SEP mission to Mars

neviden · 2007-05-07 07:42:07

What is needed:

- SEP spaceship/tug, that can be rotated to create artificial gravity (0,3 – 1 g) and have good radiation shielding. It would have to be able to make 0,5 – 1 km/s delta-v per month with isp in the range of 3000 – 7000 s. That would make it around 10 MW, 200 m x 200 m big, 30 – 100 MT. Usable for 5 – 20 years.

SEP spaceship/tug would be the most advanced part, but there is nothing too expensive about it. Two big solar wings (100 m x 200 m) attached to central “spine”, able to withstand 0 – 1 g continuous force. Always pointing toward the sun with multiple movable thrusters (direct drive hall thrusters?). Living quarters would be attached to one end of the spine, Tanks, extra radiators to the other end. At the middle of a spine: docking ports for Crew transfer ship, Mars descent/ascent vehicle, optional Mars cargo landers (more of them if there would be no crew onboard – cargo only). Basically the combination of this two ships:

http://www.entechsolar.com/SLA-SEP-WCPEC4.pdf
http://gltrs.grc.nasa.gov/reports/2006/ … 214106.pdf

- Crew transfer spaceship. It would have to be able to get from LEO to HEEO (3 km/s delta-v) and would be able to directly reenter to Earth with speed of 13-15 km/s. That would make it around 20 – 30 MT in LEO, 10 – 15 MT (empty) attached to SEP spaceship (3-5 years attached). Used once. Basically Orion + 20 MT EDS or Soyuz + 20 MT Block DM (launched on one Proton).

- Mars descent/ascent vehicle. It should be able to aero brake to Mars, ascent from Mars to Mars High Orbit, 5 km/s. 10 – 20 MT empty. Could be reused.

- Mars cargo lander. It would aero brake and land on Mars. 20 - ?? MT. Whatever would be needed on mars would be in them.

How it would be done:

Everything could be launched on existing rockets. Bigger rockets would help but they would not be necessary. SEP spaceship would probably need some assembly, but nothing too complicated. It would be assembled in LEO. If there would be any problems, crew could come from earth or ISS to fix any problems. It would get tested and parts would get send and attached to it. When ready, it would slowly cycle to high earth orbit (3 – 6 months) and wait there. More then one SEP spaceship could be assembled, cargo versions would weight more and take more time to get to HEEO.

When the time would be right for transfer to Mars, the crew would arrive with Crew transfer spaceship. SEP would fire thrusters and would escape Earth-Moon system, do 1,5 km/s delta-v needed to get on a Mars Transfer orbit, then start to slow down (1,5 km/s) so that it can get captured by Mars when they meet (3 – 6 months). It would then wait in High Mars orbit until the crew arrives back from Mars and it’s time to return to Earth (18 months later). It would then return on a course to Earth (3 – 6 months, 1,5 km/s), change trajectory to get captured into HEEO (1,5 km/s, optional). Crew would board crew transfer spaceship and return to Earth (3 days). Crew version could either wait in HEEO for another crew and propellant (sent by another SEP) or go to LEO (2-4 months, 3 km/s) to be fixed, checked or upgraded.

Cargo version would only accelerate by 1 km/s from HEEO, Mars cargo landers would detach from it (they would aero brake directly to Mars), SEP would change it’s orbit so that it flies past Mars and would reduce the speed so that it returns to HEEO (1 km/s) and then to LEO (3 km/s), ready to be reused again.

What problems do you see in this achitecture?

cIclops · 2007-05-07 08:05:57

What is needed:
- SEP spaceship/tug, that can be rotated to create artificial gravity (0,3 – 1 g)

This is often quoted but there is no evidence that this will reduce the problems of living in 0g. A rotating spacecraft may be worse than a non rotating one. This needs to be tested before such a design is used.

neviden · 2007-05-07 10:01:45

Rotation would be in the range of 1 - 3 RPM. At 200 m, the coreolis and gravity gradient problems should be minimal. I have read repots that humans could adapt up to 10 RPM if the change would be done gradually. If there would be any problems (what kind of problems?), crew could still travel at 0 g with stationary SEP tug. The tug itself does not need rotation to function.

cIclops · 2007-05-07 12:14:20

Rotation would be in the range of 1 - 3 RPM. At 200 m, the coreolis and gravity gradient problems should be minimal. I have read repots that humans could adapt up to 10 RPM if the change would be done gradually. If there would be any problems (what kind of problems?), crew could still travel at 0 g with stationary SEP tug. The tug itself does not need rotation to function.

1. It may not work. Rotation may not counteract the effects of low gravity (eg bone loss, reduced muscle strength and endurance). It needs to be proven.

2. Nobody has lived inside a rotating structure within a 0g environment before, that's why tests need to be made to discover any side effects.

3. Far better to discover problems with such a mechanism before making the long voyage to Mars and adding the useless mass.

GCNRevenger · 2007-05-07 12:21:52

I think it could best be summed up by the following: "limited benefit."

Carrying enough fuel to brake the ship back into Earth orbit will decrease the benefits a bit, as will the need for an additional launch to bring up a booster for the Orion to reach the vehicle. Assembly, ANY assembly, is not acceptable. If the vehicle cannot be reasonably assembled by docking, then this is a serious drawback. And if it can be assembled by docking, then this means each piece must be powered and maneuverable. How many pieces are there going to be?

The landers you mention are also going to be much more massive than you think, on the order of 40MT, not 20MT, and you would need at least two of them per mission. They are also not going to be reusable most likely, that would be a reach to afford for a while, and you need to send down a habitat anyway so there will still be two vehicles involved. Whether these vehicles are expended or not, they will still have to be refueled/resupplied/replaced for the next trip, and sending these up to the SEP in HEEO will consume almost as much rocket fuel as pushing them directly to Mars! You will also have to send up more Xenon and supplies for the ship too, in addition to the crew, which will take up a third 40MT "slot."

Unless of course you have two of these monster SEP vehicles, giving you an average replacement time of 4-6 years each (assuming an 8-12yr service life). For a low-energy biannual trip, that means a brand new SEP every two or three trips. The fuel for an ion-powered SEP is also ruinously expensive, and will quite likely consume the entire world supply of Xenon in the quantities required.

Trip times will not, repeat not be substantially shorter than chemical either, such a vehicle simply cannot generate enough thrust to greatly reduce trip times while lugging that kind of mass short of a monstrous solar array, particularly as you start getting away from the sun. You only have about ~70-80% of Earth-normal flux on average between here and Mars I bet.

You can also forget about making the thing spin, the solar arrays would have to reposition themselves far far too much to cope with the change in attitude needed to give any practical amount of gravity. It ain't happening, especially not with ion thrusters doing the spinning, throwing your precious Xenon overboard. Remember, the only really good reason for Ion power is to reduce propellant mass, anything that threatens this advantage threats the justification for the ship in the first place. I am also skeptical if the structure mass could be kept down in the spinning monster.

So lets summarize here, it will take at least three HLLV and two 25MT crew launchers minimum to mount a mission with the SEP, or six HLLV launchers and one crew launch for chemical. Later on, when we have a Mars base and we could have a reusable lander fueled entirely on Martian propellants, then the launches needed for chemical drops to only two HLLV and one or two crew launches. In the short term you might make an argument for SEP, but I think it makes more sense to spend the money and accelerate the long term.

There is also the issue of economies of scale, that unless the HLLV were abandoned and a smaller 40MT class launcher were built instead paired with a second SEP from LEO-HEEO, then the HLLV line will be launching many fewer times per year, but we will still be paying a lot of money for the infrastructure for it. And if we do make a 40MT class rocket, that will cost more development money as well as increased replacement rate for the LEO-HEEO SEP tug, particularly since it will spend most of its life in the Van Allen belts.

RedStreak · 2007-05-07 12:45:20

I think it could best be summed up by the following: "limited benefit."
Carrying enough fuel to brake the ship back into Earth orbit will decrease the benefits a bit, as will the need for an additional launch to bring up a booster for the Orion to reach the vehicle. Assembly, ANY assembly, is not acceptable. If the vehicle cannot be reasonably assembled by docking, then this is a serious drawback. And if it can be assembled by docking, then this means each piece must be powered and maneuverable. How many pieces are there going to be?

Sadly many bad points GCNR points out. Those ISS-sized structures have less than a snowball's chance of getting assembled on schedule especially w/o the STS in service; the Ares V could assemble chunks but it would require assembly more akin to Russia's modules with some autonomy involved.

Aerobraking wouldn't be bad to try out, but SEP or NEP will be another matter, especially the later of the two due to politics and fact we haven't flown a full-up nuclear reactor into space (those isotope batteries on space probes don't count). The only time I see where SEP might be benificial is on the cargo side and on perhaps the return leg of a Martian crewed mission - possibly the transfer SEP module used on the cargo could be reused on the crew's return trip but that's debateable as opposed to a in situ chemically-fueled escape.

I would like to see more SEP myself but don't hold your breath for it in manned exploration.

neviden · 2007-05-07 16:30:40

1. It may not work. Rotation may not counteract the effects of low gravity (eg bone loss, reduced muscle strength and endurance). It needs to be proven.

Rats raised in centrifuge on Earth had stronger bones, stronger mussels, but otherwise were healthy. But you are right, only the actual testing will give conclusive results.

Carrying enough fuel to brake the ship back into Earth orbit will decrease the benefits a bit,

That decreasing would be done with high isp engine, therefore it would not be that much of a problem fuel wise. And it would allow reuse. It would basically be mobile space station. With more than enough power, with capability for very long duration stays, with capabilities to grow it’s own food, move around, be radiation shielded.

It could return to Mars, could visit and could use/mine/move (small) asteroids, serve as a space station in moon orbit (it could easily do needed station keeping delta-v with high isp engine), serve as a staging area, serve as manufacturing station with 10 MW of electrical power and 1g (that way it could do everything that we couldn’t do in 0 g), become space tug,..

as will the need for an additional launch to bring up a booster for the Orion to reach the vehicle..

Preferable would be for Orion/Soyuz either to refuel in LEO or be capable of 3 km/s when it arrives to LEO. Separate, dockable EDS would also work. It would be light and would be the only thing that would get to HEEO with rockets. Everything else would get there efficiently, slowly with SEP.

They are also not going to be reusable most likely, that would be a reach to afford for a while, and you need to send down a habitat anyway so there will still be two vehicles involved. Whether these vehicles are expended or not, they will still have to be refueled/resupplied/replaced for the next trip

Habitats will be one way and would probably arrive on cargo SEP flights. Manned landers could be SSTO from Mars to Mars orbit (4 km/s is doable, even 5 km/s would be quite possible). They could be made either refuelable on the surface (ISRU) and therefore reusable or could be sent new from Earth every time. It would stay on Mars/Mars orbit either way.

sending these up to the SEP in HEEO will consume almost as much rocket fuel as pushing them directly to Mars!

Mars landers would have to be delivered only to LEO. There it would dock to SEP tug which would transport it to HEEO. There it would have to redock to manned SEP or that SEP could deliver it to Mars orbit/Mars.

You will also have to send up more Xenon

Yes, you would need more Xenon. Or Argon. Or Hydrogen. Or Oxygen. Or anything else that can your electric thruster can use. You have 10 MW, 600 - 1000 V of electricity. It doesn't have to be Hall.

In the long run it doesn't have to come from LEO. Fly one SEP to some nice low delta-v asteroid, process the material, return to HEEO full of propelants.

and supplies for the ship too, in addition to the crew, which will take up a third 40MT "slot."

Supplies would also come with SEP from LEO. The number of slots is not really limited. More mass would mean slower transit. If you would like to send more mass to the Mars, you can use more SEPs (they are reused). They could all wait in HEEO until planets align.

Unless of course you have two of these monster SEP vehicles, giving you an average replacement time of 4-6 years each (assuming an 8-12yr service life).

There would be more than one SEP vehicle. Once launched they would be used to transport things from LEO to HEEO, HEEO to LLO, HEEO to Mars orbit, HEEO to Asteroids,... New ones could get used for important missions (human). Older ones could be used for less important missions (cargo). The oldest could be used for power or LEO to HEEO missions. Solar cells themselves would be shielded enough to last as long as possible (preferably way more than 8-12 years).

For a low-energy biannual trip, that means a brand new SEP every two or three trips. The fuel for an ion-powered SEP is also ruinously expensive, and will quite likely consume the entire world supply of Xenon in the quantities required.

Hall's can also use Argon. It is less efficient but it is cheap. The ultimate goal will be to use something that is already in space.

Trip times will not, repeat not be substantially shorter than chemical either, such a vehicle simply cannot generate enough thrust to greatly reduce trip times while lugging that kind of mass short of a monstrous solar array, particularly as you start getting away from the sun.

It doesn't matter if they are not substantially shorter. They will be cheaper because they will not require less to be sent to LEO. They will have gravity, radiation protection, food, air and a place to wait comfortably. They will be fine.

You only have about ~70-80% of Earth-normal flux on average between here and Mars I bet.

As long as they can get enough power to make necessary delta-v, they will be fine. Manned SEPs would be lighter/faster, Cargo SEPs would be heavier/slower. On the way back from Mars SEP would be lighter (less propelant, no more Mars landers).

You can also forget about making the thing spin, the solar arrays would have to reposition themselves far far too much to cope with the change in attitude needed to give any practical amount of gravity.

SEP would become one big spin stabilized satellite. They resist changes in orientation. Propellant would have to be used once, after that it would turn forever. Any changes would be done with high isp thrusters anyway,

It ain't happening, especially not with ion thrusters doing the spinning, throwing your precious Xenon overboard. Remember, the only really good reason for Ion power is to reduce propellant mass, anything that threatens this advantage threats the justification for the ship in the first place.

propellant used for spin/de-spin would be minimal.

I am also skeptical if the structure mass could be kept down in the spinning monster.

NEP ship would weigh 233 MT of which the truss would weigh 14 MT. It seems reasonable.

So lets summarize here, it will take at least three HLLV and two 25MT crew launchers minimum to mount a mission with the SEP, or six HLLV launchers and one crew launch for chemical.

It doesn't really need HLLV (100+ MT). The biggest element (40 MT Mars crew lander?) delivered to LEO would set the smallest possible rocket launcher. HLLV would reduce number of rockets sent, but since there is nothing to boil off, there would be no hurry. SEP would act as a 1 g space station in LEO. Everything needed would dock with it. Soyuz/Orion could also dock to it so the crew could check/fix everything. Once ready it would put itself into HEEO (unmanned) and would wait for crew.

HLLV would reduce number of rockets sent, but since there is nothing to boil off, there would be no hurry. SEP would act as a 1 g space station in LEO. Everything needed would dock with it. Soyuz/Orion could dock to it so the crew could check/fix everything. Once ready it would put itself into HEEO (unmanned) and would wait for crew there.

Later on, when we have a Mars base and we could have a reusable lander fueled entirely on Martian propellants, then the launches needed for chemical drops to only two HLLV and one or two crew launches.

Actually, once there would be enough SEPs the only thing that would be needed would be propellant (Xenon, Argon) and Mars Cargo Landers delivered to LEO and Crew delivered to HEEO.

In the short term you might make an argument for SEP, but I think it makes more sense to spend the money and accelerate the long term.

SEPs would get reused. Mars transit ship would get reused. Mars Landers would get reused. SEPs could support missions to asteroids to get propellant (to be used instead of Xenon). They would even have enough electricity to split the water (from asteroids) to make Hydrogen and Oxygen (for crew transits to HEEO). They could grow their own food and live safely in 1 g.

It looks long term to me.

There is also the issue of economies of scale, that unless the HLLV were abandoned and a smaller 40MT class launcher were built instead paired with a second SEP from LEO-HEEO

That would work. It would also be cheaper since 40MT class could be evolved from current launchers.

, then the HLLV line will be launching many fewer times per year, but we will still be paying a lot of money for the infrastructure for it. And if we do make a 40MT class rocket, that will cost more development money

US government (DoD) is already paying for two rocket infrastructures. Both are vastly underused. If you didn’t need Ares V, there would be no need to develop and maintain it. Money could be better spent to build things for Mars exploration instead on third rocket system (that would be to large for anybody else to use – low launch rate).

as well as increased replacement rate for the LEO-HEEO SEP tug, particularly since it will spend most of its life in the Van Allen belts.

These solar panels could be shielded even more to reduce replacment rate.

GCNRevenger · 2007-05-07 20:22:45

Whoa now, come back down to the ground neviden

A high-Isp engine, even one with 5000sec, is not a "get out of Tiokovski free" card, you keep adding things that it is supposed to do, now it is a fuel depot, now it has greenhouses, etc and the "baseline" ship weighs 230MT already? A spacious SEP vehicle with greenhouses and living quarters and heavy radiation shielding and carrying 80-160MT of landers/stuff is going to add up, probably tipping the scales around 500MT without much trouble. Even using a tenth the propellant of chemical - with aerobraking both ways and ISRU for ascent - thats like 150MT of Xenon. Per trip. That would exhaust the world supply of the stuff and then some too. It would not "be fine."

And remember, you only get the nice 5000sec Isp with the 10MW plant if you use Xenon, performance with Argon, Cesium, or Mercury is inferior since those require more power to ionize. And if you don't use an ion engine, then what? A plasma thruster could use Hydrogen, but those have their own drawbacks, such as a ten-fold increase in tankage mass and the need for a boiloff condenser. If you use a VASIMR or other magnetic-bottle engine, you would need a much bigger condenser plant to keep the magnets cold. It would not "be fine," all the non-Xenon options include substantial mass and/or cost penalties.

Oh, and the 40MT figures are really the bare minimum for a Zubrin MarsDirect scale four-man lander. The ones in NASA's DRM-III are in the region of 80MT including decent fuel. I also reject the notion that a reusable Mars lander is an easy trivial thing; Mars has an atmosphere and significant gravity, building a vehicle that is both light, strong, and reliable will be a tall order. Such a vehicle will take years to develop and many billions of dollars to build it right.

On more logistical notes, to echo RedStreak, building any ship in the 500MT region will be ruinous because in-space assembly is one of the greatest disasters-that-worked ever. The reality of the ISS is tangible proof that relying on this is folly. The ISS will weigh 400MT and be of the same scale as the SEP, plus we won't have the space shuttle's guidance/robot arm/etc, necessitating an expendable guidance system for each piece. Even a quartet of monster 120MT class HLLV would be pushing it, much less a dozen 40MT class.

You also make lots of noise about how wonderful the reusable SEP is, but consider the competition, that small cryogenic chemical engines are dirt cheap enough to really be almost trivial, so the launch costs are where its at. Economies of scale for large rockets combined with near-zero replacement costs are not blown out of the water by the superhigh Isp of an SEP ship. An HLLV operation launching often with chemical won't be that bad price-per-kilo. Conversely, if you don't get to 5000sec, it might be even harder

I also reject the arm-waving/magic-wand assertion that you can just add more radiation shielding to the solar collectors or add redundant engines to give the SEP's whatever arbitrarily long lifespan you need to trump the competition. Adding even a little shielding to the cells of a solar array that big will add up fast, 25 more tonnes? 50? 100?

I also don't think you fully understand my problem about spinning ship + solar arrays, that building the ship so the arrays are gimbaled toward the sun but the rest of the ship spins is going to be really hard, I don't think it can be done with any reasonable solution. As with solar thermal rockets, though to lesser extent, there is also the problem of just pointing an array that big while the rest of your vehicle has to rotate relative to point its engines pro/retrograde. How will you overcome this problem?

Then there is this nonsense about mining asteroids for hundreds of tonnes of propellant: rubbish! The problems with zero-gravity digging on spinning bodies are manifold, there are not easy near-term solutions. Maybe a far-flung-future well beyond the days we could have a base more on Mars with chemical rockets.

Your whole plan about SEPs is actually almost identical to O'keefe's Lunar plan put out not long before Griffin came aboard. It was dropped because it was too complex, relying too heavily on ion propulsion, and too many small rockets. It just doesn't cost that much more to use rockets.

neviden · 2007-05-10 09:55:57

Whoa now, come back down to the ground neviden

Oh, don't worry. You all make sure I get knocked the ground with questioning every little detail and pointing out the problems. Even some that are clearly solvable only hard. And, that’s good. That forces me to look at how to improve things.

A high-Isp engine, even one with 5000sec, is not a "get out of Tiokovski free" card, you keep adding things that it is supposed to do, now it is a fuel depot, now it has greenhouses, etc and the "baseline" ship weighs 230MT already? A spacious SEP vehicle with greenhouses and living quarters and heavy radiation shielding and carrying 80-160MT of landers/stuff is going to add up, probably tipping the scales around 500MT without much trouble. Even using a tenth the propellant of chemical - with aerobraking both ways and ISRU for ascent - thats like 150MT of Xenon. Per trip. That would exhaust the world supply of the stuff and then some too. It would not "be fine."
And remember, you only get the nice 5000sec Isp with the 10MW plant if you use Xenon, performance with Argon, Cesium, or Mercury is inferior since those require more power to ionize. And if you don't use an ion engine, then what? A plasma thruster could use Hydrogen, but those have their own drawbacks, such as a ten-fold increase in tankage mass and the need for a boiloff condenser. If you use a VASIMR or other magnetic-bottle engine, you would need a much bigger condenser plant to keep the magnets cold. It would not "be fine," all the non-Xenon options include substantial mass and/or cost penalties.

If the limited world supply rules out Xenon, then our SEP would have to use something else. Ok, let us look at the baseline ship in the second link again. It’s NEP, it has 10 MW, 5000 s isp Hydrogen MDP engines. It flies on some weird Mission to Mars orbit and it’s moons, but since it delivers Crew return vehicle and habitat from LEO to Mars orbit and back with artificial gravity it’s a good starting point for a SEP long duration spacecraft. What makes up our 551 MT NEP:

- 91 MT nuclear power plant (110 W/kg)
- 32 MT Propulsion system,
- 65 MT Tanks and propellant management
- 83 MT Structures, Avionics, Habitat and Crew return vehicle
- 280 MT Hydrogen (+ 100% to the dry mass at 5000 s isp).
= 551 MT to LEO

How can we improve it? Ok, first of all, MDP engines with Hydrogen can run with isp as high as 10.000 s. Since the thrust produced with electric engines run at around 30-50 mN/KW, that means that we must get twice the power for the same thrust. That means we now have 20 MW requirement (or simply take more time to thrust with the same power). Let us replace nuclear power plant with solar cells at 500 W/kg (http://www.entechsolar.com/SPRAT05a.pdf). That means two 100 x 250 m long wings attached to the central truss. At one end of this truss we put Transhab. Around it we put smaller multilayered insulated hydrogen tanks. At the end of the truss we have one set of movable thrusters. To the other end of this 250 m long truss we put another set of movable thrusters, power converters, avionics, propellant management and bigger multilayered insulated hydrogen tanks. How does our SEP looks like now:

- 40 MT solar power plant (500 W/kg)
- 64 MT propulsion system
- 65 MT Tanks and propellant management
- 83 MT Structures, Avionics, Habitat and Crew return vehicle
- 150 MT Hydrogen (+ 60% to the dry mass at 10000 s isp?).
= 402 MT to LEO

(Out of the thin air estimate. Can anyone provide more accurate estimate?)

To carry things needed to land on Mars and actual Mars landers, you replace Habitat and Crew return vehicle with bigger tanks to get Cargo version. Cargo SEP then either delivers landers to a aerobreaked direct landing (4 km/s delta-v) or into Mars orbit (5-6 km/s delta-v). After that SEP returns to HEEO or LEO.

Oh, and the 40MT figures are really the bare minimum for a Zubrin MarsDirect scale four-man lander. The ones in NASA's DRM-III are in the region of 80MT including decent fuel. I also reject the notion that a reusable Mars lander is an easy trivial thing; Mars has an atmosphere and significant gravity, building a vehicle that is both light, strong, and reliable will be a tall order. Such a vehicle will take years to develop and many billions of dollars to build it right.

The landers themselves would be “just another cargo to be delivered to Mars”. If they stay on Mars (reusable) or they must be delivered new everytime (thrown away) is not important as far as SEP tug is concerned. Reusable would be preferable of course, but that depends on the money.

On more logistical notes, to echo RedStreak, building any ship in the 500MT region will be ruinous because in-space assembly is one of the greatest disasters-that-worked ever. The reality of the ISS is tangible proof that relying on this is folly. The ISS will weigh 400MT and be of the same scale as the SEP, plus we won't have the space shuttle's guidance/robot arm/etc, necessitating an expendable guidance system for each piece. Even a quartet of monster 120MT class HLLV would be pushing it, much less a dozen 40MT class.

402 MT Crew SEP would require 10 40 MT launches. (or 3 130 MT launchers). But i think this weight is probably way too pesimistic. My guess is that the 200-300 MT/10 MW/10K s isp, would be more realistic. That would put them in the 5-7 range. Still hard, but managable.

You also make lots of noise about how wonderful the reusable SEP is, but consider the competition, that small cryogenic chemical engines are dirt cheap enough to really be almost trivial, so the launch costs are where its at. Economies of scale for large rockets combined with near-zero replacement costs are not blown out of the water by the superhigh Isp of an SEP ship. An HLLV operation launching often with chemical won't be that bad price-per-kilo. Conversely, if you don't get to 5000sec, it might be even harder

If they are so cheap, then why is projected cost for Ares V+I 35 billion $ to develop + few billion $/year to run?

I also reject the arm-waving/magic-wand assertion that you can just add more radiation shielding to the solar collectors or add redundant engines to give the SEP's whatever arbitrarily long lifespan you need to trump the competition. Adding even a little shielding to the cells of a solar array that big will add up fast, 25 more tonnes? 50? 100?

But it’s not all in the cost of rockets. SEP would be much more expensive to develop and build, but once operational it will provide reusable but more importantly safer means of transportation then the chemical/NTP. The crew will get gravity to enable really long stays (prevents bone and muscle loss) with shielding by the Transhab wall + inner multilayer tank wall + lots of Liquid Hydrogen (excellent radiation shield) + outer multilayer tank wall. That’s a lot of shielding against even the worst solar flairs (or even against Van Allen radiation).

If extra radiation protection would weigh to much, then you can simply replace them. Or you can make special version of the wings for LEO to HEEO (or L2 if you prefer) and back to LEO Sep tug with extra radiation protection. SEPs only really have to make the trip through Van Allen belt once. After that they can stay in HEEO which is much more benign.

I also don't think you fully understand my problem about spinning ship + solar arrays, that building the ship so the arrays are gimbaled toward the sun but the rest of the ship spins is going to be really hard, I don't think it can be done with any reasonable solution. As with solar thermal rockets, though to lesser extent, there is also the problem of just pointing an array that big while the rest of your vehicle has to rotate relative to point its engines pro/retrograde. How will you overcome this problem?

One solution would be to make some complicated multi rotating ship or.. you can spin the whole ship and point it toward the sun. At the both ends of the truss you put one set of thrusters that would gimbal so they would always point to the same point. Since they would be at the opposite ends of the whole mass the effects would be the same as the one thruster at the center. The only time this would not work would be when the vector would point 90 degrees to the sun. Then one thruster would work for half rotation, wait until thruster can once again be pointed to the right way (during which the other thrusts) and start again. The solar cells would have to be strong enough not to brake of in 1 g. That’s like hanging them from a ceiling on earth (actually half of it, since in space only the outermost part of the cells would have 1 g. Closer to the center less force you would have).

To allow boarding and 0 g connections, we could use one “elevator” that would move along the truss to the center of rotation. You could even have one part of the truss heavier than the other (to keep hydrogen around transhab for as long as possible for example) and it would not matter. The tanks on the “nonliving” part of the truss could work just the same in 0.3 g or 3 g (if designed that way of course). When you would want to get the crew into Mars lander, they would enter small elevator that would attach directly to transhab. Elevator would “climb” along the truss to the center, where the lander could dock to it. The elevator could even go to the other end for repairs or supplies. Or, you could fire your thrusters to stop the spin and dock to transhab directly in 0 g.

Then there is this nonsense about mining asteroids for hundreds of tonnes of propellant: rubbish! The problems with zero-gravity digging on spinning bodies are manifold, there are not easy near-term solutions. Maybe a far-flung-future well beyond the days we could have a base more on Mars with chemical rockets.

What is so “nonsense” about asteroid mining?

- get SEP from HEEO into orbit around one asteroid. Since it can do 6 – 10 km/s there are a lot of possible destinations.
- detach one (unmanned?) “Asteroid lander” that grabs pebbles from the surface and returns to SEP (100 m/s)
- process pebbles with 20 MWe in 0 – 1g environment on SEP. (melt them? what can you do on earth with "dirt"?)
- return to HEEO

or

- get SEP from HEEO into orbit around one extinct comet core
- detach one “Asteroid lander”. Grab (= dig?) the pebbles until it gets secure grip
- drill into dirt
- melt ice, pump it to lander, detach from extinct comet core, dock to SEP
- split water with 20 MWe into H2 and O2, store
- return to HEEO

on another note...

The same design could be later “upgraded” to NEP for outer solar missions (beyond Asteroid belt). Put reactor back on the “nonliving” part of the truss, replace solar cells with radiators and you have 20 MW NEP. All of the things needed for SEP would be needed for such NEP anyway. Docking, high isp, thrusters, power, storage, rotating structures, deep space missions,.. What you don’t have to develop right now is reactor, converter and radiators. You must only know how to put solar panels on it, and we know how to do this (Deep Space 1 flew with those panels).

GCNRevenger · 2007-05-10 15:00:13

You are waving that magic wand again

10,000sec Isp for a Hydrogen engine is going to need a big complicated engine like VASIMR, which requires much much more support hardware (superconducting magnets, different voltage requirements). Also, the power requirement increase per-sec Isp is not linear over large changes, its exponential, so you are looking at 40MW and not 20MW (energy increases with the square of the velocity). I also don't buy the "optimistic" estimate for the specific power of the solar arrays, they are looking for a 70% improvement in just a few years? Uh huh, and how long will these cells last? Do not forget that they receive only ~75% the power on the average between here and Mars. Oh, and I bet this mass doesn't include the increased structure mass of the gargantuan solar arrays.

You also keep waving this notion of "if we just reduced the thrust..." repeatedly. I don't like it, if you have too little thrust then you will not have enough time to reduce velocity for gravitational capture at your destination, or at least not enough of a safety margin in the event of propulsion failure. People's lives will hinge on the engine firing as planned you know. Furthermore, even with modest thrust the departure from Earth will be awfully slow, or even the last bit of delta-V to escape velocity will be too time-consuming. Lastly, one of the greatest selling points of non-chemical propulsion is shorter trip time, which this vehicle does not furnish.

You misunderstand my misgivings about orbital construction, it is not so much the number of launches involved (which are a factor) but the need for assembly. It doesn't matter one bit if the whole vehicle could ride on a single SeaDragon mega rocket if it requires extensive assembly on orbit. Having bigger pieces with big launchers is preferable to smaller ones on a small rocket, but a ship like you are talking about would clearly need serious orbital assembly. If the thing cannot be built by simple docking of its parts then it is too complicated and will be a nightmare just like the ISS. Its not that its impossible, its just a bad thing. With DRM-III or similar mission, there is just one automated docking per vehicle, and thats it, no muss, no fuss. Even if one SEP would last several missions, this is a serious issue.

And about chemical rockets, I was referring to them as the boosters to go from LEO to Mars, not about from the ground to orbit. As these go, like J-2 and RL-10, they are dirt cheap, who cares if they are reusable or not. They will also have near-zero development costs by comparison to SEP etc. $10 billion dollars could buy you quite a few rockets, especially with economies of scale. You also take issue with the "development cost" of the Ares rockets, which is nonsense too. You must get your number from the same place as that gaetano boob, just adding up the VSE budget or something. But anyway, SEP needs these rockets too, especially if there will be no LEO-HEEO tug.

Speaking of the number of tugs, this is starting to get a little bit silly: you want one tug for crew, one tug for Mars cargo (likely 130MT per mission lets say), and enough armored SEPs for Van Allen belt transit with a one-year spiraling trip.

1 Mars crew SEP
- 200MT Hydrogen per trip
1 Mars cargo SEP
- 200MT Hydrogen per trip
1 "hardend" Van Allen SEP for 130MT Mars cargo
- 100MT Hydrogen per trip
2 "hardend" Van Allen SEPs for the 400MT Hydrogen for 1&2
- 100MT Hydrogen per trip
Total launch manifest: 700MT propellant (seven Ares-V rockets), 130MT Mars cargo (one Ares-V), and two Ares-I for crew (Orion + Centaur)

Versus NASA DRM-"IIIB"

"Stretched EDS" chemical TMI stages (3) (three Ares-V)
Mars HAB, MAV, and ERV (three Ares-V)
Crew launched by Orion (one Ares-I)

Yeah. About that whole "10,000sec ISP!" thing... And yes, I did up your fuel consumption estimates a little bit, since I think the solar arrays (inc. structure), radiation shielding, and probably propulsion are understated. Even if the fuel consumption for the SEPs is half that, it still isn't a whole lot better than chemical considering the development and SEP replacement cost.

But anyway, if these SEPs are designed to last for 20 years, then you need to replace one every four years. So that means another two or so Ares-V rockets annually plus probably one Orions/Ares-I to build the thing.

Total:
SEP - nine Ares-V, four Ares-I, three Orions/mission
Chemical - six Ares-V, one Ares-I, one Orion/mission

If you deleted the need for the "hardend" Van Allen SEPs, then this is reduced, but you still have to get the Mars cargo to HEEO somehow. One Ares-V and one Ares-I could probably provide the chemical fuel needed to get that high, which would help.
"One solution would be to make some complicated multi rotating ship or.. you can spin the whole ship and point it toward the sun. At the both ends of the truss you put one set of thrusters that would gimbal so they would always point to the same point. Since they would be at the opposite ends of the whole mass the effects would be the same as the one thruster at the center. The only time this would not work would be when the vector would point 90 degrees to the sun. Then one thruster would work for half rotation" Too complicated. Either the engines or the solar arrays would have to be on continuously moving gimbals, which will screw up the complexity considerably. A nice simple chemical engine... fire it once, throw it away. Nice and clean and easy.

Lastly I reject the notion that the SEP is somehow safer, just because it spins only eliminates the problem with zero-G physical damage and gives the crew a bit more room. It is not any safer as far as getting the crew from one planet to another.

neviden · 2007-05-10 19:20:02

You are waving that magic wand again.

Actually I am being pessimistic and conservative.. 64 MT for 20 MW MDP Propulsion system it too high. Not to mention 65 MT of tanks and condensers to store 150 MT of hydrogen. Here are some old studies. They are in the range of 300-400 MT in LEO for whole mission to Mars surface and back. I am looking more from the reuse point of view (more robust = more mass).

http://www.astronautix.com/craft/mek.htm
http://www.astronautix.com/craft/mars1989.htm
http://www.astronautix.com/craft/stcemsep.htm

10,000sec Isp for a Hydrogen engine is going to need a big complicated engine like VASIMR, which requires much much more support hardware (superconducting magnets, different voltage requirements). Also, the power requirement increase per-sec Isp is not linear over large changes, its exponential, so you are looking at 40MW and not 20MW (energy increases with the square of the velocity).

Actually, MPD are quite capable of 10.000 sec isp with Hydrogen. With more power and higher isp you also get lower alpha and higher efficiency. And since we also have smaller power plant and carry less propellant the 20 MW would get us about the same thrust. 10 MW would probably work.

I also don't buy the "optimistic" estimate for the specific power of the solar arrays, they are looking for a 70% improvement in just a few years? Uh huh, and how long will these cells last?

Those numbers don’t look too optimistic to me. Most of their weight goes to the concentrator and radiator. Solar cells are highly efficient and well shielded already. Put more efficient solar cells in them, you get more power out and need less radiators to radiate heat away from cells. Then you can increase the concentration and you get smaller core that can have even thicker layer of glass on it. They were almost a year in Van Allen belt and they barely degraded. 1000 W/kg seems quite possible, but 500 W/kg will get you some tough cells.

Do not forget that they receive only ~75% the power on the average between here and Mars. Oh, and I bet this mass doesn't include the increased structure mass of the gargantuan solar arrays.

Yes, I know they receive less. If installed solar panels would not be enough, you could do simple trick. Unfurl aluminium foil at both ends, the centripetal force will keep them stretched, you only need few thin cables that will keep it at the right angle to increase solar concentration by up to twice. That way you can have 100% electric power at Mars. When you return closer to Earth you relax the cables to prevent overheating. Very thin foil and thin steel wire doesn’t weigh a lot.

Structure mass. I think it does. For more structure, add more wire. Rotation takes care of the rest.

You also keep waving this notion of "if we just reduced the thrust..." repeatedly. I don't like it, if you have too little thrust then you will not have enough time to reduce velocity for gravitational capture at your destination, or at least not enough of a safety margin in the event of propulsion failure. People's lives will hinge on the engine firing as planned you know. Furthermore, even with modest thrust the departure from Earth will be awfully slow, or even the last bit of delta-V to escape velocity will be too time-consuming.

If you start to slow down even before you enter Mars orbit you get captured with very low delta-v. There is a limit how low can you go, and if this would be a problem you could fire small chemical rocket for final insertion. Most of the delta-v and time is spent in cycling away to HEEO. Once in HEEO you can easily get on a trajectory that will take you on a Moon fly-by and that will give you nice boost. If your crew starts “the clock” in HEEO, then it’s comparable to chemical. One month more or less doesn’t mean a lot on a mission that will last 3 or more years.

Lastly, one of the greatest selling points of non-chemical propulsion is shorter trip time, which this vehicle does not furnish.

No, but it furnishes something that is as fast as the chemical but safer.

You misunderstand my misgivings about orbital construction, it is not so much the number of launches involved (which are a factor) but the need for assembly. It doesn't matter one bit if the whole vehicle could ride on a single SeaDragon mega rocket if it requires extensive assembly on orbit. Having bigger pieces with big launchers is preferable to smaller ones on a small rocket, but a ship like you are talking about would clearly need serious orbital assembly. If the thing cannot be built by simple docking of its parts then it is too complicated and will be a nightmare just like the ISS. Its not that its impossible, its just a bad thing. With DRM-III or similar mission, there is just one automated docking per vehicle, and thats it, no muss, no fuss. Even if one SEP would last several missions, this is a serious issue.

If you have Ares V then you have 3 parts which can be docked. If you don’t have Ares V then you have no choice and you must do construction anyway.

And about chemical rockets, I was referring to them as the boosters to go from LEO to Mars, not about from the ground to orbit. As these go, like J-2 and RL-10, they are dirt cheap, who cares if they are reusable or not. They will also have near-zero development costs by comparison to SEP etc. $10 billion dollars could buy you quite a few rockets, especially with economies of scale. You also take issue with the "development cost" of the Ares rockets, which is nonsense too. You must get your number from the same place as that gaetano boob, just adding up the VSE budget or something. But anyway, SEP needs these rockets too, especially if there will be no LEO-HEEO tug.

You can try to get “economies of scale” by building more, but we are talking about higher numbers. “economies of scale” is not launching rocket few times per year. “economies of scale” is not having 10 launch systems for 100 satellites per year. I take issue with “development cost” of the Ares rocket, because it is redundant (US already has two launch systems) and because it actually harms the development. Why develop anything that will reduce cost in the long term, if you can simply build more of the same things and throw them away

That way of the thinking will get us to Mars.. in 30 years.. 4 crews.. for two years.. which will get canceled because it’s pointless (which it will be). Space construction is “hard”, therefore we should just dump everything, right?

Speaking of the number of tugs, this is starting to get a little bit silly..

What if you found “something” you could use for propelant (700 MT of hydrogen? Oxygen? “dirt”?), already in space? What would be cheaper then? What if you wanted to send 50 people to Mars? Would you send them in HABs? How many MT would you need to put into LEO? How much of that would be propellant?

Yeah. About that whole "10,000sec ISP!" thing... And yes, I did up your fuel consumption estimates a little bit, since I think the solar arrays (inc. structure), radiation shielding, and probably propulsion are understated. Even if the fuel consumption for the SEPs is half that, it still isn't a whole lot better than chemical considering the development and SEP replacement cost.

The whole isp thing is irrelevant, once you can use mass already in space. Good thruster would be one, that could use anything at any isp at low alpha.

A nice simple chemical engine... fire it once, throw it away. Nice and clean and easy.

And this is longterm?

Lastly I reject the notion that the SEP is somehow safer, just because it spins only eliminates the problem with zero-G physical damage and gives the crew a bit more room. It is not any safer as far as getting the crew from one planet to another.

How is keeping the crew from getting killed by radiation or keeping them healthy not safer?

GCNRevenger · 2007-05-10 19:32:52

The big problem with asteroid mining is how do you dig? Mining in zero gravity is so unlike Earth that most people just casually gloss over the simple physics problems involved.

Scoops, drills, etc all require being able to apply pressure against the surface of the asteroid, which you simply do not have because there is no gravity. You push against the surface, you get pushed off, and there is much less pressure than you would think.

Also, you are talking about an awful lot of Hydrogen here, you would have to process a great deal of asteroid ice to make 100s of tonnes of H2.

SpaceNut · 2007-05-10 20:58:28

Here are a few efforts to get to digging off world...

Teams will compete in robot challenge

NASA is offering $250,000 to the team whose system can excavate and deliver as much lunar regolith - moon dirt - as possible in 30 minutes.
Competitors' machines must use less than 30 watts of power, weigh less than 88 pounds, and excavate more than 330 pounds of the simulated moon dirt.

Lockheed scientists hope to draw oxygen from lunar dust to sustain astronauts

Engineers hope that mission will include some version of the Lockheed digger robot and soil cooker - which can mine gritty, gray lunar dust and turn it into breathable oxygen.
"We could make enough oxygen to keep three, four astronauts alive," said Larry Clark, a Lockheed senior manager and engineer. "We want to get this stuff cranking out oxygen before humans get there."
Two digging robots, each less than 4 feet by 2 feet, stood on the floor, with water-wheellike contraptions to lift lunar soil into a loader. The soil is then drawn into a cylindrical cooker

neviden · 2007-05-11 05:10:42

OK, it is the future. We have been on Mars. We have few SEPs in various orbits flying around, moving things slowly but surely. But the cost of all those rockets that must send bulky (and full) hydrogen tanks to LEO is starting to get a little high. We look around for nice big floating rocks in the range of 10-100 m, that are probably extinct comets (we can check them before with unmanned small probes like DS1). There are literally millions (http://spaceguard.esa.int/NScience/neo/ … number.htm) of them and some have delta-v from HEEO in the range of 1-3 km/s (http://neo.jpl.nasa.gov/ca/). We can use one Manned SEP that has returned back to HEEO. To it we attach all those (now empty) tanks that we had to use to bring hydrogen from Earth. We have 250 m long truss onto which we can attach them. Next, we also bring “asteroid lander” from Earth which we redock to our Manned SEP. Preferably more than one (they don’t have to be that big). Our “Mission to get Propelant” is ready to begin. The crew arrives from Earth, boards the ship, SEP fires it’s thrusters and after few months it arrives near one rock and orbit’s it in a nice distance to be safe and not have it’s wings polluted by the mining activities. The crew is completely safe and can remotely operate those “asteroid landers”. Our solar wings could even provide shade to prevent explosions of suddenly expanded gases/water.

The big problem with asteroid mining is how do you dig? Mining in zero gravity is so unlike Earth that most people just casually gloss over the simple physics problems involved.

Mining in 0 g would be like grabbing stones in water. No Asteroid is completely smooth. Most of them are rubble piles that are are loosely held together. Open your scoop (http://www.lifting-world.co.uk/Drapak%2 … 202%20.jpg), fire your thruster (10 m/s) to “land” on the asteroid. Close the scoop. If the material is loose you will get only small amounts of pebbles and dust in them. That's ok. Repeat this lot's of times and you have lots of dirt and pebbles that can be delivered to SEP for processing.

But, you can specifically try to “land” on top of a boulder that can fit inside the scoop. You either grab onto it (it is mechanically strong enough that it doesn’t brake off) or you get one rock inside the scoop. When you get the rock (or pebbles) inside the scoop, you fire your thrusters (20 m/s), return to SEP, transfer material via “elevator” (that can move from 0g to 1g on the truss) to 1 g area.

Scoops, drills, etc all require being able to apply pressure against the surface of the asteroid, which you simply do not have because there is no gravity. You push against the surface, you get pushed off, and there is much less pressure than you would think.

What if your grabbed rock is not a loose material? Well. You are in affect attached to the surface. You can drill/dig against it now. If your supporting rock breaks of, then it’s no big deal. Transfer it to SEP, return empty back to asteroid to repeat everything. And if you don’t break off, then you can drill and dig for as long as you want.

Also, you are talking about an awful lot of Hydrogen here, you would have to process a great deal of asteroid ice to make 100s of tonnes of H2.

Your “space mining problem” is now reduced to: “Here is a (1 g) room full of dirt and ice. What can you do with it with 20 MW of electricity?”.

To get 100 MT of H2 you would need 900 MT of water. How to get water from ice is a known problem (add heat). Apply electricity, split water into H2 and 02, liquefy. Electrolysis of the water is simple (remember those 20 MW of electricity?). You can do it at your home if you wanted. Liquefaction is harder. But, luckily, what do you know: We already have liquefaction equipment onboard for our propulsion H2. Fill our empty tanks with Hydrogen, Oxygen. You can keep the dirt and deliver it to HEEO or simply dump it (or use it for propulsion if you can). Or even better: Simply fill the tanks with water. Split what you need to return to HEEO (http://www.permanent.com/t-theory.htm), the rest would be easily stored for as long as you wanted.

After one year, you return to HEEO with enough water to supply enough Hydrogen and Oxygen for space transportation. When you spend it just go to another asteroid and do the same thing. Or you can go to Mars moons Phobos and Deimos and to that.

In my SEP tug I used MPD with Hydrogen. MPD can use various propellants. To date they have used Xenon, Argon, Neon, Hydrazine, Lithium, Hydrogen,... It’s not really picky about what you use. Isp would be in 1000-10000. And that would be good enough. And if MPD would not last long enough, there are number of other plasma thrusters that are even longer lasting and can use anything for propellant (including oxygen)..

Cost of the mission (that is not already payed for): 1 crew exchange, few small remotely operated "asteroid landers", propelant to get in orbit around asteroid (200 MT Hydrogen), Machineries to crush/melt/electrolyze dirt and water (100 MT).

Benefit: Billions in saved costs for building rockets on earth to deliver Hydrogen into LEO.

SpaceNut · 2007-05-13 19:46:28

Here are a few efforts to get to digging off world...
Teams will compete in robot challenge
NASA is offering $250,000 to the team whose system can excavate and deliver as much lunar regolith - moon dirt - as possible in 30 minutes.
Competitors' machines must use less than 30 watts of power, weigh less than 88 pounds, and excavate more than 330 pounds of the simulated moon dirt.
Lockheed scientists hope to draw oxygen from lunar dust to sustain astronauts
Engineers hope that mission will include some version of the Lockheed digger robot and soil cooker - which can mine gritty, gray lunar dust and turn it into breathable oxygen.
"We could make enough oxygen to keep three, four astronauts alive," said Larry Clark, a Lockheed senior manager and engineer. "We want to get this stuff cranking out oxygen before humans get there."
Two digging robots, each less than 4 feet by 2 feet, stood on the floor, with water-wheellike contraptions to lift lunar soil into a loader. The soil is then drawn into a cylindrical cooker

While failure can be a disapointment one can also call it a learning experience. Posting article complete article since it is short...
No winners in NASA's moon-dirt digging competition

Four teams and some strange machines competed for a quarter-of-a.m.illion dollars from NASA, but all walked away empty-handed.
NASA's Regolith Excavation Challenge invited teams to build machines for digging mock moon dirt, or regolith, in a competition held in a one-ton sandbox on Saturday.
But all the teams fell well short of the winning requirement of 330 pounds of regolith deposited in a container in 30 minutes, and no one claimed the $250,000 purse.
An excavator built by Technology Ranch of Pismo Beach did the best, collecting just over 65 kilograms in half an hour. All the other machines broke down while digging.
The other three teams were from Berkley, Michigan; Rolla, Missouri and Rancho Palos Verdes.
The prize rolls over to next year's competition, which will be worth $750,000.

neviden · 2007-05-27 04:49:53

New Mars Forums

Announcement

#1 2007-05-07 07:42:07

Re: SEP mission to Mars

#2 2007-05-07 08:05:57

Re: SEP mission to Mars

#3 2007-05-07 10:01:45

Re: SEP mission to Mars

#4 2007-05-07 12:14:20

Re: SEP mission to Mars

#5 2007-05-07 12:21:52

Re: SEP mission to Mars

#6 2007-05-07 12:45:20

Re: SEP mission to Mars

#7 2007-05-07 16:30:40

Re: SEP mission to Mars

#8 2007-05-07 20:22:45

Re: SEP mission to Mars

#9 2007-05-10 09:55:57

Re: SEP mission to Mars

#10 2007-05-10 15:00:13

Re: SEP mission to Mars

#11 2007-05-10 19:20:02

Re: SEP mission to Mars

#12 2007-05-10 19:32:52

Re: SEP mission to Mars

#13 2007-05-10 20:58:28

Re: SEP mission to Mars

#14 2007-05-11 05:10:42

Re: SEP mission to Mars

#15 2007-05-13 19:46:28

Re: SEP mission to Mars

#16 2007-05-27 04:49:53

Re: SEP mission to Mars

Board footer