have the speed you had on previous orbits plus the additional
speed of your rocket burn at 200 km plus the additional speed due to
the Oberth effect, that is the additional speed due to you slowing down
less while going up. If you had done your rocket burn at 40,000 km
you would only get your speed plus your delta-v due to the rocket
burn.

You get that even if the planet was a rogue planet not around a star.
The fly-by gravity assist is a different thing. If you don't do a rocket
burn low in the gravity field of a lonely planet you don't really get
an extra push from going into the gravity field of that planet. You
come back out with the same speed you went in, just in another
direction. If the planet is around a star, you again get out with the
same speed relative to the planet, just in another direction. But that
can mean a greater speed relative to the star. For example, if you
had zero speed relative to the star, you had a large speed relative
to the planet. Now changing the direction of that large speed gives
you a large speed relative to the star. So you go from no speed relative
to the star to a large speed relative to the star with no rocket burn.

There is also another component to gravity assist, in that you actually
change very slightly the orbit of the planet, either stealing energy
from the planet or giving it some energy. That energy goes into the
spacecraft. This doesn't need any rocket burn at all. The Oberth effect
does require an acceleration deep in the gravity well.


Perhaps I'm all screwed up here (hey, I'm retired and it's early
morning), but I'm missing some things in this discussion.

1) 'Gravity slingshot' isn't just about changing the direction of your
velocity vector, is it? In a lot of cases these are designed to
'steal' orbital velocity from the planet by 'falling' from the back
side of the planet's velocity vector so you get 'dragged' along as you
fall inward, aren't they? So you get increased velocity 'free'.


Changing the direction of your velocity vector relatively to one
body (let's say a planet) can be very much the same thing as
increasing your orbital velocity relatively to another body
(let's say the star). And changing the velocity vector is done by
changing (so very slightly) the velocity vector of the planet.
So the two are kind of the same thing. (In a previous post, I
was implying they are different, I was wrong.)

If you have zero velocity relatively to a star, you can have
a high velocity relatively to a planet orbiting that star in
the direction opposite to the orbital motion of the planet.
If you change that to a high velocity relatively to the planet
in the same direction as the orbital motion of the planet,
you went from zero velocity relatively to the star to twice
the orbital velocity of the planet relatively to the star.
And you are on your way to being ejected from the solar system.

2) Oberth Effect really only applies for orbits in the same plane,
doesn't it? If you want to do something like an orbital plane change,
those are actually 'easier' and 'more efficient' if you are higher up
(the opposite of Oberth Effect) and they will actually raise the
apoapsis of the orbit and do the plane change burn at maximum distance
from the planet and then recircularize.


Yes and no, depends on your point of view. In a previous post I
explained how you can use the Oberth Effect while staying in orbit
in the gravity well. I was giving that example only to show the
difference between the Oberth Effect and gravity slingshot. That
is not the classical example of using the Oberth Effect. Usually
when you use the Oberth Effect, you start outside a gravity well,
go deep into it, fire your rockets and comeback out much faster
than if you had simply fired your rockets without a gravity well.

You could do that to do a plane change. Let's say you want to
go into a polar orbit around the Sun at a distance similar to
Venus' orbital distance. You can go deep into Venus' gravity
well, fire your rockets deep in the gravity well, and use
that to make your plane change. I'm not saying that is the
best way to do it. One would have to evaluate different scenarios
to know what is best. A good candidate would be to go to
Jupiter, use a gravity assist from Jupiter to get into a
polar orbit with perigee near Venus (no need for a rocket
burn here, Jupiter is massive enough to do the plane change
without using the Oberth Effect). Then use the Oberth effect
at Venus to circularize your orbit.

So, yes you are correct. If you want to do a plane change
in a simple gravity well, you do your burn high up, not low
in the gravity well as when using the Oberth Effect. But
if you are in a gravity well with secondary gravity wells,
you can very well use the Oberth Effect in those secondary
gravity wells to help you in your plane change.


Alain Fournier

 




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