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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 15:06 
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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 15:59 
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Seems like the "fuel" itself is a payload that could easily tolerate the extremely high g load of a rail gun launch. I wonder if some day fuel delivery sats will be resupplied with that method?

Tell me how you rail gun something into orbit.

If you shoot it "up", it goes away from the planet and then falls back towards it.

If you shoot it "out", it goes horizontally through a lot of atmosphere and the drag prevents it from reaching escape velocity.

If it changes direction in mid flight, well, then that's not a rail gun any more, you need rocket engines.

Mike C.


The idea is to impart much of the kinetic energy needed to reach orbit from sources on the ground. I assume some form of steering and guidance is used to transition from high incline to something appropriate for orbital insertion. Obviously if there is no internally generated thrust, most changes in the inclination would have to occur while still in the upper atmosphere. Alternatively there could be an ability to generate some thrust for modifying the trajectory.

AFaIK multiple entities have been looking at this for several years including NASA.

https://ntrs.nasa.gov/citations/19830026744

There are also some proposals to do a hybrid "rail launcher" and scramjet or rocket although I suspect the g force in those concepts is much lower than a electromagnetic rail gun.
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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 17:17 
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What might make even more sense are tanker sats that stay with the main satellite for long periods of time. They could have enormous fuel capacity, refuel the main sat tanks periodically, ...

What would make this advantageous versus just putting a big-ass fuel tank on the main sat in the first place?

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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 17:21 
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What might make even more sense are tanker sats that stay with the main satellite for long periods of time. They could have enormous fuel capacity, refuel the main sat tanks periodically, ...

What would make this advantageous versus just putting a big-ass fuel tank on the main sat in the first place?

They could be launched by separate boosters.

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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 17:58 
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They could be launched by separate boosters.

OK. Basically you are spec-ing out a multi-part satellite. i.e. something along the lines of the ISS.

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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 18:00 
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What would make this advantageous versus just putting a big-ass fuel tank on the main sat in the first place?

Don't know, but could be related to dynamics (too much fluid sloshing about), overall weight, or even fuel degradation over time.

Fuel may be only part of the problem, batteries aging out are also a problem for satellites. Not sure how you swap that out.

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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 18:24 
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What would make this advantageous versus just putting a big-ass fuel tank on the main sat in the first place?

Don't know, but could be related to dynamics (too much fluid sloshing about), overall weight, or even fuel degradation over time.

Fuel may be only part of the problem, batteries aging out are also a problem for satellites. Not sure how you swap that out.

Mike C.

Probably a "few" details yet to be worked out. :D
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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 18:36 
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At present, it seems the telescope could operate for about 20 years. That's a good long run, especially since it was designed in the 1990s and uses tech mostly from that era.

As we've noted earlier in this thread, several space telescopes that will incorporate newer sensors, computers, and other hardware are in various stages of development.

And when we start seeing the data from Webb, the discoveries are likely to prompt designs for or changes to new space-based observatories that will complement and then eventually supplant Webb.

By the 2040s, I suspect we'll have more than gotten our money's worth from Webb, even given its cost overruns and delays. Money that might be spent trying to extend Webb's working life is probably better allocated to those new efforts.

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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 19:39 
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Username Protected wrote:
I wonder if some day fuel delivery sats will be resupplied with that method?

Tell me how you rail gun something into orbit.

If you shoot it "up", it goes away from the planet and then falls back towards it.

If you shoot it "out", it goes horizontally through a lot of atmosphere and the drag prevents it from reaching escape velocity.

If it changes direction in mid flight, well, then that's not a rail gun any more, you need rocket engines.

So, “no” then.
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 Post subject: Re: James Web Telescope
PostPosted: 26 Jan 2022, 23:26 
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I looked at this some, along with [readacted] big tech company that was looking at the possibility of a rail-gun like launcher.

Nowhere close to practical. Insanely expensive no matter how you look at the numbers. The high launch accelerations mean only ultra rugged cargo. If you want to launch directly to orbit, you need to launch near horizontal - and spend a long time in the atmosphere at Mach 20 - losing a lot of your energy.

If you launch straight up, you still ned most of the total delta- v to get into an orbit, but now your engine and tanks need to survive 1000G accelerations.

The entire idea is silly. The fuel cost for rockets is a tiny fraction (< 1%) of the total cost, saving energy doesn't change the cost equation.

Same for the crazy spin-launch like ideas, and don't get me started on space elevators.


The only non- standard launch scheme that looks at all reasonable is air launch, where you drop a rocket from an aircraft at ~FL350. The thin air causes several things to scale better (including the engines) and the minimum practical rocket size is a lot smallr. For ~1 ton payloads to low earth orbit, it has the potential to be cheaper than conventional rockets. (Note, the extra energy from the altitude and velocity isn't important - unless you want to make a 2020s version of the B-70, where it starts to matter).






Username Protected wrote:
Tell me how you rail gun something into orbit.

If you shoot it "up", it goes away from the planet and then falls back towards it.

If you shoot it "out", it goes horizontally through a lot of atmosphere and the drag prevents it from reaching escape velocity.

If it changes direction in mid flight, well, then that's not a rail gun any more, you need rocket engines.

Mike C.


The idea is to impart much of the kinetic energy needed to reach orbit from sources on the ground. I assume some form of steering and guidance is used to transition from high incline to something appropriate for orbital insertion. Obviously if there is no internally generated thrust, most changes in the inclination would have to occur while still in the upper atmosphere. Alternatively there could be an ability to generate some thrust for modifying the trajectory.

AFaIK multiple entities have been looking at this for several years including NASA.

https://ntrs.nasa.gov/citations/19830026744

There are also some proposals to do a hybrid "rail launcher" and scramjet or rocket although I suspect the g force in those concepts is much lower than a electromagnetic rail gun.


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 Post subject: Re: James Web Telescope
PostPosted: 27 Jan 2022, 07:17 
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So little fuel was used to position Webb that a 20+ year mission is now expected! The red arrow is pointing to the James Webb telescope.
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 Post subject: Re: James Web Telescope
PostPosted: 27 Jan 2022, 23:02 
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Username Protected wrote:
Same for the crazy spin-launch like ideas, and don't get me started on space elevators.

Well, besides that the materials are 1-2 orders of magnitude away in the required strength to weight ratio, what you got against space elevators?


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 Post subject: Re: James Web Telescope
PostPosted: 28 Jan 2022, 00:23 
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Same for the crazy spin-launch like ideas, and don't get me started on space elevators.

Well, besides that the materials are 1-2 orders of magnitude away in the required strength to weight ratio, what you got against space elevators?


How about the 20-40 mile elastic stretching of the cable?
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 Post subject: Re: James Web Telescope
PostPosted: 28 Jan 2022, 23:01 
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Other than the impossible materials. The launch cost of putting >20,000 miles of impossible materials in orbit, the need to deal with the many-mile length change from the tides, the difficulty of building climbers that move fast enough to be useful (20,000 miles is a LONG way to travel - these are not at elevator speeds, the weather issues, the meteor issues, the need to dodge low earth orbit satellites, there are the really amazing failure modes to consider, from the warping around the planet to the pretty awesome slignshot possibilities.

Oh and it being an incredibly difficult way to do something we already know how to do.

But sure, if someone wants to fund it, it sounds like fun.



Username Protected wrote:
Same for the crazy spin-launch like ideas, and don't get me started on space elevators.

Well, besides that the materials are 1-2 orders of magnitude away in the required strength to weight ratio, what you got against space elevators?


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 Post subject: Re: James Web Telescope
PostPosted: 29 Jan 2022, 10:20 
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Aren't there also some orbital mechanics issues as well. If you run your unobtainium cable from the ground to a "station" in geostationairy orbit the mass of the cable not to mention the mass of any payload or "car" would be applying a force to the station with that force affecting the orbit of the station? Also I'd think that the car and payload would exert a lateral force on the cable when the car was moving vertically since the path of the car wouldn't be ballistic given the rotation of the Earth?

Username Protected wrote:
Other than the impossible materials. The launch cost of putting >20,000 miles of impossible materials in orbit, the need to deal with the many-mile length change from the tides, the difficulty of building climbers that move fast enough to be useful (20,000 miles is a LONG way to travel - these are not at elevator speeds, the weather issues, the meteor issues, the need to dodge low earth orbit satellites, there are the really amazing failure modes to consider, from the warping around the planet to the pretty awesome slignshot possibilities.

Oh and it being an incredibly difficult way to do something we already know how to do.

But sure, if someone wants to fund it, it sounds like fun.

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