Cycler Ships

[RainDreamer]

Uh, the capsule's main engines? Because, like, that's how orbital rendezvouses are done; accelerate to target, match trajectory, and bleed off relative velocity as the capsule gets closer. After that, line up the docking ports together and slowly close the distance (less than .5 m/s), and let the ports click into place.

If you spend all that dV to match the trajectory of the cycler and kill relative velocity, wouldn't it be basically the same as just launching yourself to mars with the same dV? Or is there something I fundamentally misunderstood here? I am not very knowledgable about astrophysics.

My original scifi idea was just simply using a mass driver to shoot a capsule straight up to the cycler, cycler capture capsule with tractorbeam/forcefield and drag it along to the other location. Almost no further propulsion is needed aside from what the cycler need for course correction.

[peadar1987]

If you spend all that dV to match the trajectory of the cycler and kill relative velocity, wouldn't it be basically the same as just launching yourself to mars with the same dV? Or is there something I fundamentally misunderstood here? I am not very knowledgable about astrophysics.

My original scifi idea was just simply using a mass driver to shoot a capsule straight up to the cycler, cycler capture capsule with tractorbeam/forcefield and drag it along to the other location. Almost no further propulsion is needed aside from what the cycler need for course correction.

A tractor beam or forcefield would still exert a force on the capturing spacecraft. Newton's third law, and conservation of momentum. No way around that.

The big advantage is that if you're sending lots of crews to Mars, you can house them in relative comfort in a big living space. You only need to launch that once. Any spacecraft making a rendezvous with the cycler does have to put itself on a Mars transfer orbit, but it only has to be able to keep the crew alive till that rendezvous, instead of all the way to Mars, so it can be made lighter.

Basically, you can launch one big, heavy cycler, and rendezvous with many small, light transfer vessels, or launch many medium sized direct transfer vessels and no cycler. At some point, there is a break-even where the mass you save on transfer vessels cancels out the mass you used on launching the cycler.

[Rakaydos]

If you spend all that dV to match the trajectory of the cycler and kill relative velocity, wouldn't it be basically the same as just launching yourself to mars with the same dV? Or is there something I fundamentally misunderstood here? I am not very knowledgable about astrophysics.

It's not the astrophysics that's the problem with mars missions- it's the ecosystem. The Cycler sets up a permanant lifesupport infrastructure for travel between worlds- so you can literally launch to a mars transfer orbit in a spacesuit, board the cycler, and spend the next few months in spacius hotel accomidtions that you only had to accelerate once in over 20 years of use. Then when you reach mars, you put your spacesuit back on and jump out, while the cycler keeps going, ready for the next trip.

[KerikBalm]

Yeah but you need that 748 m/s twice, once to break from the transfer and once to boost back into it.

Ummm... I highly doubt that it is that high. That was to break from mars, and capture into low Earth orbit, as we're not going to LEO, and we can use aerobraking, its considerably less than that.

360 m/s to eject from Earth to Mars.

< 748 to capture to Mars, aerobraking should help considerably here

< 748 to eject again

360 to capture again, aerobraking should help considerably here

I don't have exact numbers, but my point is that this is sort of an intermediate that would be more efficient than what we might call a simple "apollo style" mission.

1500 m/s is a very significant chunk of dV - a 100 tonne mothership would require 58 tonnes of USDH/N2O4 for that.

which is why I suggested propulsion systems with much higher ISP. One doesn't need massive acceleration to acheive these relatively small dV changes.

Also the Aldrin cycler takes crew to mars once every 2.1 years - which is the same frequency as the Hohmann transfer orbit window opens. Fair enough you need 2 cyclers (inbound and outbound) but it's not a staggering investment.

I didn't realize cyclers were that frequent, I was thinking somewhere more around 10 years... that certainly makes this intermediate less appealing.

Besides the flaws in your mothership argument, which Fuzzy quite succinctly and effectively points out, you seem to be under some fundamental misconceptions about which type of vessel would require which type of propulsion...

A "mothership" vessel, since it would be manned, would need to rely on "low ISP chemical propellant", so as to keep the acceleration and trip time to a reasonable length

Considering the burns would be at most 700 m/s at a time, even a modest acceleration would be sufficient. I don't have the numbers to what sort of acceleration you could get with an ion drive and a reactor comprising XX% of ship mass, but I'd think even a small NTR could handle that. I do know that an NTR has much lower ISP, but its still much better than chemical.

[shynung]

If you spend all that dV to match the trajectory of the cycler and kill relative velocity, wouldn't it be basically the same as just launching yourself to mars with the same dV? Or is there something I fundamentally misunderstood here? I am not very knowledgable about astrophysics.

dV requirement is not reduced. Propellant requirement is reduced, because rather than dV-ing an entire habitation module into transfer orbit, one would simply dV a small capsule carrying only crew and supplies, without the actual hab module itself (which is presumed to be lighter). At the destination, the capsule aerobrakes or otherwise decelerates itself to enter the planet's orbit, while the cycler continues on to intercept the planet-of-origin again.

My original scifi idea was just simply using a mass driver to shoot a capsule straight up to the cycler, cycler capture capsule with tractorbeam/forcefield and drag it along to the other location. Almost no further propulsion is needed aside from what the cycler need for course correction.

The tractor beam would pull the cycler towards the capsule as it closes in. In order to keep itself on the same trajectory, it would have to expend propellant. There's no other way around that.

A much easier, and simpler way is to keep the cycler where it is (as in, don't pull/push other things with tractor beam, or fire the engines), and have the capsule do the final approach on RCS. In a scifi setting, using arcjet thrusters as RCS blocks wouldn't seem out of place.

Edited December 10, 2014 by shynung

[NERVAfan]

I have a feeling that this is something that might make a lot of sense for a regular Earth-Mars service, but not early on.

IMO early expeditions will accept a quite high radiation risk (less shielding mass) and go with very small habitation volumes (so it probably won't be NASA, but somebody less risk-averse, like a private group or China or maybe somebody else like India).

[Fuzzy Dunlop]

Ummm... I highly doubt that it is that high. That was to break from mars, and capture into low Earth orbit, as we're not going to LEO, and we can use aerobraking, its considerably less than that.

It is that high: At the apohelion of an Earth-Mars Hohmann transfer orbit our spaceship will be moving about 2.5 km/s slower than mars. We can massivley reduce the dV required to match orbits by using the Oberth effect. At best, with an instantaneous insertion burn while scraping the martian surface we can cut the dV to a little under 750 m/s. This will leave us in a highly eccentric orbit with a very low periapsis where our spaceship will be travelling just under escape velocity.

The nearest example I can think of is India's Mars Orbiter Mission which used 1098.7 m/s of dV to insert into a 423 x 80,000 km orbit

360 m/s to eject from Earth to Mars.

< 748 to capture to Mars, aerobraking should help considerably here

< 748 to eject again

360 to capture again, aerobraking should help considerably here

I don't have exact numbers, but my point is that this is sort of an intermediate that would be more efficient than what we might call a simple "apollo style" mission.

which is why I suggested propulsion systems with much higher ISP. One doesn't need massive acceleration to acheive these relatively small dV changes.

Leaving the mothership in an almost escape orbit isn't a bad idea, and would probably be better than braking it into a very low orbit.

High ISP propulsion systems may be less useful than you think. Currently avaliable systems would struggle to slow our spaceship before it shot past Mars and started falling back towards Earth. At the very least you wouldn't be able to take full advantage of the Oberth effect. Of course if you have a NTR then that's great but at the moment we don't.

Areobraking from a transfer orbit is called 'Areocapture' and is much more challenging than traditional areobraking. Normally you skim through the very top of a planet's atmosphere to shave off a tiny sliver of velocity at a time - often this is repeated 100s of times. This spreads the heat load over a great deal of time and also allows for more control (the density of the upper atmosphere varies unpredictably, hence the amount of braking you get from each pass is pretty random, but with multiple passes you can correct for this). With Areocapture you only get one chance, which means much higher tempuratures (probably requiring heavy protection) and flying on a knife edge in terms of avoiding crashing into the planet or shooting off into space. There's a reason we've done a lot of areobraking but never attemped an areocapture.

I didn't realize cyclers were that frequent, I was thinking somewhere more around 10 years... that certainly makes this intermediate less appealing.

There are a lot of different cycler orbits, some have lower dV, some allow the same cycler to be used for both legs, some are faster, some are slower.

Considering the burns would be at most 700 m/s at a time, even a modest acceleration would be sufficient. I don't have the numbers to what sort of acceleration you could get with an ion drive and a reactor comprising XX% of ship mass, but I'd think even a small NTR could handle that. I do know that an NTR has much lower ISP, but its still much better than chemical.

If you have an NTR it's probably not a good idea to have that coming back to earth, it's a bit risky. Once they're switched on they contain a huge amout of radiation, they're also fairly big and very dense so they'll re-enter intact, and then hit the ground, vapourise and render a large area uninhabitable. Certainly areocapture is out of the question.

[peachoftree]

The thing about cargo cyclers is that they only save you so much mass because you still have to have life support at the destination. To make them totally feasible you need a base at the mars side of things so that you don't have to carry several months worth of life support with you for your stay on mars, making them useless for exploration missions where there is no existing infrastructure such as a station or a base.

[sndrtj]

The thing about cargo cyclers is that they only save you so much mass because you still have to have life support at the destination. To make them totally feasible you need a base at the mars side of things so that you don't have to carry several months worth of life support with you for your stay on mars, making them useless for exploration missions where there is no existing infrastructure such as a station or a base.

That's why they'll prolly not be used for early exploration. Once there is a base, however, it's a very attractive option. With a series of cyclers, you basically get a shuttle service from Earth to Mars.

[Fuzzy Dunlop]

The thing about cargo cyclers is that they only save you so much mass because you still have to have life support at the destination. To make them totally feasible you need a base at the mars side of things so that you don't have to carry several months worth of life support with you for your stay on mars, making them useless for exploration missions where there is no existing infrastructure such as a station or a base.

Firstly, Cargo Cyclers? - cargo is litterally the worst possible use for cyclers. If you just wanted to send something like a rover, or a crane or a hab module to the martian surface you would be much better off just shooting it directly at Mars on a Hohmann transfer. Cyclers are for stuff you don't need on Mars but do want on the journey (like radiation shielding and the capsule that lands you back on earth).

Secondly, well of course you want life support on Mars, but that's probably going to be seperate from the life support on your transfer vehicle in any case. You're not going to want to launch your life support system back up from the martian surface but you'll need something for the journey back.

Most concepts for a manned Mars mission involve putting some infrastructure on the surface robotically first.

[peadar1987]

For cargo you're probably best using a tether system. Have it a decent way above the atmosphere so it can boost multiple payloads in a launch window without deorbiting, then it has plenty of time between windows to raise its orbit via electrodynamic propulsion ready for the next one.

[lincourtl]

...if the rendezvous will take less than 36 hours, I think it's perfectly reasonable to ask the ask the astronauts to sit in their space suits and space diapers, hooked up to oxygen tanks in an unpressurized capsule, to save literally hundreds of thousands of dollars on the mission cost...

You're an airline executive in real life, right? ;-)

[m4rt14n]

Unpressurized Capsule?? What a waste.. Command seats weigh much less

[Northstar1989]

Basically, you can launch one big, heavy cycler, and rendezvous with many small, light transfer vessels, or launch many medium sized direct transfer vessels and no cycler. At some point, there is a break-even where the mass you save on transfer vessels cancels out the mass you used on launching the cycler.

The break-even point for a pair of full cyclers (ones that can operate perpetually- unlike semi cyclers, which are more comparable to free return trajectories) is at just 2 manned missions.

Each cycler takes about 50% more Delta-V to launch than placing the same mass on a Mars Hohmann trajectory. However neither has to make the return-burn to Earth/Mars (you place the cyclers so that one has the short leg of its cycle Earth --> Mars and the other Mars --> Earth), meaning it actually costs less Delta-V to place a single Cycler on a cycle trajectory than it does to accelerate the same mass of habitat/radiation-shielding/life-support systems to Mars, capture it there (mostly by propulsion- aerocapture is extremely difficult, although you might still set a periapsis through the edge of the atmosphere to maximize the Oberth Effect...), return it to Earth, and let it burn up in Earth's atmosphere...

Since you need two cyclers for the system to work, and a single cycler is still going to take more than 50% of the propellant of a Mars round-trip on a conventional transfer vehicle, you need to perform at least 2 manned missions for the cycler to pay for itself.

A cycler will basically always pay for itself after 2 cycles. You can either use it for 2 manned missions (and launch it with crew onboard and chemical engines), or launch it unmanned and spend the first cycle and several years just slowly achieving the cycle orbit with ion engines and solar sails... (it costs less mass to launch 2 cycler ships on cycle trajectories using engines with ISP over 19,000 seconds like the Dual Stage 4 Grid ion thruster being worked on in real life, than it does to launch a single transfer+return vessel on a round-trip with chemical engines that can max around 480 seconds on the outbound trip with LH2/O2 and only around 380 seconds on the return trip if you use Mars-derived Methane/LOX, so great are the propellant-costs)

Regards,

Northstar

P.S. To launch a cycler with electric engines, you would probably want to power them with Microwave Beamed Power, as that is BY FAR the lightest way to get the necessary power to them. A nuclear reactor or solar panels will be MUCH heavier. You will also need to use a huge cluster of ion engines (probably over 20 engines) if you can't develop a multi-MW thruster with better ISP than VASIMR... (which CAN scale up to multiple MW)

Edited December 11, 2014 by Northstar1989

[Red Iron Crown]

Don't forget that the "tender" ships, the ones that go between cycler and planet, also have to pay that 50% penalty in delta-V to rendezvous with the cycler and likely to capture at the destination, so it will take more than two trips to break even. Doesn't matter much as it would be foolish to use a cycler when only a handful of missions are planned; cyclers are suitable for regular, scheduled transfers suitable for some sort of continuous habitation on Mars rather than exploratory missions.

[Northstar1989]

Don't forget that the "tender" ships, the ones that go between cycler and planet, also have to pay that 50% penalty in delta-V to rendezvous with the cycler and likely to capture at the destination, so it will take more than two trips to break even.

I didn't forget- but it still only takes two trips to break even. The fuel-savings from using a more than 40x more efficient engine (19300 s vs 480 s) more than pay for the tiny cost of fuel of accelerating the "tender" ships. The size of the Cycler ship would easily dwarf the "tender" ships, so it only takes 2 missions to break even. (to be precise, I think the cycler has to be about 3x as massive as the "tender" ship in order for the break-even point to be 2 missions or less)

Doesn't matter much as it would be foolish to use a cycler when only a handful of missions are planned; cyclers are suitable for regular, scheduled transfers suitable for some sort of continuous habitation on Mars rather than exploratory missions.

Cyclers are actually great for exploratory missions as well. The single greatest strength of a Cycler Ship is that you can accelerate it to its cycle orbit with low-thrust, high-ISP electric engines (or solar sails) over the course of *several* years. There's no need to get to the cycle orbit quickly, because there are no crew onboard yet during the initial acceleration to the cycle orbit.

This advantage holds just as well for a pair of just two exploratory missions as it does for regularly-scheduled "shuttle" service between planets (and in fact is MORE advantageous for exploratory missions, as by the time you have regularly-scheduled "shuttle" service you should have the capability to refuel craft in Mars orbit, and send them off from Earth with things like magnetic tethers...)

Regards,

Northstar

Edited December 11, 2014 by Northstar1989

[NERVAfan]

It's not the astrophysics that's the problem with mars missions- it's the ecosystem.

Eh, Mars is close enough that you don't really need awesome recycling. People need about 5 kg/day of life support, most of which is water.

I'm still wondering how this intercept thing would work. You launch from the surface or mars and accelerate up to match the speed of the Cycler as it whips by. But that means you are on an escape trajectory without the ability to turn around ... in a ship not capable of making the journey. If you miss the rendezvous, you are dead.

What are the chances of that happening, though? Seems pretty unlikely.

Ok, you could argue "But you wont have enough ÃŽâ€V to return to Earth!". Why would you use a spacecraft not capable of returning to Earth in case of an emergency?

Because putting in that much delta-v would likely make the whole thing impractical.

I suspect it is. The cycler is a strictly orbit-to-orbit ship, with plenty of time for maneuvering burns. We can fit it with more efficient engines than typical chemical engines, like nuclear thermal or nuclear electric engines that, despite being ultimately heavier, can lower the propellant needs for maneuvering burns at each pass.

Your point is correct, but Mars is close enough to the Sun that solar-electric would almost certainly win out over nuclear-electric. Solar panel technology is advancing fast and nuclear technology... mostly isn't.

IMO nuclear-electric is only for Jupiter and outwards.

[shynung]

Your point is correct, but Mars is close enough to the Sun that solar-electric would almost certainly win out over nuclear-electric. Solar panel technology is advancing fast and nuclear technology... mostly isn't.

IMO nuclear-electric is only for Jupiter and outwards.

Large solar panels powerful enough to run something like a VASIMR wouldn't be lightweight. It could be lighter than a nuclear reactor of comparable output, but I think not by much. It would still be quite advantageous, though, since every gram of extra weight would eat into the dV budget.

[RuBisCO]

Delta-V for a cycler depends greatly on the cycle used, a cycle is possible purely on gravity assistance, which hypothetically zero delta-v in fuel needed (surely ~150 m/s is needed to attitude control and aiming). S1L1 cycle can achieved this theoretical zero delta-v, with earth to mars transit times of 5 months, Mars to earth transit time of 30.9 months, and 20.5 months intercycle period, 51.4 month long cycles. Clearly only useful for one way to mars as it takes 2.5 years to get back to earth on this cycler. The amount of delta-v needed to get to the cycler is not much more then Hoffman transfer to mars, or equal to a 5 month hoffman transfer to mars.

[Dkmdlb]

All cyclers do have a serious flaw. In order to transfer fuel, crew, cargo and whatever you'll need to dock. And to dock you'll have to match velocities. If you match velocities with a cycler to for example Mars you will be on a trajectory to Mars.

You are missing part of the story - nobody wants to go to Mars cramped into a LM-sized closet. Put a big hab on a cycler orbit, and then just send up the crew in something the size of a Gemini capsule. Let the capsule dock with the cycler, and then let the crew spend the next 7 months relaxing in Hotel Hohmann. Then let them jump back into their tiny capsule for landing on Mars.

That's just one example. There are others. Carrying Hydrogen, for example - put a heavy zero boil-off tank in a cycler orbit, and then launch your hydrogen in a lighter tanks before transferring the hydrogen to the cycler tank. That way you don't have to send up that heavy zero boil-off equipment every time you want to send your hydrogen to Mars.

[Northstar1989]

You are missing part of the story - nobody wants to go to Mars cramped into a LM-sized closet. Put a big hab on a cycler orbit, and then just send up the crew in something the size of a Gemini capsule. Let the capsule dock with the cycler, and then let the crew spend the next 7 months relaxing in Hotel Hohmann. Then let them jump back into their tiny capsule for landing on Mars.

That's just one example. There are others. Carrying Hydrogen, for example - put a heavy zero boil-off tank in a cycler orbit, and then launch your hydrogen in a lighter tanks before transferring the hydrogen to the cycler tank. That way you don't have to send up that heavy zero boil-off equipment every time you want to send your hydrogen to Mars.

You mirrored my thoughts exactly. Except, embarrassingly, I didn't think of the zero-boiloff tanks for cryogenic fuels (such as Liquid Hydrogen) part.

Boil-off of cryogenic fuels is one of the BIGGEST problems facing Mars mission plans today.

A rocket using LH2/LOX has to carry extremely heavy insulation and active cooling equipment to keep its fuel from boiling off. If you use Kerosene/LOX instead, you have problems withe the Kerosene coking onto the engines with repeated engine use. If you use hypergolics, there are issues with toxicity and low ISP. Methane/LOX is probably the best option, but there are no space-grade engines designed specifically to burn that mixture yet... (a couple are in development, and several have been retro-fitted to work with Methane, though)

Carrying the heavy insulation and cooling equipment on the Cycler Ship lets you use LH2/LOX (Hydrolox) without needing nearly as much insulation (and no cooling equipment) on the vessels that will actually be having to make a capture-burn at Mars. It also lets you use Hydrolox on the lander- although you might be better off using a Methane/LOX engine there for a more compact form, and the greater ease of refueling the Methane via ISRU (you *can* find Hydrogen on Mars- but you're going to have to dig down and melt water-ice in the soil to get it), unless you can design a dual-use engine that can burn both Meth/LOX and Hydrolox on a single mission. Hydrogen can also be used as feedstock for the Sabatier Reaction and ISRU on Mars...

Finally, if you switch to Microwave Thermal Rocketry, pure Hydrogen is the highest-ISP propellant for that (and is even harder to store than Hydrolox- as Oxygen is far less cryogenic than Hydrogen). If you carry a nuclear reactor on the Cycler Ship and beam the power to the Mars capture vehicle and lander (or even just emplace a Microwave Relay on the Cycler to re-focus microwaves beamed from Earth), then having access to Hydrogen without needing heavy insulation on the capture vehicle and lander is a great boon for efficiency...

So, having the ability to carry Hydrogen out to Mars without having to accelerate heavy insulation and cooling equipment there over and over is a HUGE advantage of using a Cycler Ship... (which probably already has impressive electricity-generation capabilities anyways: to power electric thrusters and life-support)

Regards,

Northstar

[Northstar1989]

Mars is close enough to the Sun that solar-electric would almost certainly win out over nuclear-electric. Solar panel technology is advancing fast and nuclear technology... mostly isn't.

IMO nuclear-electric is only for Jupiter and outwards.

Large solar panels powerful enough to run something like a VASIMR wouldn't be lightweight. It could be lighter than a nuclear reactor of comparable output, but I think not by much. It would still be quite advantageous, though, since every gram of extra weight would eat into the dV budget.

Actually, a *proper* nuclear reactor (as opposed to, say, a Stirling Engine nuclear reactor or an RTG) has much higher power-density than any solar panel except thin-film panels (which can only be coated onto existing hard surfaces- such as the entire sun-facing side of a Cycler Ship...) Meaning, you get more electric power per kg of weight with the nukes.

Of course the BEST option from a mass-efficiency standpoint is just to *beam* the power at the Cycler Ship via microwaves, and receive it with a rectenna (which is extremely lightweight). That same infrastructure is *also* really useful for launching rockets or spaceplanes from the surface of the Earth in the first place (using Microwave Thermal Rocketry), and could provide the kind of baseline power-demand on the Microwave Transmitters that would allow them to *quickly* pay for themselves without having to launch 100+ rockets a year... (and at times where there are no active Mars missions, those same Microwave Transmitters could be utilized to launch rockets/spaceplanes from Earth...)

Regards,

Northstar

P.S. Microwave Transmitters are *already* economically viable for rocket and spaceplane-launches without needing to power a Cycler Ship. The ISS consumes up to 80 metric tons in consumables (includes life-support, fresh clothes, etc.) a year- so that demand alone is capable of supporting a 1000 MW array of Microwave Transmitters (which could launch 1 metric ton of payload to LEO per launch using rockets, or significantly heavier payloads using spaceplanes- would cost about $2 billion if transmitter costs remained fixed at $2 million/MW-capacity, and would last about 10 years before starting to wear out... Current launch costs are about $10,000/kg, so the system would need to launch at least 20 rockets/year to pay for itself...)

P.P.S. Just IMAGINE what electric thrusters could do with 1 GW of electric power, and only the weight of a small rectenna (1 GW is enough to power a Dual-Stage 4-Grid Ion Thruster with a thrust of 10 kN and ISP of 19,300 seconds, to give a rather absurd example... And with electric thrusters, the lower your ISP, the higher your thrust.) Now you're starting to see why Microwave Electric is the highest-performance option for a Cycler Ship...

Edited December 13, 2014 by Northstar1989

[Kibble]

Is zero boil-off even possible? I was under the impression that even with perfect cooling, hydrogen atoms are small enough to just leak thru the molecules of the tank!

[RuBisCO]

Depends on the tanks, hydrogen can permeate metals, but not so much non-metals.

[NERVAfan]

Actually, a *proper* nuclear reactor (as opposed to, say, a Stirling Engine nuclear reactor or an RTG) has much higher power-density than any solar panel except thin-film panels (which can only be coated onto existing hard surfaces- such as the entire sun-facing side of a Cycler Ship...) Meaning, you get more electric power per kg of weight with the nukes.

Maybe... but I'd like to see the numbers compared to modern solar panels.

But it's irrelevant, since you can use thin film panels without hard surfaces - see the Japanese IKAROS solar sail, which had ultra-thin (25 micrometers!) solar cells in the sail.