space plane + space tug = cheap flight?

[enroger]

Hi everyone! I have been thinking if we can combine space plane and space tug to create a really cheap access to space:<br /><br />First we have a permanent space tug in LEO, then we have our suborbital space plane like spaceshipone. Then when our space plane take off from earth with a detachable payload canister (with fuel tank), the space tug will decelerate from LEO to rendezvous with the space plane when the plane reach highest altitude. Next they have probably a few minutes for the space plane to transfer the cargo with fuel tank to the space tug. And then the space tug burn and reach LEO. Space plane return Burt Rutan style.<br /><br />This scheme will not save fuel. But it divides the problem of building an all powerful space plane orbiter into two smaller and manageable problem of building a suborbital space plane and a space tug. The most difficult part is probably the cargo transfer in a few minutes thing.<br />

 

[kelvinzero]

We had a raging argument about this a while back. There are a bunch of people on this site who really do know their rockets and they were unanimous that it would not work.<br /><br />The big problem is that getting up to orbital speed is much harder than getting up to orbital height. If a craft has to decelerate and accelerate you have at least doubled the problem. Then, yeah the docking maneuver would be scary.<br /><br />However it is very similar to another idea that is often considered and that is having a tug from low earth orbit to higher orbits. Possibly we can do this with something like VASIMR which is much more efficent than chemical rockets but cannot get us to orbit. Also you have the saving that the tug is designed purely for vaccum. You are not towing the unnecessary mass of the shuttle's heat shielding or wings higher than necessary.<br /><br />http://www.adastrarocket.com/vasimr.html

 

[enroger]

I agree that the tug having to decelerate and accelerate, the whole launch would spend more fuel than ordinary rocket with same payload. But base on the fact that a space shuttle has to do exactly the same thing too, it's pretty safe to assume the space tug scheme would consume less fuel than space shuttle, not having to carry heat shield and such. So maybe re-usability can justify the extra fuel cost?<br /><br />Great point about the plasma drive, higher ISP, lower thrust, longer burn. Though the thrust must be large enough or the tug might end up touching the upper atmosphere. Nuclear rocket is also a good choice.

 

[eniac]

One of the problems here is that the craft must match speed in order to rendezvous, which will cost fuel. This has made me think of the following scheme: A massive space station lowers a line down towards the boundary of the atmosphere. The payload comes up on a suborbital space plane and catches the line. It is pulled out of the hold and accelerates dragging behind the station until it matches speed. The line is let go to maintain the proper acceleration, after orbital velocity is obtained the load is reeled in.<br /><br />It would require a very fast line, and about 1,000 km of it, but who wants to say it is impossible? It would certainly save a LOT of fuel. <br /><br />Call it the Gone Fishing launch method. <br /> <div class="Discussion_UserSignature"> </div>

 

[scottb50]

It would require as much fuel to return the station to it's orbit as it would to take the payload to orbit. <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>It would require as much fuel to return the station to it's orbit as it would to take the payload to orbit.<p><hr /></p></p></blockquote><br />True, unless you were to use a high ISP ion drive or an electrodynamic tether to gradually reboost the station between launches.<br /> <div class="Discussion_UserSignature"> </div>

 

[scottb50]

Either way you still have to expend the energy needed to increase the speed of the payload to that of the orbiting body, plus bring it up to the same altitude.<br /><br />Look at what it takes to re-boost the ISS when it 's orbit deteriorates. <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

Suborbital is a no-go.<br /><br />However there is something to be said in favor of a scheme employing a reusable orbital spaceplane + a reusable spacetug. Just google selenianboondocks for interesting discussion.

 

[j05h]

A spaceplane such as an evolved X-15 or SABRE/HOTOL craft combined with orbital tugs makes a lot of sense. Sub-orbital does not cut it. The only reasonable proposals using suborbital anything involve "bolo" tethers, and even those require near-orbital speeds on the lower end.<br /><br />Deorbitting your tug or space station to get payloads is impossible/wasteful. Similar to the thing proposed on SB&T a few months ago where the payload would "zoom" up to a station and meet it in one shot. Nothing like docking with several 1000 km/hr difference in velocity.<br /><br />J <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>Either way you still have to expend the energy needed to increase the speed of the payload to that of the orbiting body, plus bring it up to the same altitude.<p><hr /></p></p></blockquote>Yes, that is true. In space, however, energy is more available than propellant, and you can save an enormous amount of propellant by shifting most of the work to a high ISP engine. In essence, by using a high ISP drive you defang the rocket equation, and the mass fraction (payload vs. propellant) can be an order of magnitude better than with chemical boosters only. <br /><br />Plus, you can explore alternative means of boosting, such as electrodynamic tethers that work without any propellant.<br /><br />Plus, you have a choice of energy sources, including solar and nuclear.<br /><br />Now, the fishing reel described may not work for mechanical reasons, but there are a variety of other concepts that involve catching suborbital payloads, the most developed of which, I think, is the rotating momentum exchange tether: http://spacetethers.com/ . Another one that I like is the electrowheel: http://www.islandone.org/LEOBiblio/SPBI130.HTM .<br /><br /><br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>The only reasonable proposals using suborbital anything involve "bolo" tethers, and even those require near-orbital speeds on the lower end.<p><hr /></p></p></blockquote>I don't think they do. Bolos (aka rotating momentum exchange tethers), can come in all sizes and tip velocities, up to full orbital, material strength permitting. With currently available materials, you can reasonably achieve 3-4 km/s, enough to eliminate most of the propellant and use a single stage reusable suborbital launch vehicle.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[nexium]

My guess is space tugs are useful for very large space projects, but not for go fetch, because, Low Earth Orbit has thousands? of times more volume than the human biosphere. On Earth tug boats rarely leave the harbor, nor tow anything more than a few miles. Neil

 

[scottb50]

I think you are taking the term Tug too literally. The ability to reach various orbits in LEO from LEO would allow much more flexibility and capability for the hardest part, getting from the surface to LEO.<br /><br />A Tug would also facilitate smaller and more numerous Stations that could be used for specialized purposes that would be affected by other uses on a larger Station. Another factor would be corporate protection of proprietary projects. <div class="Discussion_UserSignature"> </div>

 

[nexium]

If we have ten million habitats in LEO with fewer than 20 people each, we should have a thousand hospital tugs which make the rounds. The hospital tugs could provide many other services when there are no urgent errends of mercy.<br />Why less than 20 people? If a habitat has 5 pounds per square inch of oxygen, and a million square inches of inside surface; the pressure trying to explode the habitat is 5 million pounds. With currently available materials, this is close to the limit of practicality, but a million square inches is claustphobic with more than 10 or 15 people. A sphere with an inner radius of 275 inches has 949,850 square inches of inner surface.

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>If a habitat has 5 pounds per square inch of oxygen, and a million square inches of inside surface; the pressure trying to explode the habitat is 5 million pounds. With currently available materials, this is close to the limit of practicality<p><hr /></p></p></blockquote>This argument is incorrect. Material strength needed for pressurized enclosures is independent of size. Increased stress is balanced by increased wall thickness, if wall thickness is kept proportional to container size. <div class="Discussion_UserSignature"> </div>

 

[scottb50]

It also makes more sense to build Stations with a number of independent Module rather then a single vast expanse. Any failure could be isolated much easier. Being independent each Module could be fairly light weight.<br /><br />What I have in mind is using the Upper Stage tanks from a TSTO Launcher as Modules for Stations construction as well as propellant tanks and crew quarters for Tugs. This would allow both one or two engine Tugs, with independent propellant tank, or additional propellant for longer missions and more massive payloads. <div class="Discussion_UserSignature"> </div>

 

[nexium]

Hi Eniac: Does size mean radius, surface area or volume? If one millimeter of steel gives a good safety factor, at 5 psi, for a one meter radius sphere; a two meter radius sphere has 16 times 3.14 square meters and 4/3 times 8 times 3.14 cubic meters. That is 0.2cm or 5.3 cm of steel or 33.5 cm of steel? Perhaps not as the one meter radius sphere has 12.56 square meters and 4.2 cubic meters? Neil

 

[eniac]

nexium: I don't follow all your numbers, too confusing for me. However, imagine a sphere with a seam around it, like the Magdeburg hemispheres http://en.wikipedia.org/wiki/Magdeburg_hemispheres . As the radius increases, the force at the seam increases in proportion to the surface area, quadratically with the radius. At the same time, both wall thickness and circumference increase linearly, so the cross sectional area at the seam also increases quadratically. Thus, the required material strength (force/area) is independent of scale.<br /> <div class="Discussion_UserSignature"> </div>

 

[15203700700579]

Cheap Flight that is an oxymoron.

 

[josh_simonson]

Actually tug and barge ships do full ocean voyages, the seam between the tug/barge is just so well and closely mated that it looks like a regular freighter - but the tug can detatch from the barge and attach a different type of barge (cargo, oil, ect) if need be. Tug and barge is also used to pull trains of barges up and down rivers, and seaways... Also remember that garbage barge that was towed up and down the coast when no-one would take it back in the 80's? <br />http://www.newsday.com/media/thumbnails/photo/2003-08/9128723.jpg<br /><br />ISS is currently boosted by hypergolic fuels in the russian segment (and the STS), spare OMS fuel is basically free, and the russians fly theirs cheaply. There hasn't yet been sufficient need for ISS to be reboosted by high ISP engines or an electrodynamic tether. One hopes that the research station will be used to demonstrate such technologies eventually. <br /><br />I believe the MXER tether + interceptor is one of the more ingenious methods of getting to space and is the best architecture similar to that proposed by the OP. I hope someone demonstrates electrodynamic propulsion in LEO sometime soon because infinite ISP propulsion in LEO would be an enormous boon.