Author Topic: Rotovator help with re-entry  (Read 1244 times)

Offline Hop_David

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Rotovator help with re-entry
« on: 05/26/2016 08:15 PM »
Most of the LEO rotovator schemes I've seen entail catching a payload in LEO and tossing it to a higher orbit.

Catching from a lower orbit and tossing to a higher orbit are two actions that deplete a tether's orbital momentum.

Here's a possible way to restore some of the tether's lost momentum and make reuse of the upper stage less difficult.

1) Rotovator catches upper stage and payload.
2) Rotovator throws payload to higher orbit.
3) Rotovator drops upper stage to lower orbit.

Step 3 accomplishes two things: restores orbital momentum to tether. And reduces upper stage re-entry velocity. Lower re-entry velocity is desirable should we try to re-use the upper stage.

Pic attached.
« Last Edit: 05/26/2016 08:18 PM by Hop_David »

Offline JasonAW3

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Re: Rotovator help with re-entry
« Reply #1 on: 05/26/2016 08:27 PM »
Most of the LEO rotovator schemes I've seen entail catching a payload in LEO and tossing it to a higher orbit.

Catching from a lower orbit and tossing to a higher orbit are two actions that deplete a tether's orbital momentum.

Here's a possible way to restore some of the tether's lost momentum and make reuse of the upper stage less difficult.

1) Rotovator catches upper stage and payload.
2) Rotovator throws payload to higher orbit.
3) Rotovator drops upper stage to lower orbit.

Step 3 accomplishes two things: restores orbital momentum to tether. And reduces upper stage re-entry velocity. Lower re-entry velocity is desirable should we try to re-use the upper stage.

Pic attached.

     It seems that you'd still loose some of your orbital momentum, although not quite as much as without the stage drop.  Transferring some of the fuel, for boosting the rotovator itself, or refiring the booster stage to add to the orbital momentum, would be a viable solution.
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Offline Asteroza

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Re: Rotovator help with re-entry
« Reply #2 on: 05/27/2016 01:26 AM »
Isn't the momentum loss cheaper if you grab only payload though? Holding the upper stage not only increases the temporary momentum loss, but also shifts the CG towards the payload, lowering spin rate which penalizes an orbit raising toss.

Offline AlanSE

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Re: Rotovator help with re-entry
« Reply #3 on: 05/27/2016 01:52 AM »
Most of the LEO rotovator schemes I've seen entail catching a payload in LEO and tossing it to a higher orbit.

Catching from a lower orbit and tossing to a higher orbit are two actions that deplete a tether's orbital momentum.

Here's a possible way to restore some of the tether's lost momentum and make reuse of the upper stage less difficult.

1) Rotovator catches upper stage and payload.
2) Rotovator throws payload to higher orbit.
3) Rotovator drops upper stage to lower orbit.

Step 3 accomplishes two things: restores orbital momentum to tether. And reduces upper stage re-entry velocity. Lower re-entry velocity is desirable should we try to re-use the upper stage.

Pic attached.

     It seems that you'd still loose some of your orbital momentum, although not quite as much as without the stage drop.  Transferring some of the fuel, for boosting the rotovator itself, or refiring the booster stage to add to the orbital momentum, would be a viable solution.

For almost any orbital launch assist infrastructure, I strongly favor separation of responsibilities between payload and station-keeping propellant delivery launches. In the quoted comment here, we're talking about a launch that contains some payload, and some propellant that is used to help boost the rotovator. It will probably be logistically superior to catch a launch which has no extra propellant, and then (at some regular ratio) catch a launch that is strictly a propellant delivery mission to the station.

I would concede, however, that using the boosters on a reusable upper stage could make sense - for the purpose of avoiding either a fuel transfer, or a transfer of a fuel vehicle from the tip to the hub. Transferring fuel would be spectacularly complicated because the rotovator would need extremely strong materials to start out with. You would either transfer propellant into tanks on climbers, or you could lay a pipe over the entire length of the tether, which would destroy your specific-strength economics. Allowing no climbers, whatsoever, would make this make sense, because controlled firings of the upper stage would serve to maintain the orbit and angular momentum of the tether (putting aside the problem of oscillations).

In this vision, there would probably be engine firings over several rotations before the upper stage vehicle is released again.