Author Topic: Space tethers and SSTT  (Read 28951 times)

Offline Airlock

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Space tethers and SSTT
« on: 02/18/2012 04:30 PM »
For the purposes of this discussion, let us assume that there is a 200km long tether in orbit rotating with an an angular velicity of 3.5km/s, which (from what I've read anyways) should be sufficient to sling vehicles to EML-1.  Now, because the end of the tether on the earth-facing side is traveling 3.5km/s slower than orbital velocity, that means that any vehicle wanting to use the tether only needs 6km/s of delta V from the earth's surface (I'm assuming ~9.5km/s delta V for a normal orbital insertion).  The tether should remain in orbit with proper stationkeeping and if we are using the tether for both leaving orbit and returning to earth.  Assuming all of what I said is reasonable (if it is not please explain why not), my questions are:

What are the implications of a system like this on SSTO vehicle designs? Cutting the delta-v requirements by 1/3 should open up a lot of possibilities for vehicle design.

Are there any existing or soon-to-exist vehicles that could be adapted without a significant level of effort to fly this type of mission?

When you use the tether to return from EML-1, you reduce your velocity back to 6km/s which causes you to de-orbit. I suspect that this would result in quite a steep angle of attack. Same trajectory would happen if you missed the tether entirely at the beginning of the mission.  Could this problem be overcome by correction burns and/or vehicle design?

EDIT: Changed portion of thread title from "SSTO" to "SSTT" per comment from user DeltaV.
« Last Edit: 02/18/2012 05:15 PM by Airlock »

Offline deltaV

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Re: Space tethers and SSTO
« Reply #1 on: 02/18/2012 04:52 PM »
Look up "single stage to tether".

Offline Jim

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Re: Space tethers and SSTO
« Reply #2 on: 02/18/2012 04:53 PM »

Are there any existing or soon-to-exist vehicles that could be adapted without a significant level of effort to fly this type of mission?


What is the altitude? 
how would they link up? 
How long do they have to link up?
What happens if a link up is missed?
What is the maneuvering delta V requirements?
Now compare with current rendezvous and docking processes and timelines.

What is the outcome of the above comparisons?


Offline Airlock

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Re: Space tethers and SSTO
« Reply #3 on: 02/18/2012 05:13 PM »

Are there any existing or soon-to-exist vehicles that could be adapted without a significant level of effort to fly this type of mission?


What is the altitude? 
how would they link up? 
How long do they have to link up?
What happens if a link up is missed?
What is the maneuvering delta V requirements?
Now compare with current rendezvous and docking processes and timelines.

What is the outcome of the above comparisons?



I will take that as a no.  I was curious if anyone had a good "maybe" answer to this question, however realistically we are probably talking about a different kind of vehicle from anything else being built currently.

Also, an attempt to answer your questions:

What is the altitude? There are various proposals, but typically the altitude would be a compromise between travel distance for the SSTT vehicle and atmospheric drag on the tether.

how would they link up? Some type of graple mechanism.  It would need to be a lot faster than a docking ring, but we're not trying to connect two pressurized vehicles so something faster and more low-tech is feasible.

How long do they have to link up? Not very long.

What happens if a link up is missed? The vehicle performs a nominal de-orbit and re-entry (assuming the angle of attack issue is solved).

What is the maneuvering delta V requirements? Depends on how long you allow for a launch window.  Ideally, you would want to do 1 second launch window like NEO missions, and throttle up/down throughout the launch based on the real-time position of the tether.

Offline Jim

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Re: Space tethers and SSTO
« Reply #4 on: 02/18/2012 06:09 PM »

Are there any existing or soon-to-exist vehicles that could be adapted without a significant level of effort to fly this type of mission?


What is the altitude? 
how would they link up? 
How long do they have to link up?
What happens if a link up is missed?
What is the maneuvering delta V requirements?
Now compare with current rendezvous and docking processes and timelines.

What is the outcome of the above comparisons?



I will take that as a no.  I was curious if anyone had a good "maybe" answer to this question, however realistically we are probably talking about a different kind of vehicle from anything else being built currently.

Also, an attempt to answer your questions:

What is the altitude? There are various proposals, but typically the altitude would be a compromise between travel distance for the SSTT vehicle and atmospheric drag on the tether.

how would they link up? Some type of graple mechanism.  It would need to be a lot faster than a docking ring, but we're not trying to connect two pressurized vehicles so something faster and more low-tech is feasible.

How long do they have to link up? Not very long.

What happens if a link up is missed? The vehicle performs a nominal de-orbit and re-entry (assuming the angle of attack issue is solved).

What is the maneuvering delta V requirements? Depends on how long you allow for a launch window.  Ideally, you would want to do 1 second launch window like NEO missions, and throttle up/down throughout the launch based on the real-time position of the tether.

Grapple?  Not a really good way to connect several tons.  There needs to be some rigidity

Deorbit?  It is not in orbit

As for time, that is really over looked.  It is has different requirements than an ASAT.

Throttling up and down during ascent  is not viable. 

Offline deltaV

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Re: Space tethers and SSTO
« Reply #5 on: 02/18/2012 06:20 PM »
Throttling up and down during ascent  is not viable. 

The space shuttle begs to differ. Can you please clarify your point?

Offline Jim

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Re: Space tethers and SSTO
« Reply #6 on: 02/18/2012 06:25 PM »
Throttling up and down during ascent  is not viable. 

The space shuttle begs to differ. Can you please clarify your point?

Context
Not for rendezvous adjustment

Offline deltaV

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Re: Space tethers and SSTO
« Reply #7 on: 02/18/2012 06:33 PM »
Throttling up and down during ascent  is not viable. 

The space shuttle begs to differ. Can you please clarify your point?

Context
Not for rendezvous adjustment

Airliners adjust throttle in real time to help rendezvous with a runway. Moon landers adjust throttle in real time to rendezvous with a landing site. Launch vehicles rendezvous with an orbit (although not very accurately). Missiles rendezvous with fighter aircraft and satellites in real time (although not using throttle and with no control of relative velocity). What about a tether rendezvous distinguishes it from these demonstrated rendezvous types?
« Last Edit: 02/18/2012 06:39 PM by deltaV »

Offline Jim

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Re: Space tethers and SSTO
« Reply #8 on: 02/18/2012 07:53 PM »
Launch vehicles rendezvous with an orbit

No, they get into orbit.  They do not use it to adjust arrival time nor a specific point in space.  Because launch vehicles have little margin to work with and non scheduled throttling affects the ability to reach orbit or a specific velocity
Your analogies are not applicable, especial ASAT, and landing.  The aircraft can go around and has excess energy and the ASAT doesn't care about arrival velocity. Moon lander doesn't have to adjust for winds and it is a fixed trajectory with excess energy.

The launch vehicle is not going to using powered flight all the way to the tether, it is going to be coasting.






« Last Edit: 02/18/2012 07:55 PM by Jim »

Offline mmeijeri

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Re: Space tethers and SSTO
« Reply #9 on: 02/18/2012 07:56 PM »
No, they get into orbit.  They do not use it to adjust arrival time nor a specific point in space.

What about those hypothetical Shuttle missions to rendez-vous with Soviet spy satellites?
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Offline Andrew_W

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Re: Space tethers and SSTT
« Reply #10 on: 02/18/2012 08:29 PM »

What is the altitude? 
how would they link up? 
How long do they have to link up?
What happens if a link up is missed?
What is the maneuvering delta V requirements?
Now compare with current rendezvous and docking processes and timelines.

What is the outcome of the above comparisons?



http://www.niac.usra.edu/files/library/meetings/annual/jun01/391Grant.pdf
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Online QuantumG

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Re: Space tethers and SSTT
« Reply #11 on: 02/18/2012 09:05 PM »
One of Kirk's nice videos:



I can't seem to find the videos he made of the scale grapple hardware they tested (http://selenianboondocks.com/2010/02/a-tether-technology-anniversary/) but you can see the animation at the 3 minute mark in that video.

I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Offline deltaV

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Re: Space tethers and SSTO
« Reply #12 on: 02/18/2012 09:46 PM »
No, they get into orbit.  They do not use it to adjust arrival time nor a specific point in space.  Because launch vehicles have little margin to work with and non scheduled throttling affects the ability to reach orbit or a specific velocity

Your analogies are not applicable, especial ASAT, and landing.  The aircraft can go around and has excess energy and the ASAT doesn't care about arrival velocity. Moon lander doesn't have to adjust for winds and it is a fixed trajectory with excess energy.

A vehicle with tight final position (~1 m) and velocity tolerances such as a moon lander naturally needs a greater delta vee margin for maneuvering than a vehicle with looser tolerances such as a launch vehicle (~1 km). A SSTT vehicle would obviously be designed with enough margin for the rendezvous. The fact that current launch vehicles don't provide enough margin doesn't make providing such margin impossible.

Quote
The launch vehicle is not going to using powered flight all the way to the tether, it is going to be coasting.

Good point, RCS would be needed for adjustments after burnout.



Jim: your reason for discounting the possibility of SSTT seems to be based on your engineering judgment of the difficulty of the required GNC R&D. (Many ideas can be shot down with back of the envelope calculations using first principles, but this doesn't seem to be one of them.) I'll only be convinced if I trust your engineering judgment on this more than the teams that have studied SSTT in the past. Are you experienced in GNC R&D?

Offline Jorge

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Re: Space tethers and SSTO
« Reply #13 on: 02/19/2012 02:11 AM »
No, they get into orbit.  They do not use it to adjust arrival time nor a specific point in space.

What about those hypothetical Shuttle missions to rendez-vous with Soviet spy satellites?

The key word is "hypothetical".

The single-orbit polar rendezvous mission from VAFB (Baseline Reference Mission 3B, or BRM-3B), as described in 1975 documentation (the latest available before the requirement was dropped), required a hypothetical shuttle ascent guidance algorithm that could control for downrange distance as well as inertial velocity, flight path angle, and target plane (IY).

Shuttle powered explicit guidance (PEG), as actually implemented, could not control for downrange distance relative to an orbiting body.

Therefore the shuttle, *as built*, was not capable of performing BRM-3B.
JRF

Offline colbourne

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Re: Space tethers and SSTT
« Reply #14 on: 02/19/2012 03:16 AM »
Other factors that need to be considered is how much G- force the payload can withstand and whether it is possible to have another attempt at docking if the first attempt is aborted.

Finding a clear space for the tether to orbit in is also important to avoid existing satellites.

I personally think that rotating tethers will have a big part to play in space transport especially on other worlds where there are very few satellites  to be avoided. Accurate guidance and positioning of the sub-orbit craft is required but should be achievable.

This could also be used for fast inter-continental transport.

Luckily tethers can be tested at slow speed (rotation) and at small scale initially

Offline mmeijeri

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Re: Space tethers and SSTO
« Reply #15 on: 02/19/2012 08:12 AM »
Shuttle powered explicit guidance (PEG), as actually implemented, could not control for downrange distance relative to an orbiting body.

Therefore the shuttle, *as built*, was not capable of performing BRM-3B.

Interesting, thanks. Was the requirement dropped because it was considered too difficult or just not useful enough to justify the effort?
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Offline Jim

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Re: Space tethers and SSTO
« Reply #16 on: 02/19/2012 12:21 PM »

Jim: your reason for discounting the possibility of SSTT seems to be based on your engineering judgment of the difficulty of the required GNC R&D. (Many ideas can be shot down with back of the envelope calculations using first principles, but this doesn't seem to be one of them.) I'll only be convinced if I trust your engineering judgment on this more than the teams that have studied SSTT in the past. Are you experienced in GNC R&D?


The issue isn't GNC, it is real world affects on launch vehicle trajectories.
Of all the simulations done for each launch, the one trajectory that isn't flown, is the one that is planned. 

If this is to work, then the launch vehicle will need higher structural and propellant margins than existing vehicles.

For example, current vehicles react to wind shear by flying with it vs fighting it. They don't care when they make or where they make orbit on macro scale.

Offline mmeijeri

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Re: Space tethers and SSTO
« Reply #17 on: 02/19/2012 12:37 PM »
For example, current vehicles react to wind shear by flying with it vs fighting it. They don't care when they make or where they make orbit on macro scale.

What about ICBMs? Aren't they close to orbital? I have a thick tome on missile guidance, but it hasn't made to the top of my to read list yet...
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Offline Jim

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Re: Space tethers and SSTT
« Reply #18 on: 02/19/2012 01:41 PM »
One of Kirk's nice videos:

I can't seem to find the videos he made of the scale grapple hardware they tested (http://selenianboondocks.com/2010/02/a-tether-technology-anniversary/) but you can see the animation at the 3 minute mark in that video.

But MXER is for a vehicle already in orbit

Offline Andrew_W

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Re: Space tethers and SSTT
« Reply #19 on: 02/19/2012 09:49 PM »
I'm not buying the claim that launching for a precise time and place just outside of the Earth's atmosphere is a serious challenge compared to other aspects of SSTT.
http://en.wikipedia.org/wiki/USA-193#Destruction

ASSTT (Assisted SSTT (air launch)) would be a still easier option as the ascent to space starts above most of the atmosphere and weather.
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