top ten reasons there'll be faster progress

In article ,
Fred J. McCall wrote:

:Again, the atmosphere is near vacuum at 100 kilometers.

There is a difference between "near vacuum" and "vacuum", particularly
at the speeds being discussed.

Not a significant difference at that altitude, unless you're planning a
long-term orbit.

Perhaps you think that a rendezvous at close to 9,000 miles per hour
with something NOT traveling in a straight line is simple.

Why do you keep harping on the ground speed? It doesn't matter. Both I
and my house are whizzing around the Sun at 30 km/sec (to say nothing of
how fast we're whizzing around the center of the Earth), yet I'm able to
rendezvous with it easily at the end of each day.

The only speed that matters is the relative speed, and how long a window
you have to bring that relative speed to zero and get in the right
position. Achieving the right position and velocity in a short window
may be difficult, but not because of how fast the ground is going by 100
km below.

How difficult that rendezvous will be remains to be seen. But I think
the coming crop of suborbital craft will give us good data on that.
They'll be flying a LOT -- several launches per week or so (combined) --
and I bet they'll keep good data on their trajectories. A company
interested in the rotovator approach could try flying computer-corrected
trajectories for a while, and see if hitting a narrow
space/time/velocity box is possible. If not, they abandon the rotovator
idea (at least for that vehicle). If so, they move forward.

Best,
- Joe

Fred J. McCall wrote:

Hop David wrote:

:Fred J. McCall wrote:
:
: Hop David wrote:
:
: :Fred J. McCall wrote:
: :
: : (Wayne Throop) wrote:
: :
: : :: Fred J. McCall
: : :: For which you have to do a 4 km/s rendezvous and cargo transfer, with
: : :: your surface-to-rotovator vehicle having to precisely match the path
: : :: of the rotovator tip for however long it takes to get the cargo
: : :: transfer accomplished.
: : :
: : :How long does it take to engage a hook and fire some explosive bolts?
: : :Or whatever similar system there may be.
: :
: : Engaging a hook may take more time than you have. Changing the
: : aerodynamics of your vehicle that radically in hypersonic flight is
: : going to be ugly.
: :
: : :Basically, you're talking a rendesvous vaguely similar to a
: : :mid-air refuel, and once you've got the end of the cable docked
: : :to the cargo module you're boosting, Bob's your uncle.
: :
: : Very vaguely similar, since we only do that sort of thing at
: : relatively low speeds. We don't do supersonic IFRs, much less
: : hypersonic ones.
: :
: :Again, the atmosphere is near vacuum at 100 kilometers.
:
: There is a difference between "near vacuum" and "vacuum", particularly
: at the speeds being discussed.
:
:The "near vacuum" that burns up stuff traveling at mach 25 or higher is
:the mesosphere which ends at about 80 km.
:
:I'm skeptical that traveling 4 km/sec at 100 km is a big aerodynamic
roblem.
:
:Why don't you provide some cites?

You're the one claiming it's no issue.

I don't recall making such a claim. I don't know how much of issue it
will be. But the paper cited was written by engineers at Boeing, Robert
Forward and a university prof. I think those guys are aware of
aerodynamic issues and probably chose 100 km as their rendesvous
altitude for a reason.

I do recall you claiming "Changing the aerodynamics of your vehicle that
radically in hypersonic flight is going to be ugly."

Why don't you? Keep in mind

Even though I don't know much about it, I find this a fascinating topic.
Here a couple of things to read:

http://www.nasa.gov/centers/dryden/p...ain_H-2553.pdf

Figure 7 page 18 gives information on shuttle entry. It shows the
slowing and heating start at around 80 kilometers altitude.

Also interesting is

http://www.ux1.eiu.edu/~cfjps/1400/atmos_struct.html

Neither of these settles the issue but they're a start.

Do you know of any relevant papers? If you teach me something new,
you'll have my gratitude.

in your analysis that your vehicle is going to have to be under
acceleration to match the movement of your rotovator tip.


That is a different issue. But I'll talk about it. In the Boeing paper
the rotavator tip is feeling 1.6 gees from centrifugal force. At
rendesvous there's also nearly a full gee of acceleration coming from
the earth for a total of 2.6 gees. So it seems to me the plane would
have to maintain an acceleration of 2.6 gees during the rendesvous. A
minute of this acceleration would be a delta vee of about 1.5 km/s. If
the plane stays beside the rotavator tip for three minutes, it'd have
used enough fuel to change it's 3.7 km/sec suborbital velocity to an 8.2
km/sec orbital velocity.

It would have to be a very quick rendesvous or the expenditure of fuel
would negate the advantages of suborbital pickups. To be frank, I do see
this as a problem. Whether it's a show stopper, I don't know.

Hop

In article ,
Hop David wrote:

That is a different issue. But I'll talk about it. In the Boeing paper
the rotavator tip is feeling 1.6 gees from centrifugal force. At
rendesvous there's also nearly a full gee of acceleration coming from
the earth for a total of 2.6 gees. So it seems to me the plane would
have to maintain an acceleration of 2.6 gees during the rendesvous.

That's an overly conservative assumption, I think. Assuming the
rendezvous is quick (i.e. as easy as touching a big electromagnet to the
craft or cargo's docking plate), you only have to match the tangential
velocity of the rotovator tip -- not its actual rotation. To even
partly match its rotation, you'd have to actually start out above the
rendezvous altitude and be executing a dive-and-pull-up, which would be
very difficult to do with only a RCS.

You also don't have to add 1G for Earth's gravity, unless you plan on
lounging around the rendezvous area for an extended period of time. But
I don't think that's what most proponents are assuming. I think the
goal is to not be under ANY acceleration during the rendezvous; just at
the top of your suborbital trajectory, in freefall, at precisely the
right time and place when the rotovator tip comes along.

It would have to be a very quick rendesvous or the expenditure of fuel
would negate the advantages of suborbital pickups. To be frank, I do see
this as a problem. Whether it's a show stopper, I don't know.

That's pretty much my feeling about it too.

Best,
- Joe

Joe Strout wrote:

:In article ,
: Fred J. McCall wrote:
:
: :Again, the atmosphere is near vacuum at 100 kilometers.
:
: There is a difference between "near vacuum" and "vacuum", particularly
: at the speeds being discussed.
:
:Not a significant difference at that altitude, unless you're planning a
:long-term orbit.

Which means you would get no control advantage from it, either. That
may actually make it HARDER.

However, that actually turns out not to be true. At an altitude of 60
kilometers that 4 k/sec velocity amounts to Mach 12. If there is a
local speed of sound and I can get Mach numbers, there's enough air to
worry about at Mach 12 relative to the air mass (which is sort of tied
to that 'ground speed').

: Perhaps you think that a rendezvous at close to 9,000 miles per hour
: with something NOT traveling in a straight line is simple.
:
:Why do you keep harping on the ground speed? It doesn't matter. Both I
:and my house are whizzing around the Sun at 30 km/sec (to say nothing of
:how fast we're whizzing around the center of the Earth), yet I'm able to
:rendezvous with it easily at the end of each day.

Both you and your house are in the same orbit and have a relatively
low ground speed with respect to the Earth and it's surrounding air
mass, which is also in the same orbit.

:The only speed that matters is the relative speed,

With lots of relatives. You have to worry about speed with regard to
the air mass (which you still have enough of to have a speed of sound
and hence are going to have to deal with interference between two
hypersonic shockwave fronts). You also have to worry (a lot) about
relative speed between your ascending vehicle and your rotovator tip.
Since the rotovator tip is in an accelerated frame, matching
velocities with it is not going to be trivial.

You want to treat this as if it is an orbital rendezvous. It won't
work like that.

:and how long a window
:you have to bring that relative speed to zero and get in the right
osition. Achieving the right position and velocity in a short window
:may be difficult, but not because of how fast the ground is going by 100
:km below.

Yes, ignoring air is a nice simplifying factor, but you don't get to
do it in the real world. If you want to ignore the air, you need to
get somewhat higher than the edge of space.

:How difficult that rendezvous will be remains to be seen. But I think
:the coming crop of suborbital craft will give us good data on that.
:They'll be flying a LOT -- several launches per week or so (combined) --
:and I bet they'll keep good data on their trajectories. A company
:interested in the rotovator approach could try flying computer-corrected
:trajectories for a while, and see if hitting a narrow
:space/time/velocity box is possible. If not, they abandon the rotovator
:idea (at least for that vehicle). If so, they move forward.

Uh, that's not how you do things. First you look at the numbers. Then
you try to simulate things (and we already have a pretty good idea of
what the atmosphere behaves like). Then you start costing things out.

But before you do any of that, you look at the business case. There
is none for a rotovator, so far as I can tell. By the time there is,
I think conventional access to space will be so cheap that nobody will
even consider such a thing.

--
"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

"Jordan" wrote:

Back in the 1960's - early 70's, there were only two countries engaging
in orbital flight, America and Russia. Thus when Russia conceded,
America had no competitor engaging in Lunar exploration. This made it
very easy for Nixon to cancel the program for what amounted to personal
spite against Kennedy (yes, I know Kennedy had been dead for 9 years,
Nixon still felt the animosity).

This paragraph alone, your other bat-in-the-belfry mutterings
nonwithstanding, shows your near total disconnect from reality.

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL

Hop David wrote:

:Fred J. McCall wrote:
:
: Hop David wrote:
:
: :Fred J. McCall wrote:
: :
: : Hop David wrote:
: :
: : :Fred J. McCall wrote:
: : :
: : : (Wayne Throop) wrote:
: : :
: : : :: Fred J. McCall
: : : :: For which you have to do a 4 km/s rendezvous and cargo transfer, with
: : : :: your surface-to-rotovator vehicle having to precisely match the path
: : : :: of the rotovator tip for however long it takes to get the cargo
: : : :: transfer accomplished.
: : : :
: : : :How long does it take to engage a hook and fire some explosive bolts?
: : : :Or whatever similar system there may be.
: : :
: : : Engaging a hook may take more time than you have. Changing the
: : : aerodynamics of your vehicle that radically in hypersonic flight is
: : : going to be ugly.
: : :
: : : :Basically, you're talking a rendesvous vaguely similar to a
: : : :mid-air refuel, and once you've got the end of the cable docked
: : : :to the cargo module you're boosting, Bob's your uncle.
: : :
: : : Very vaguely similar, since we only do that sort of thing at
: : : relatively low speeds. We don't do supersonic IFRs, much less
: : : hypersonic ones.
: : :
: : :Again, the atmosphere is near vacuum at 100 kilometers.
: :
: : There is a difference between "near vacuum" and "vacuum", particularly
: : at the speeds being discussed.
: :
: :The "near vacuum" that burns up stuff traveling at mach 25 or higher is
: :the mesosphere which ends at about 80 km.
: :
: :I'm skeptical that traveling 4 km/sec at 100 km is a big aerodynamic
: roblem.
: :
: :Why don't you provide some cites?
:
: You're the one claiming it's no issue.
:
:I don't recall making such a claim. I don't know how much of issue it
:will be. But the paper cited was written by engineers at Boeing, Robert
:Forward and a university prof. I think those guys are aware of
:aerodynamic issues and probably chose 100 km as their rendesvous
:altitude for a reason.

It's the traditional 'edge of space'. People like round numbers.
However, there is still some amount of air there (so that 4 km/sec
amounts to a bit over Mach 12 given the pressure altitude). If
there's enough air to have a speed of sound it seems to me that being
hypersonic, no matter how thin the air, is going to give you a
shockwave to deal with.

:I do recall you claiming "Changing the aerodynamics of your vehicle that
:radically in hypersonic flight is going to be ugly."

Think about it. I'm at Mach 12, which means I've got a nice
hypersonic shockwave going. What do YOU think a sudden change in
vehicle configuration is going to do?

: Why don't you? Keep in mind
:
:Even though I don't know much about it, I find this a fascinating topic.
:Here a couple of things to read:
:
:http://www.nasa.gov/centers/dryden/p...ain_H-2553.pdf
:
:Figure 7 page 18 gives information on shuttle entry. It shows the
:slowing and heating start at around 80 kilometers altitude.

Actually that's not how I read it. Look at the descent profile on 18.
The reason peak heating is at 80 km is because they scream down to 80
km in the first few minutes of descent and then hold there because
they're getting to their maximum desired heat load until they lose
more speed.

:Also interesting is
:
:http://www.ux1.eiu.edu/~cfjps/1400/atmos_struct.html
:
:Neither of these settles the issue but they're a start.
:
o you know of any relevant papers? If you teach me something new,
:you'll have my gratitude.
:
: in your analysis that your vehicle is going to have to be under
: acceleration to match the movement of your rotovator tip.
:
:
:That is a different issue. But I'll talk about it. In the Boeing paper
:the rotavator tip is feeling 1.6 gees from centrifugal force. At
:rendesvous there's also nearly a full gee of acceleration coming from
:the earth for a total of 2.6 gees. So it seems to me the plane would
:have to maintain an acceleration of 2.6 gees during the rendesvous. A
:minute of this acceleration would be a delta vee of about 1.5 km/s. If
:the plane stays beside the rotavator tip for three minutes, it'd have
:used enough fuel to change it's 3.7 km/sec suborbital velocity to an 8.2
:km/sec orbital velocity.
:
:It would have to be a very quick rendesvous or the expenditure of fuel
:would negate the advantages of suborbital pickups. To be frank, I do see
:this as a problem. Whether it's a show stopper, I don't know.

I didn't work the numbers, but intuitively I thought that you wouldn't
have to push a whole lot harder to get to orbit once you expended fuel
to match the rotovator tip for long enough to do anything. It may not
be a show stopper, but is there really enough margin there for it to
be worth the added issues of needing a specialized vehicle and cargo
transfer mechanism?

Again, I think that by the time we can be seriously considering
something like a rotovator that putting mass on orbit will be so cheap
it won't be worth building one.

--
"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

:: Hop David
:: In the Boeing paper the rotavator tip is feeling 1.6 gees from
:: centrifugal force. At rendesvous there's also nearly a full gee of
:: acceleration coming from the earth for a total of 2.6 gees. So it
:: seems to me the plane would have to maintain an acceleration of 2.6
:: gees during the rendesvous.

: Joe Strout
: That's an overly conservative assumption, I think. Assuming the
: rendezvous is quick (i.e. as easy as touching a big electromagnet to
: the craft or cargo's docking plate), you only have to match the
: tangential velocity of the rotovator tip -- not its actual rotation.

Two points. The rotavator itself, as a whole, is freefalling.
Therefore, relative to freefall, you need only 1.6g. Relative to
level flight, you need .6g.

More importantly, you don't necessarily need to match velocity with the
tip, especially not vertical velocity. So you don't need the .6g at
all; possibly not even the 1 (though it seems advisable). Consider the
retreival mechanism that's been used, with a balloon lofting a cable,
which is hooked on the fly by a special-purpose plane, lifting packages
or people to be reeled in. I do not pretend that solves the problem as
a whole. But I don't think the matching of velocities is as big a deal
as all that. Hypersonic shockwaves seem to me to be the larger problem.
The cabletip is leaving a shockwave you need to fly through. nd it limits
what you can stick out into the airstream to hook the cabletip. Mind you,
that doesn't prove it's impossible; there are shockwaves, and shockwaves,
and a shockwave in very rarified gas won't apply as much force, or as
much problems from turbulence. But it seems the larger problem.


Wayne Throop http://sheol.org/throopw

Derek Lyons wrote:
"Jordan" wrote:

Back in the 1960's - early 70's, there were only two countries engaging
in orbital flight, America and Russia. Thus when Russia conceded,
America had no competitor engaging in Lunar exploration. This made it
very easy for Nixon to cancel the program for what amounted to personal
spite against Kennedy (yes, I know Kennedy had been dead for 9 years,
Nixon still felt the animosity).

This paragraph alone, your other bat-in-the-belfry mutterings
nonwithstanding, shows your near total disconnect from reality.

???

It's pretty well historically attested that Nixon hated Kennedy, and
that this hatred persisted past Kennedy's death. It's semi-speculative
that this hatred was one of Nixon's reasons for cancelling Apollo.
However it's difficult to explan otherwise why the program was not only
cancelled but cancelled in such a way as to make it difficult for a
successor to restart. Saturn V rockets already produced were broken
up, and blueprints either intentionally destroyed or stored in ways not
conducive to protecting them. This is not the normal way of ending a
rocket production program!

The proximate reason Nixon hated Kennedy was because he believed, with
much justice, that Kennedy stole the 1960 election. The deeper reason
why Nixon hated Kennedy is that he considered him a spoiled shallow
rich kid with no actual understanding of policy issues, who was
throughout his life cut breaks by everyone because he was good-looking
and well-spoken. Nixon continued to complain about this (privately, of
course) into the 1970's (*).

If you want to criticize the other things I've said, have the guts to
name them -- simply referring to them as "bat-in-the-belfry mutterings"
and dismissing them unspoken is an argumentative technique likely to
work only on the exceedingly simple-minded. But then, I guess everyone
judges others by their own example.

- Jordan

(*) Please don't be dim enough to ask me how we know what Nixon
complained of privately in the early 1970's ...

On 3 Jul 2006 11:41:32 -0700, in a place far, far away, "Jordan"
made the phosphor on my monitor glow in such
a way as to indicate that:


Derek Lyons wrote:
"Jordan" wrote:

Back in the 1960's - early 70's, there were only two countries engaging
in orbital flight, America and Russia. Thus when Russia conceded,
America had no competitor engaging in Lunar exploration. This made it
very easy for Nixon to cancel the program for what amounted to personal
spite against Kennedy (yes, I know Kennedy had been dead for 9 years,
Nixon still felt the animosity).

This paragraph alone, your other bat-in-the-belfry mutterings
nonwithstanding, shows your near total disconnect from reality.

???

It's pretty well historically attested that Nixon hated Kennedy, and
that this hatred persisted past Kennedy's death. It's semi-speculative
that this hatred was one of Nixon's reasons for cancelling Apollo.

It's completely speculative, and based on bad data, since Nixon didn't
cancel Apollo. Johnson did.

However it's difficult to explan otherwise why the program was not only
cancelled but cancelled in such a way as to make it difficult for a
successor to restart. Saturn V rockets already produced were broken
up, and blueprints either intentionally destroyed or stored in ways not
conducive to protecting them.

This is urban myth.

In article ,
Fred J. McCall wrote:

Joe Strout wrote:

:In article ,
: Fred J. McCall wrote:
:
: :Again, the atmosphere is near vacuum at 100 kilometers.
:
: There is a difference between "near vacuum" and "vacuum", particularly
: at the speeds being discussed.
:
:Not a significant difference at that altitude, unless you're planning a
:long-term orbit.

Which means you would get no control advantage from it, either. That
may actually make it HARDER.

However, that actually turns out not to be true. At an altitude of 60
kilometers that 4 k/sec velocity amounts to Mach 12.

And this relates to conditions at an altitude of 100 kilometers... how,
exactly?

But before you do any of that, you look at the business case. There
is none for a rotovator, so far as I can tell. By the time there is,
I think conventional access to space will be so cheap that nobody will
even consider such a thing.

Well, I hope you're right about that latter point. I doubt it, but I
hope so.

Best,
- Joe