Zero burn return to planets surface

[Seegull]

Scenario: An orbiting station or habitat. A small vessel wishes to leave and return to the planet's surface.

Initial assumptions: The habitat rotates to produce 1g acceleration for inhabitants. The rotation axis of the planet and habitat are parallel. The habitat orbits on the equatorial plane. The orbit can be circular or eliptical. The habitat orbits at less than geo-synch altitude. The planet has negligible atmosphere. The planet may have equivalent dimensions to earth (bigger/smaller & heavier/lighter would be of interest in later discussions.

Intention: The small craft is docked on the periphery of the habitat. At departure it simply undocks. No reaction engine burn. The velocity / speed of the vessel is now other than the ideal orbital conditions for that altitude.

Question: could an appropriate choice of departure direction (from around the habitat) ensure a flightpath that intersects with the surface of the planet.

Note: I call it a zero burn scenario but also speak of negligible atmosphere. I am assuming that a last minute burn just before landing is neccesary to avoid bug-splatting on the surface.

Bonus question: Is this the cheapest (in terms of fuel use) solution or would a different burn scenario be even cheaper.

I hope this all makes some degree of sense.

 

[statickid]

"...

Note: I call it a zero burn scenario but also speak of negligible atmosphere. I am assuming that a last minute burn just before landing is neccesary to avoid bug-splatting on the surface.
..."

Without an atmosphere the landing shuttle will be traveling at orbital speed when it reaches the surface, so a retrograde burn is still needed to slow the vehicle down for a landing...

unless the surface is spinning at near orbital velocities...? but that sounds more like a an protoplanet to me and IMO not a good place to have a base :lol:

Something I just thought of:
Maybe this would be possible in a pseudo-orbit around a moonlet like the "orbit" that comes with the stock mars scenarios of interest.

 

[paddy]

Like the idea as I see rate of descent such that it would use almost no hover fuel. Down side would be it would take so very long to land that the returning crew could write a book! Still would end up spending fuel to slow down but that is a fixed cost come what may.

 

[Hielor]

You can use one of the online calculators to determine how much linear velocity the ship will get depending on the attributes of the habitat. Such as: http://www.artificial-gravity.com/sw/SpinCalc/

As has already been pointed out, on an airless world you'll still need to kill all your velocity anyway in order to land, so you don't gain much there.

For Earth orbit, you only need approximately 67 meters/second delta-v to de-orbit from a 193km orbit (citation needed).

However, if you plug that in to the SpinCalc, you find that a habitat maintaining 1g would need to be about 457m in radius in order to have a rim speed of 67m/s. If you were docked to the rim of such a structure, and released at the point of the structure's spin where you were moving "backwards" against the direction of the orbit, you'd end up de-orbiting.

So, for low earth orbit, if you had an absolutely massive habitat that was nearly a kilometer across, it would be possible.

Of course, in order to dock with the rim of such a station you'd need to apply that much delta-v in order to match speeds with it, and that would mean docking would have to be a fairly fast affair (since you'd rapidly drift away from the station due to orbital differences). In real life, docking is a very slow and very methodical process, so you wouldn't want to do this.

I guess you could do the docking procedure with the hub of the station and then have the ship moved to the rim in order to undock, but at the point your technology is able to do that (or even build such a structure in the first place) you probably don't care about saving the 67m/s of delta-v.

 

[RisingFury]

Scenario: An orbiting station or habitat. A small vessel wishes to leave and return to the planet's surface.

Initial assumptions: The habitat rotates to produce 1g acceleration for inhabitants. The rotation axis of the planet and habitat are parallel. The habitat orbits on the equatorial plane. The orbit can be circular or eliptical. The habitat orbits at less than geo-synch altitude. The planet has negligible atmosphere. The planet may have equivalent dimensions to earth (bigger/smaller & heavier/lighter would be of interest in later discussions.

Intention: The small craft is docked on the periphery of the habitat. At departure it simply undocks. No reaction engine burn. The velocity / speed of the vessel is now other than the ideal orbital conditions for that altitude.

Question: could an appropriate choice of departure direction (from around the habitat) ensure a flightpath that intersects with the surface of the planet.

Note: I call it a zero burn scenario but also speak of negligible atmosphere. I am assuming that a last minute burn just before landing is neccesary to avoid bug-splatting on the surface.

Bonus question: Is this the cheapest (in terms of fuel use) solution or would a different burn scenario be even cheaper.

I hope this all makes some degree of sense.

In general yes, you can do this. If the planet has no atmosphere, you can always place the station in a low enough circular orbit that even the slightest reduction is velocity would lead the projectile to intersect with the surface.

Also be in mind that while you throw your projectile into a lower elliptical orbit, the station itself gets shot into a higher elliptical orbit.

unless the surface is spinning at near orbital velocities...? but that sounds more like a an protoplanet to me and IMO not a good place to have a base :lol:

Such a planet would not be stable, as gravity could not hold it together:
Remember, the other most bit of the planet on the equator is already spinning fast enough that a rock on the surface could just float, so such a planet could not coalesce together from primordial particles.

 

[statickid]

I can see a story plot device in an action movie where this is being done as an emergency evacuation procedure where the shuttle doesn't have enough fuel to deorbit and land, but they use the station spin to deorbit, giving them just enough fuel to survive a splashdown or something.

 

[Hielor]

Such a planet would not be stable, as gravity could not hold it together:
Remember, the other most bit of the planet on the equator is already spinning fast enough that a rock on the surface could just float, so such a planet could not coalesce together from primordial particles.

What if the planet is made of magnets? :lol:

 

[Seegull]

This is what I was getting at.

Without an atmosphere the landing shuttle will be traveling at orbital speed when it reaches the surface, so a retrograde burn is still needed to slow the vehicle down for a landing...

Yes, I understand that that must be true.

My thinking was as follows:

I keep reading that the most efficient place to burn fuel is when the vehicle is going at its fastest. In the 'lazy de-orbit' scenario I imagined, the vehicle would be going fastest just before hitting the surface. This would then be a single MASSIVE retrograde burn just before touch down. A bit like hitting the brakes in an express elivator just before the ground floor arrives. :hotcool:

 

[Face]

I keep reading that the most efficient place to burn fuel is when the vehicle is going at its fastest.

Where do you keep reading that?
It really depends. E.g. if you want to change planes, it is more efficient to burn when the vehicle is going slowest. In an elliptical orbit, this would be at apoapsis.

 

[Seegull]

Please excuse my over simplistic statement.

My mind was trapped in '2d in plane orbit / de-orbit thinking'

---------- Post added at 02:58 PM ---------- Previous post was at 02:52 PM ----------

Down side would be it would take so very long to land that the returning crew could write a book!

I read into this that the more delta-v the quicker the flight to the surface.

Coincidentally, I am writing a sci-fi book myself. Maybe I should do it whilst riding in the proposed shuttle vehicle.

---------- Post added at 03:01 PM ---------- Previous post was at 02:58 PM ----------

Also be in mind that while you throw your projectile into a lower elliptical orbit, the station itself gets shot into a higher elliptical orbit.

Good point. However, if the re-docking logic has the opposite polarity then it should average out to a zero-sum effect in the end, shouldn't it?

---------- Post added at 03:07 PM ---------- Previous post was at 03:01 PM ----------

So, for low earth orbit, if you had an absolutely massive habitat that was nearly a kilometer across, it would be possible.

I guess you could do the docking procedure with the hub of the station and then have the ship moved to the rim in order to undock, but at the point your technology is able to do that (or even build such a structure in the first place) you probably don't care about saving the 67m/s of delta-v.

Reading between the lines, higher orbits would require greater delta-v to de-orbit so the habitat would have to get stupidly big.

Putting on my engineer's hat I think I can imagine a few 'low budget' ways of getting this done without the shuttle being treated like a base ball during re-docking.