Author Topic: Space Elevator Initiative  (Read 5695 times)

Offline Paul451

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Re: Space Elevator Initiative
« Reply #20 on: 06/15/2018 04:57 PM »
SEI,

Re: Pick-up and release.

Given that you have a >100km tether on the "OPUM", and it's rotating during pick-up fast enough for the tip-velocity to counter orbital-velocity, then why wouldn't it just continue to rotate around a full circle, and release the payload at orbital+rotational velocity. It could fling the payload well beyond escape velocity (and by adjusting the release radius, any transfer orbit under that. Eliminating transfer burns between OPUM and Dock, and Dock and Summit.) The concept in the video requires the tip to accelerate to ~7km/s, pickup the payload, then decelerate 7km/s again.

[edit: fixed the acronym and added the bit about transfer burns.]

Re: Lake Impossible.

There's no point in having the base move. At 50 or more km height, the top would be able to move many kilometres back and forth even if the base is static. Indeed, preventing such movement from lower altitude winds would be a bigger issue. Which means that you'd want the tether under tension to prevent wobbles, which means you need positive net buoyancy and a strongly anchored base.



If you can get a floating structure that extends up to 60 km, extending it down a few extra kilometers will be many, many times easier than literally building a mountain.

Especially when the connections between the platforms already involves tethers tens of kilometres long.

Re: Thermal/re-entry issues for the pickup tether.

The pickup seems to be clumsily based on a rotovator. So the entry speed is enormously reduced, sub-sonic ideally, and is almost entirely vertical (the horizontal component mostly cancelled out. Mostly.) Of course, that doesn't explain the orbital mechanics shown in the video.
« Last Edit: 06/15/2018 05:11 PM by Paul451 »

Offline Paul451

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Re: Space Elevator Initiative
« Reply #21 on: 06/15/2018 05:05 PM »
Off-topic aside:



Re: Orbital Ring.

Ring spins at orbital velocity while structure stays stationary.

Nope. The ring has to spin faster than orbital velocity, or there's no momentum to transfer to the stationary structure.



Re: Orbital Ring.

As one example, your ring needs to be [...] have a very large tension on it to support the excess speed it needs to support the station and remain in orbit

No, the superorbital ring isn't under tension. It doesn't even need to be solid and is usually modelled as a particle stream. (Indeed, I'm not sure that a solid ring would be a good idea.)

[edit: typo]
« Last Edit: 06/15/2018 05:14 PM by Paul451 »

Offline SEI

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Re: Space Elevator Initiative
« Reply #22 on: 06/17/2018 08:18 AM »
Thank you meberbs and Paul 451,

A lot to think about indeed.
This video is now unfortunately too old, so the whole 100 km tether and the pick-up
aren't relevant anymore.  I will update this in the next video after having isolated
a few things first. I'm still investigating that at this point and will share.

Regarding the flinging of the pay-load, the original idea was to 'elevate' the pay-load
along the tether. The SPACE DOCK stage is there really as a platform for anything that needs
to be in low-orbit, such as satellites etc... The SUMMIT is for space travel.
You have introduced an interesting concept though, maybe some pay-loads just simply by-pass
the SPACE DOCK and go straight to the SUMMIT when necessary.

It hasn't been discussed thoroughly yet but there will definitely be room for figuring out
the transition between OPUM - DOCK - SUMMIT.

In the interest of methodology, I would like to compartmentalize the discussion a little bit.
Here are my thoughts/questions regarding firstly the BASE so that we can reach some sort of
consensus:

The mountain of waste:

The cost as far as I have calculated is very high and not necessarily eco-friendly which out-ways
the benefit of a communal landfill. I am however, still investigating a few options, mainly pipelines.
The thoughts have been based on all the garbage converging to a single point on the planet,
but perhaps multiple 'space elevators' might relax that eventually.
In any case, meberbs is right regarding the actual gain of creating a mountain versus dropping
more cable.

The lake:

I admit, I only introduced a lake because I had seen it in other videos. I assumed that it was
to allow for the base to have some wiggle room. I do like your idea of having to anchor the BASE
which was part of my next study. If the solution is to have an anchored base, then perhaps the idea
of a mountain might have some new purpose, such as to get as close as possible to the stratosphere
where the winds are more stable.

Offline meberbs

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Re: Space Elevator Initiative
« Reply #23 on: 06/17/2018 03:29 PM »
The mountain of waste:

The cost as far as I have calculated is very high and not necessarily eco-friendly which out-ways
the benefit of a communal landfill. I am however, still investigating a few options, mainly pipelines.
The thoughts have been based on all the garbage converging to a single point on the planet,
but perhaps multiple 'space elevators' might relax that eventually.
In any case, meberbs is right regarding the actual gain of creating a mountain versus dropping
more cable.
I have nothing specific to add right now, but I just want to thank you for actually listening to advice. All too frequently people come by with ideas, asking for thoughts, but end up being completely inflexible when confronted with issues with their plans.

Offline Paul451

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Re: Space Elevator Initiative
« Reply #24 on: 06/17/2018 07:51 PM »
You have introduced an interesting concept though, maybe some pay-loads just simply by-pass
the SPACE DOCK and go straight to the SUMMIT when necessary.

You might have missed my point. I wasn't saying to eliminate the other orbital stations, I said to use the tether to eliminate the rocket burn that is required to move the payload to another orbit. Any orbital change requires delta-v. Using the tether's velocity to not only capture, but also release, allows you to "cheat". (You can also use smaller rotovators at the higher orbits to capture the payload, avoiding even the small circularisation burns. Similarly, a rotovator in low-lunar orbit can exchange payloads between TLI and the lunar surface.)

Your concept seems to be recreating the rotovator concept, so you might as well take advantage of it. (Reading up on rotovators will give you a short-cut to understanding the pros & cons, and the calculations used.) The thing added by your concept is the floating platform at intercept altitude. Most use high-altitude aircraft or suborbital rockets to reach the intercept height.

Offline meberbs

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Re: Space Elevator Initiative
« Reply #25 on: 06/17/2018 09:21 PM »
I wasn't saying to eliminate the other orbital stations, I said to use the tether to eliminate the rocket burn that is required to move the payload to another orbit.
Just as a point of clarification, depending on design, this eliminates the need for chemical rocket burns. Some system is needed to keep this type of apparatus in orbit and spinning. As long as the mass of the payload is not too large a fraction of the on-orbit system, this allows a lower thrust, but more efficient propulsion method.

Offline SEI

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Re: Space Elevator Initiative
« Reply #26 on: 06/18/2018 10:59 AM »
I see.
I'm not entirely sure yet if this will apply to the new design of the OPUM stage.

Rotovators are definitely interesting and they may be a good solution for
the SPACE DOCK and SUMMIT stages to send and receive.

Offline alexterrell

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Re: Space Elevator Initiative
« Reply #27 on: 06/18/2018 12:09 PM »
In terms of balloons ...

This sci fi short story: http://space.nss.org/settlement/MikeCombs/bridge.htm
Has balloons holding up a linear accelerator at an altitude of 24km.

This was written before SpaceX and well before BFR which has taken the steam out of elevators etc - as in $100/kg is good enough.

Offline Paul451

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Re: Space Elevator Initiative
« Reply #28 on: 06/19/2018 06:05 AM »
Just as a point of clarification, [...] Some system is needed to keep this type of apparatus in orbit and spinning.

Yeah, but that applies to any momentum exchange tether, including SEI's. So I assume it'll already have something. Flinging a payload still reduces your fuel requirements by 80% or more.

Offline SEI

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Re: Space Elevator Initiative
« Reply #29 on: 06/19/2018 03:54 PM »
Here are some thoughts so far for the BASE and the first floor (20KM):

-Base is anchored down at altitude of 5 km                           
-cables go to 20Km high where they meet the first floor


__________________________________________
1) The balloons would be geodesic spheres:

The spheres would be made of some graphene-based material.
The sphere would be a double-layered geodesic sphere with
hydrogen filling the 'sandwiched' layer
Inside the sphere would be a partial vacuum/hydrogen mix
to reduce implosion risk.

--------
Equation to determine neutral buoyancy: R = 3TD/A
R: radius cm
T: skin thickness cm
D: Density = (2.2g/cm3)
A: Density air

R = 3 X 0.1 X 2.2 / 1.2 X 10^-3 = 550 cm

a vacuum sphere with a diameter of 11m, 0.1 cm's worth of
graphene shell at sea level would be neutrally buoyant.

--------
Now, Assuming:
30 M diameter geodesic vacuum sphere:
-0.1 cm worth of Graphene for double-layer
-20Km altitude, the winds are calmer.

Volume of sphere: V = 4/3pi r3
air displaced at 20 km = 0.088

LIFT = 1244.07 kg

--------
Rounding off:

-30 M sphere of hydrogen would weigh at sea level : 
 14137.17 X 0.083 = 1173.385 kg   
 lift would only be -> 70 kg

-The exact vacuum/hydrogen ratio or amount of graphene needed
 are unknown to me at this point in time but let's assume a
 contingency number of 20% which would bring the lift capacity
 of the geodesic sphere to:

LIFT = 1,000 kg (a round-number starting point for now)

--------
Average lift of column of geodesic spheres from 5-20km

at 05 km, lift = 10404.98  kg
at 10 km, lift =  5838.66  kg
at 15 km, lift =  2742.62  kg   
at 20 km, lift =  1244.07  kg

average lift = 5057.5825
If we remove 20% contingency and round off
average lift = 4000 KG

if
15000 / 30 = 500 spheres stack
then
500 spheres with average lift capacity of 4000kg

COLUMN OF GEODESIC SPHERES LIFT = 2000 tons from 5-20km


_______________________________
2) The weight of the structure:


-weight of cable:
   -If Kevlar:
      kevlar = 8 g/cm^3 = 8000000 = 8 tons / m^3
   
      1 cable size = 0.1 X 0.1 X 15,000 = 150 m^3
      1200 tons of kevlar
      Assume 5 cables (unknown):
      6000 tons of kevlar

   -If graphene:
      graphene = 2.2 g/cm^3 = 2200000 = 2.2 tons / m^3
      graphene 40 times stronger than kevlar

      If was equivalent in graphene:
      1650 tons of graphene
      but graphene being 40 times stronger => 41.25 tons

   -lift apparatus and pay-load = (unknown), assume 100 tons

   -structure : (unknown), assume almost equivalent to cable: 5900 tons   

-TOTAL WEIGHT: 12,000 tons if structure in kevlar

thought: this is just a starting point of an estimate, need to figure out structure,
other materials etc...
How much cable is actually needed ?
Is kevlar the right material?
What role can graphene have?

____________________________________
3) The drag of the geodesic spheres:

   If
   surface of 30m circle = 706.858 m^2
   Then total surface of a column of geodesic spheres is:
   500 X 706.858 = 353429 m^2
   Add
   10% contingency = 388771.9
   round off to 390000 m^2
   and
   3 bands of 5 km -> 390000 m^2 / 3 = 130000 m^2   

   Highest wind speed to withstand = 140 m/s = 504 km/h

   D = 1/2PV^2 DA

   05-10km: D = 1/2 X (0.7362 + 0.4127 / 2) X 140 X 0.5 X 130000 = 2613747 N
   10-15km: D = 1/2 X (0.4127 + 0.1937 / 2) X 140 X 0.5 X 130000 = 1379560 N   
   15-20km: D = 1/2 X (0.1937 + 0.0880 / 2) X 140 X 0.5 X 130000 =  640867 N

TOTAL WIND DRAG PER COLUMN OF SPHERES = 4634174 N

_________________________
4) Lifting the structure:

   If
   structure weight = 12,000,000 tons
   then one configuration could be:
   -4 X column of geodesic spheres = 8000 tons of lift
      Total drag would be: 18536696 N
      (drag of structure not included)
   -Array of 4000 geodesic spheres to make up first floor = 4000 tons of lift
      Drag of first floor to be determined depending on shape

TOTAL OF 6000 GEODESIC SPHERES

____________________
5) Further thoughts:

-The height of the mountain can be altered to help the equation.

-I'm thinking a first floor would be useful for stability, maintenance
etc... This may be an 'old school' way of thinking about this structure though.
Do we need floors or is there an umbilical cord only approach to reach eventually
the FLOATING DOCKS ?

-Is 20 km the optimum first floor? This seems to be where the winds quiet down,
 but perhaps there needs to be a lower floor than that.

-The column of spheres holding the cable and structure would probably help with
 safety. If the elevator was severed, the column of spheres would drop in altitude
 at first until it reached a neutral buoyancy altitude and bob, preventing crashing
 to the ground.

-How do we stabilize the structure? What are the solutions? fans that constantly
 re-adjust the position etc...




This is only a first draft to start plugging in some numbers.
I am missing quite a bit of knowledge on structural design and engineering if anyone
can point me in the right direction to help with the numbers and the accuracy of
my approach. And of course, let me know if some numbers or facts are incorrect.

Offline Paul451

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Re: Space Elevator Initiative
« Reply #30 on: 06/21/2018 02:32 AM »
SEI,

Why do you think a 1mm thick wall of graphene would withstand around 1400 tonnes of pressure? (Remember, it's not tensile strength that matters for an evacuated shell, but bending resistance.)

Offline SEI

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Re: Space Elevator Initiative
« Reply #31 on: 06/22/2018 02:44 PM »
Spot on question Paul451, admitedly I was originally focusing on these
spheres at higher altitude for the FLOATING DOCK and I seemed to have
concentrated only on lift capacity when going back down towards the BASE.
I forgot to think about pressure and buckling.

That's only part of the problem here though as I am struggling with all the
parts of the equation that I need to find out about. I have found a graphene
composite with a flexural strength of 116 N/mm^2 (the same as perspex) so far.


Here was the reasoning behind the numbers and how they came about.

1-This first document:

https://pdfs.semanticscholar.org/c050/8576da956a3e01c44c4b13c58728c9d33b51.pdf

According to his research, a vacuum sphere with a diameter of 250 cm and a thickness
of 0.1 cm of perspex would have neutral buoyancy.

2-From this patent, the double-layered approach that is being explained using more exotic
materials seem to strengthen the structure greatly.

http://akhmeteli.org/wp-content/uploads/2011/08/vacuum_balloons_cip.pdf

3-Both these approaches were based on spheres, so the stronger geodesic sphere structure
should help even more. (not entirely sure how to demonstrate the amount though)

4-I have allowed 20% of contingency for a mix of hydrogen/vaccum and possibly thickness
of the shell. knowing that I was missing elements to the equation, this was specifically
to help the buckling problem.

5-Other things I have missed include:

-mentioning that maybe more internal structure would be needed
-calculation is based on the mass of pure graphene, not a composite
-no evacuation device has been accounted for to create the vacuum
-inversely, regulation of the hydrogen level
-effect of temperature
-no paint weight has been accounted for

It may well be that the spheres for each altitude will have to have their own sets of optimal
parameters. Any thoughts on how to approach this?

Offline SEI

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Re: Space Elevator Initiative
« Reply #32 on: 07/12/2018 09:57 AM »
Here is the new video to recap what's been discussed and to introduce some new ideas as they stand




There are still quite a lot of unknowns and caveats but let's continue the discussion

Offline Paul451

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Re: Space Elevator Initiative
« Reply #33 on: 07/12/2018 11:53 AM »
SEI,

You're still burning a lot of fuel decelerating and re-accelerating the OPUM part. And the whole thing seems unnecessary, given your other assumptions.

If you think you can accelerate the payload to near-orbital velocity, why not orbital velocity? (Even if it includes a rocket-engine and carries itself into orbit, you'd be wasting less fuel than you will with OPUM's tragectory.)

Worse, the g-load on that launch oval-ring is insane. The turns have a 5km radius, I don't know what "near" orbital velocity you mean, but at 7km/s you are pulling 1000 g's. At just 5km/s, you are doing 500 g's. Anything about 2.25km/s, you exceed 100 g's on the turns.

But the thing is, if you think your payloads can tolerate 1000 g's, you would only need a 3km linear track to reach orbital velocity. With 50km of linear track, same length as your oval ring but straightened, you can reach orbital velocity at just 60 g's. There's absolutely no advantage to having an ring/oval track. Same length of track, vastly, vastly lower g-loads.

Aside 1: You may have an exaggerated view of atmospheric "skip". It doesn't save you any energy. All it means is that you come in shallow enough that re-entry drag doesn't lower your apogee all the way into the atmosphere. You still experience massive drag (lowering your apogee somewhat) and you still have to raise your perigee (and then apogee) using high-thrust rockets.

Aside 2: Geosync is not "Free of Earth's gravity". It's just a regular old orbit than happens to have an orbital period the same as Earth's rotation. To transfer between LEO and GTO still requires a 4km/s burn. (Plus a 1km/s circularisation burn.)

Not trying to be insulting, but I think you need to spend time playing with orbital mechanics until it becomes more intuitive. People rave over Kerbal Space Program. Also pick up the basic physics equations for linear and rotational acceleration (or find a trustworthly online calculator).



Aside 3, your previous reply to me: Why do you think a geodesic is stronger than a sphere for compression?

Offline SEI

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Re: Space Elevator Initiative
« Reply #34 on: 07/13/2018 09:21 AM »
I had looked at the linear acceleration and one of my original designs for the ramp was a slightly bent straight line.
However, to reach 7km/s at a tolerable 5Gs for humans, you need approximately 490 Km of track.
Not just that, assuming the Pay-load is 5,000 Kg, the Power needed would be approximately 850,000 kW to
accelerate the Pay-load in 140 seconds.
This is where the idea for the oval ramp came about; to get more track through revolutions and to relax the
amount of Gs during the acceleration process.
I have however failed to think about the rotational acceleration and yes... 1000 Gs is insane!
Side note: The added advantage of the oval track was the safety feature to decelerate in case the connection
with the OPUM didn't happen.

I had also started questioning the need for the OPUM stage.
My reasoning for it was that the Pay-load (at 5,000Kg) could be solely used for cargo/passengers, extra fuel
and as a heat-shield. The OPUM was there to be the mechanism part of the vehicle.
If the Pay-load needs to be self-propelled to reach the SPACE DOCK, then it probably would need to be bigger
and it would be even more demanding on the ramp.

From what I've read on the Buckminster Fuller Insitute page, geodesic domes seem to be structurally more strong. I thought the straightness of the triangular shapes is what gives the geodesic dome it's structural strength, ...and extrapolating from that the same for a sphere.
Or am I misinterpreting square panels VS triangular ones only? Would a perfect sphere be the optimal structure?

Offline Paul451

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Re: Space Elevator Initiative
« Reply #35 on: 07/13/2018 02:44 PM »
My reasoning for it was that the Pay-load (at 5,000Kg) could be solely used for cargo/passengers, extra fuel
and as a heat-shield. The OPUM was there to be the mechanism part of the vehicle.
If the Pay-load needs to be self-propelled to reach the SPACE DOCK, then it probably would need to be bigger
and it would be even more demanding on the ramp.

No. Each payload would need to carry enough fuel to restore OPUM from that dip and lift (the latter including the mass of the payload) which is necessary for each pick-up. Think about the mass of OPUM (and its rocket engines and payload-capture system), vs the mass of a single upper-stage type engine for the payload sled.

Moreso, if you merely accelerate the payload a little faster, to orbital velocity, then you only need a small engine to circularise the orbit at apogee.

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