LEO to TMI - discuss

SBird · 2004-03-24 20:47:09

OK, I have the feeling that the Earth to LEO discussion is approaching a wind-down so I wanted to start up a discussion about getting from a Low Eath parking Orbit to a Trans Mars Injection trajectory. Basically, how do we get from being in low orbit to heading for Mars?

I list this as seperate from actually getting to Mars because the physics of getting around near Earth are very different from the interplanetary space between Earth and Mars. For one thing, the solar wind isn't present in this region of space and you are still deep in Earth's gravitational well.

How can we get from LEO to TMI using as little energy as possible and in a reasonable amount of time?

Here's the various technologies that I see:

Chemical - old, kinda dumpy but it works.

NTR - NERVA drive that uses a nuclear reactor to heat up fuel - about twice the performance of chemical. Has the disadvantage of using a heavy engine that isn't terribly useful for power generation.

STP - basically the same idea as NTR but using the sun to heat the propellant. Not as good as NTR but avoids the negative political connotations of nuclear power.

Ion propulsion - either powered by nuclear reactors or solar power. Very low thrust but with the advantage of using very little fuel. Most estimates for using ion engines to push a Mars Direct-sized craft from LEO to TMI take a year of slowly spiraling upwards from Earth.

Rotovators - giant rotating tethers with counterweights. By grabbing the end as it swings past you, the rotovator flings you up into a higher orbit. In return, the rotovator drops to a lower orbit and has to be reboosted somehow if it is to be reusable. The problem is that the counterweight should be at least 5 times the mass of the spacecraft being boosted - this means that a Mars Direct spacecraft would require a 500-700 ton rotovator which is quite impractical.

Electromotive Tether Propulsion (ETP) - similar to the rotovator (in fact most rotovator designs use ETP to reboost its orbit after giving a boost to a spacecraft) This uses Earth's magnetic field and a conductive tether like a motor, trading power for motion. Most ETP designs use solar panels for power. The advantage is that no propellant is used. The disadvantage is that thrust is low. However, ETP should scale to higher thrusts more easily than ion engines.

Magsail - although there is no solar wind here fora magsail to use, the large magnetic field can interact with the Earth's to produce thrust. However, since most of the Earth's magnetic field is perpendicular to the surface, your ability to navigate and gain useful momentum is badly limited. This is primarily a useful technique when used in a polar orbit. Because of these disadvantages, it is unlikely that magsails will be useful here.

Solar sail - like the magsail, a solar sail is very limited near Earth. For one thing, the constant changing angle of the spacecraft's direction with respect to the direction of light make it difficult to consistently add velocity to an orbit. This can be partially handled with tacking against the light flux but still will represent a tough challenge, especially with delicate solar sail materials that have trouble handling the stresses of continually changing orientation.

Space elevator - as before, this is a highly speculative proposal but climbing an elevator to the top gets you most of the way to TMI in terms of energetics.

Also, as mentioned recently in the Earth to LEO thread, considerable performance increases can be achieved by refueling an unfueled spacecraft in LEO using SSTOs or other smaller launch vehicles - this allows for either a smaller hevy lift launcher to put the spacecraft in orbit or a much larger spacecraft payload to Mars.

Also, we should consider the utility of space tugs. These are reusable independent spacecraft that boost the Mars spacecraft to TMI but stay behind for reuse. If the tug uses fuel, it will have to be refueled. Rotovators and ETP tugs would not need to refuel.

ERRORIST · 2004-03-24 20:57:16

LINEAR PHOTON-----> ION-------> ACCELERATION

GCNRevenger · 2004-03-24 21:50:04

Ummm I think linear laser reaction mass acceleration rockets have been pretty well poked full of holes in previous posts...

Chemical isn't too bad if you can use cryogenic fuels, it can get the job done if you can get stuff into orbit with better-than-now efficency. Light weight engines, relativly light weight tanks, but a fuel hog.

Solar thermal, unproven, hard to scale up, only somewhat better thrusts than ion, loses power with distance from Sun rapidly, presents maneuvering difficulties, won't work in shadow.

Ion power is too slow except for later-term bulk and heavy nonperishable cargos to Mars, though the Moon isn't that far off for them. Nuclear power is preferable over solar arrays for the return trip, solar losing efficency beyond Earth distances, and would scale up.

VASIMR electroplasma, would require an ungodly amount of electricity to pull off. Will probably have to wait for advances in magnets and vapor core nuclear reactors to reach reasonable weights. Large system weight, only moderate thrust, but excelent fuel economy.

Anything with tethers make me nervous, presenting alot of cable to debries impact over any length of time, and unless you intend to stop the thing for each payload I have my doubts that you can link up with it reliably... I'm really not liking the whole idea in general, with all the dynamics and development and such involved. Even the ETP will be pretty heavy. Space elevator materials are questionable, among other reasons.

Sails of all sorts are only good for very light weight craft that aren't in a hurry to get anywhere, plus are very hard to maneuver. Might be good for very long range probes, but not for people or cargo.

Now for nuclear options, for which there are plenty more than NTR...

-NERVA NTR, solid core reactor, liquid hydrogen heated to 3000K. A tested, available technology, the easiest and lowest investment needed. 1000Isp and high thrust, marginal benefit, modest engine mass.

-GCNR, my personal favorite, the gas core nuclear rocket. Liquid hydrogen vortex contains a critical uranium vapor cloud, releases huge amounts of energy. High thrusts, low fuel masses, but slightly beyond current technology. 3,000Isp for early and 6,000Isp+ for mature possible.

-The wacky Orion drive, that the rocket rides on the blast wave produced by small nuclear bombs. Very high thrust, Isp varies depending on the size of pusher plate, usually between 5,000-9,000Isp if memory serves for a large plate.. Serious political implications.

-NSWR nuclear salt water rocket, uses a solution of uranium in water of concentration that induces criticality when enough is brought together, not that hard to do with current technology but fuel poses extreme environmental and high handling hazards. High fuel tank mass, and probably high engine mass, though similar or slightly lower performance as Orion without the political nightmare.

Gennaro · 2004-03-24 21:56:18

Good topic. In fact, I'd like to know more about trajectories generally, especially in regard to various velocities (propulsion systems).

To Mars everyone probably agrees that a conjunction trajectory is the way to go, but how much can you practically bend a conjunction trajectory (and get a shorter travel distance) and still not add ludicrous amounts of thrust to your vehicle?

Also what risks are there with higher velocities in regard to orbit entry and aero-braking?
To the Moon for example the Apollo rockets went at a quite leisury pace in order not to miss being captured by the low gravity alltogether, or so I've understood it.

GCNRevenger · 2004-03-24 22:05:31

Aerobraking is not all its cracked up to be... that heat shield has a nontrivial mass, it limits the transit speed you can have, and requires a high degree of trajectory accuracy... if you use a high-ISP propulsion system, propulsive capture would be preferable.

Gennaro · 2004-03-24 22:25:20

Interesting that you say so. Just had an idea regarding absence of aero-breaking. Say it's early colonization time on Mars (the base building phase is over). More and more people want to go there and the demand for goods and cargo in both directions is steadily increasing, although we are far away from being able to build for example orbital infrastructure around Mars.
If we built a space station around Earth on the other hand, would it be practical to introduce a sort of interplanetary SSTO which could go directly from the Mars surface to the Terran space station and back, refuelling at both points of departure?
To be able to haul payloads in both directions without having to build new craft all the time would be a benefit. The Martian surface Delta-V to escape is only 5.5 km/s. Say it used a simple H2/O2 chemical engine. The way I understand the rocket equation (I'm really not very good at math) such a spaceship would get a mass fraction of somewhat less than 3.
Getting propellant at both points would be easy: ferried up from Earth on other SSTO's and produced by catalyzation of water on Mars.
Question is, could such flights be made frequent enough to pay off or differently put, how much could one deviate from the conjunction trajectory?

GCNRevenger · 2004-03-24 22:30:43

4.4km/sec Delta-V I think is for best allignment orbital transfer, and then you have to also consider the higher Delta required to get from LEO to the surface of Mars. If you wanted to make this kind of flight on a regular basis outside of favorable launch windows, I think such a system would require one of the more exotic >1,000ISP engines for orbital transfer and a second set of chemical engines for landing.

Gennaro · 2004-03-24 22:42:58

Of course the Terran LEO to Mars surface would have a somewhat higher Delta-V and worse mass fraction, but it would still be pretty good, no?
Okay I see, about Isp 1000. The example was chosen in order to avoid fission drives, which would likely be trouble to handle at both destinations.

GCNRevenger · 2004-03-24 23:07:40

The use of somthing with better Isp than chemical engines is a must for this sort of Mars Shuttle... I'm wondering if such a vehicle could have regular chemical engines for final landing/pad-to-altitude takeoff, and then kick in the GCNR engine for the TEI/TMI.

SBird · 2004-03-25 01:48:49

GCNR, I agree with most of what you said earlier. Sails, ion engines, rototethers, and solar thermal are all non-starters as far as I'm concerned.

I think that ETP has potential - the mass of the tether for ETP is trivial as it has to carry very little force as opposed to a rototether. Getting enough power to run the thing at any appreciable speed is difficult, however. If there were a way to run a 5 MW nuclear reactor in Earth orbit, you could boost a 140 ton Mars mission to TMI in about 1.5 months IIRC. The weight involved with ETP would require that it operate in an Earth tug configuration. There's no reason to take it along as it's not going to work at Mars with no magnetic field there anyways. While it is a bit slow and requires a seperate drive tech, it has an infinite ISP for all practical concerns, which is nothing to sneeze at.

NTP gives some benefit, basically cutting the fuel fraction of a Mars Direct mission by about 2/3.

GCNR is promising but the technical concerns of getting it running reliably might put it out of the timeframe of initially getting to mars. 30 - 40 years down the road, it does have the potential to cut fuel fractions to almost nothing.

Orion/NSWR - I think that the probability of anyone letting you fire either of these drives off in LEO can be nicely approximated as 0.

I'm dubious about a Mars to LEO SSTO. Too many design conflicts between a good SSTO and good interplanetary freighter. The Martian SSTO would probably use either CH4/O2 or NTR for takeoff and has to be able to withstand aerodynamic forces. The LEO to LMO freighter probaly uses ion engines (for freight) or something equivalent and has all sorts of big radiators and other ungainly stuff hanging off it.

It just doesn't make much sense to have a heavy, aerodynamic vehicle go interplanetary. I think that a custom built Mars SSTO could ferry stuff up to LMO where it can be loaded up by a dedicated interplanetary freighter that cycles back and forth. You'll get much better performance out of both.

Also, we should look at these new 0-energy transit orbits. I've ben reading a little more about them and it appears that you can use them to go from any L point to any other L point in the solar system for pretty much no delta V. I think that initially getting to a particular L point from a planetary surface has to be done the old fashioned way but going back down to the planetary surface can be done with a 0 energy trajectory. Check out the NASA Genesis experiment for an example of such a trajectory. The example Earth to Mars trajectory actually looks fairly straightforward. If we use this scheme, it looks as if the only delta V we have to provide is LEO to moon L1. If we're able to use something like an ETP tug, LEO to Mars surface can be done for essentially free. 140 tons to LEO becomes 140 tons to Mars surface.

Rxke · 2004-03-25 02:22:44

a bit off-topic, but about the refuelling in orbit...

Anyone remember PROFAC? It turned out to be a no-go, because at the time they intended to use a nuclear reactor to make up for the orbital decline, but now we have ion engines, and maybe tethers...

A cryopump mounted on a satellite orbiting the Earth at the altitude of 160 km collects oxygen and nitrogen. At this altitude one cubic kilometer of the atmosphere has the mass of one kilogram. One half of the mass is oxygen, while the other half is nitrogen. The gasses are stored as cryogenic liquids. In a few months the mass of the collected gasses is the same as the mass of dry satellite. The gasses are loaded into last stage of a rocket launcher. Electrodynamic tether or ion engine restores orbital energy. The ion engine accelerates nitrogen ions to a velocity several times greater than the orbital velocity. This engine consumes more solar power than the electrodynamic tether, but is more convenient to use, because it can operate at any altitude.

(most of this comes from [http://www.islandone.org/LEOBiblio/SPBI101.HTM]here .)

So harvesting oxygen around the earth, but still in need of hydrogen or something similar... Would it be possible?

Gennaro · 2004-03-25 08:29:09

It just doesn't make much sense to have a heavy, aerodynamic vehicle go interplanetary. I think that a custom built Mars SSTO could ferry stuff up to LMO where it can be loaded up by a dedicated interplanetary freighter that cycles back and forth. You'll get much better performance out of both.

That may be true, that may be true... especially since it's now understood such a vehicle would actually have to have a dual propulsion system in order to be utilized on a regular basis and not to be confined to narrow launch windows.

Though if you send the vehicle all the way to Mars surface you'll save one spacecraft and you'll make it servicable on both destinations (Earth LEO, provided a space station, and on Mars), not just refuellable.
Also, don't blame GCNRevenger for the the Mars SSTO Cycler, if you think it's real lousy. He's just answering questions, I'm the one coming up with the bad ideas.

GCNR is promising but the technical concerns of getting it running reliably might put it out of the timeframe of initially getting to mars. 30 - 40 years down the road, it does have the potential to cut fuel fractions to almost nothing.

But that's a perfect timeframe for being ready at the Mars Colonization phase!
Personally, I like GCNR. It has a lot of power and could push huge amounts of cargo for a liberal choice of trajectories.

Question: For attitude change (costing Delta-V), could there possibly be advantages for high thrust systems? I mean, Ion and such needs a long time to utilize its Isp potential, but a GCNR for example would have lots of power readily available to change direction at an early time.

So harvesting oxygen around the earth, but still in need of hydrogen or something similar... Would it be possible?

Sounds like a great idea. Only having to ferry up the hydrogen would certainly ease off SSTO operations. But for nuclear cyclers the usefullness would be limited naturally, since they have no need for oxygen at all.

GCNRevenger · 2004-03-25 10:07:05

There is an advatage for high-thrust systems in general, that they don't need to spend alot of time accelerating, which is one of the reasons why low-thrust systems will be bound to a much longer travel time even with similar or superior fuel efficencies.

Though I think GCNR would be ideal, and am still sold on the concept until somthing better comes along, I am warming a little to the concept of the NSWR system... If you can launch the Uranium salt and the water seperatly with chemical launchers, that would eliminate alot of the environmental trouble, and if the liquid plume can be tailored to go critical far enough outside the engine, then the engine itself could be made very reliable. The fuel would be noncryogenic and pretty dense too, solving some of the issues of pure hydrogen. Water is also easier to get in space, generally speaking. I'd like to see some figures on fuel tank mass fraction though.

Though my heart be with GCNR, the pragmatist in me sees merit in NSWR...

I'm not liking the ETP concept too much, even getting 100kWe of power in space is a big pain, and I don't see such a reactor coming available for a while. Nasa only has the capacity to build a 300-400 kilowatt reactor at the moment, and would take a looong time to make a multimegawatt model. It would be fairly huge to... nothing like the ETP has ever been tried before either, and even if it were pulled off, getting stuff to Mars fast at any day of the year would be a big issue.

An ion powerd LEO "fuel scoop" is also out of the question, if for no other reason than the reactor could come back down too easily and solar pannels would make alot of drag.

dicktice · 2004-03-25 10:12:24

Re. tether acceleration: What wrong with straight momentum exchange, that you-all seem to ignore in favour of electrical versions in magnetic fields. If the winch is carried, so the tether can be retracted for reuse later? All that is needed is a 10X mass (say) in LEO, such as a depleted-fuel tank from which be transferred from to excape velocity.

Bill White · 2004-03-25 10:23:09

Also, we should look at these new 0-energy transit orbits. I've ben reading a little more about them and it appears that you can use them to go from any L point to any other L point in the solar system for pretty much no delta V. I think that initially getting to a particular L point from a planetary surface has to be done the old fashioned way but going back down to the planetary surface can be done with a 0 energy trajectory. Check out the NASA Genesis experiment for an example of such a trajectory. The example Earth to Mars trajectory actually looks fairly straightforward. If we use this scheme, it looks as if the only delta V we have to provide is LEO to moon L1. If we're able to use something like an ETP tug, LEO to Mars surface can be done for essentially free.
140 tons to LEO becomes 140 tons to Mars surface.

I have little to add, I just don't want this lost in the clutter.

Probably won't work for crewed flights (time spent in space is too long) but for supply drops and rovers and big bulldozers this seems literally revolutionary.

John Creighton · 2004-03-25 10:46:30

It just depends on how we travel. Remember the L1 space station? Maybe we will eventually learn how to build a space station that doesn't leek, recycles 99.9% of its wast, protects the crew from radation and micro meteriotes, has an ion thruster and cost under a billion to build.

John Creighton · 2004-03-25 10:48:16

and the space station either rotates or we have another way to counter act zero g.

Gennaro · 2004-03-25 11:06:32

You bet it rotates!

Launch the entire centre frame with one huge NSWR rocket. Make the toroid wheel out of inflatables and coat it with boron doped polyethylene, though place it in LEO rather than L1 so it can be reached by SSTO.

It'll cost more than a billion, of course!

John Creighton · 2004-03-25 11:14:33

If it is in LEO it will take it forever to go to L1 with an ion engine. Requiring the station to rotate means that the materials used to make it must be stronger (For instance the solar panels). If it cost more then a billion to build there will be very few ever made.

GCNRevenger · 2004-03-25 11:15:25

And the question remains... WHY would you want to build this station?

John Creighton · 2004-03-25 11:54:39

In such a ship you could:
-explore the entire solar system in luxory
-transport people between the L points of planets with miniamal energy
-learn how to live in space
-have a huge labratory for research into medical an material science.
-have a safe haven to escape to when exploring any planet

SBird · 2004-03-25 12:55:46

Remember the L1 space station?

Oh, yeah - especially the time that guy from Brazil got 0-G sick all over the shuttle control panel just as we were going into the final docking manuevers with the station! Or that Italian lady that do those cool juggling tricks with the Coriolis effect from the station rotation? Ah yeah...good times....

John Creighton · 2004-03-25 13:00:28

I liked drinking carona on the lieto deck by the pool, well sun tanning under the UV lights.

Gennaro · 2004-03-25 13:22:00

And the question remains... WHY would you want to build this station?

Well... I thought it could be a good place to store fuel, cargo and people changing transport systems and to service cyclers.

I got big plans you see.

Anyhow, I might have a fair guess what you're about to say: cyclers can be refuelled and exchange cargo with SSTO's directly on orbit etc, etc, so spacestations are only juvenile rubbish.

But it is what Flash Gordon would have built, dammit!

Gennaro · 2004-03-25 13:39:19

If it is in LEO it will take it forever to go to L1 with an ion engine. Requiring the station to rotate means that the materials used to make it must be stronger (For instance the solar panels).

Indeed but how else are you going to reach it? SSTO won't do. Earth surface access is everything. Maybe something to tug whatever to L1? Maybe not use ion engines?

Solar panels? I thought you could use nuclear reactors.

If it cost more then a billion to build there will be very few ever made.

I'd be happy to build one.

Yet okay, space stations are maybe just too sci-fi...

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Re: LEO to TMI - discuss

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