Why we need fast flights to Mars.

kbd512 · 2020-08-11 13:03:00

Calliban,

For this to work, we need a very powerful electromagnet to operate efficiently inside of a very high thermal power output fission reactor. That's a pretty tall order. The electrical resistance of the best conductors increases dramatically by the time you reach typical operating temperatures. If that operational issue wasn't a particularly difficult problem to solve, which it is, then repair or replacement of such coils would be.

tahanson43206,

The entire point behind implementing high-Isp fusion rockets and minimum energy trajectories is to deliver the most payload tonnage using the least propellant tonnage. There's no technical reason why we can't move a 5,000t or a 25,000t ship using low-Isp chemical propellants, but the ship would necessarily become a giant thin-skinned gas tank.

Supplying the power is the actual problem. The lightest practical solution to supply 10MWe continuously for 2 days appears to be Sr90 RTGs combined with SCO2 gas turbines and hull plating as the radiator / heat sink. That means there are no insurmountable engineering challenges.

I've ball-parked the system masses for the following solutions:
1. PV only - array must flex or be so rigid that it doesn't flex very much and can't spin at the size required
2. PV plus Lithium-ion - ship becomes a giant battery rather than a giant gas tank
3. PV plus Carbon fiber flywheels - takes a couple of months to spin up the flywheels
4. CSP only - same problems as massive PV array and likely impractical for ship orientation during thrusting
5. CSP plus Carbon fiber flywheels - solves CSP only problems
6. PV and regenerative PEMFC using LOX/LH2 - requires storing low 100s of tons of cryogens, cryocooling
7. Sr90 RTG plus SCO2 gas turbines - quite heavy for the overall power-to-weight provided, yet still better than the alternatives
8. LEU UO2 fueled fission reactor* - very heavy shielding required, no better than the PV and fuel cell solution

*Note: I was using numbers from a past NASA design with a traditional BeO reflector, traditional NaK coolant with secondary SCO2 loop for power takeoff, and traditional shielding materials like Water and Tungsten, under the auspices of "use what's been well-proven to work in the past".

The end goal here is to move lots of people and cargo to other planets at reasonable speeds while providing artificial gravity and adequate radiation and space debris protection, not to go broke on fuel costs associated with shipping. Aluminum is readily available here and on other planets. Here in America, we can collect the Aluminum waste we toss into the trash every year and vaporize it for propulsion. I want to power our space fleet using trash.

CO2 is readily available for coolant loop top-up and splitting into CO/O2 for retro-propulsive landings. Sr90 and D-T are readily available waste products from fission reactors. There's no fission reactor aboard the smaller vessels and no reason to think we can't make RTG units "reentry proof", same as all other existing RTG units used in space.

Random Thought:

If we built RTG-powered cruisers (25,000t ships) and sent cargo only on CSP / flywheel powered frigates (5,000t ships) we could reduce the number of nuclear powered ships and increase the cargo tonnage delivered by deleting the giant gravity wheels from the uncrewed or minimally crewed frigates. Conceivably, orbital CSP solar power stations could "recharge" the flywheels of the frigates in orbit around Earth / moon / Mars / Venus to eliminate the expensive photovoltaic panels and their life limitations due to radiation degradation. Lockheed-Martin had the right general idea with the "Mars Base Camp" concept. We need orbital power facilities to reduce total IMLEO / IMLMO tonnage. If we did that, then we may not need RTG or fission at all, which would remove another technological hurdle towards implementation. We could also have "fast frigates" for continuous cargo delivery of high priority cargo between crewed missions, irrespective of the orbital position of Mars in relation to Earth.

Calliban · 2020-08-11 16:05:17

kbd512 wrote:

Calliban,
For this to work, we need a very powerful electromagnet to operate efficiently inside of a very high thermal power output fission reactor. That's a pretty tall order. The electrical resistance of the best conductors increases dramatically by the time you reach typical operating temperatures. If that operational issue wasn't a particularly difficult problem to solve, which it is, then repair or replacement of such coils would be.

That's a good point. Are we talking superconducting coils that need to be kept at cryogenic temperatures? Or copper or aluminium alloy? Either way, the coils would likely need to be insulated and cooled.

A Sr-90 RTG is a much simpler idea, if it is possible to reprocess sufficient spent fuel to extract it and lifting it into orbit can be achieved at a reasonable cost. A 1GWe light water reactor will produce about 50kg Sr-90 each year. But there is plenty of strontium-90 sitting in spent fuel ponds around the world right now. It is satisfying to think that far from being a problem, this isotope is a resource that could permit space colonisation.

Last edited by Calliban (2020-08-11 16:09:59)

tahanson43206 · 2020-08-11 18:14:32

For SpaceNut re new topic for Interplanetary Ship Design topic:

This is one of many images of the famous "Big Wheel" toy.

I found an article reporting on the 50th anniversary of the "Big Wheel".

(th)

Last edited by tahanson43206 (2020-08-11 18:15:02)

SpaceNut · 2020-08-11 19:49:14

list post number to split out to create new gyro ship topic with respect what remains making sense to fast flights to mars....
nice human pedal powered "bigwheel"

kbd512 · 2020-08-11 21:48:26

Calliban,

I finally broke down and computed thrust using mass flow and Isp. The engine produces an average thrust of 343kN / 77,109lbf. That's similar to a large NTR engine, meaning TWR is pitiful, just not quite as abhorrent as an ion engine. My 5,000t ship would have 37 engines, so 2,853,033lbf. MSNW LLC's concept vehicle has a TWR nearly identical to mine. Acceleration values are quite low, so "landing" on something like Phobos or Deimos would be feasible, but not the moon or any other significant gravity well. In theory you could get off the moon using a small FDR powered lander with a carbon fiber flywheel energy storage system, but payload would be less than a conventional chemical rocket due to the neutron shielding and heat rejection mass. If you could feasibly double the pulse repetition rate, then you might have something worth looking into. You have to figure out how to dump all that waste heat, though. It could feasibly be an upper stage for a reusable booster, though. We could use LOX/LCO to reduce embodied energy in the manufactured booster propellant (Isp of LOX and Liquid Carbon Monoxide is no better than solids, but the propellants are storable and you get lots of thrust). We might be able to use flywheels and resistive heating elements to heat pure LCO2. That'd maximize our technology investment in FDR and flywheels and reduce energy expenditure on propellant manufacturing. It's simple compression and liquefaction of CO2 at that point.

So far as I know, the coils in MSNW LLC's FDR lab device used 7075 Aluminum alloy (sufficient strength and didn't appreciably deteriorate with use, but not strong enough for higher field strengths), with some mention of using SiC composite for flight quality coils to better withstand neutron radiation and to reduce weight. Superconducting electromagnetic coils were seen as too complicated and recharging the super capacitor bank from the coils was likewise seen as overly-complicated and unnecessary (it's perfectly feasible, though), given the low input power required. The 180kW / 200W/kg (ATK MegaFlex) photovoltaic array was capable of a super capacitor recharge rate sufficient for 14 fusion pulses per second using a 0.5kg liner (mixed references to using 1100 series Aluminum and Lithium foil; both fuels have their own advantages). That means it would consume 10,080kg of fuel per day.

The radiators were sized to reject 360kW of waste heat (10% of the fusion yield would be thermalized into the device). In my concept, I would sink that heat into the ship's hull plates (radiator mass integrated with hull mass). Their concept had 120m^2 of radiator area for their rocket.

To provide 49km/s of dV for the 210 day round-trip mission, 120g of Tritium and 80g of Deuterium were required. A single heavy water plant produced 1,600t of Deuterium per year, so a single fission reactor is sufficient for an entire fleet of FDR ships. Tritium production is a real problem, but in 1958 the US managed to produce 25.7kg in a single year, so I think it's time to step up our Tritium game. Each commercial fusion reactor would require 100kg to 200kg per year, so there are other reasons to step up our game. Then there's uber-important stuff like making Mickey Mouse watches glow in the dark, quite possibly the most important reason to produce more Tritium. We really need a large fission reactor on Phobos or Deimos to make this work as well as it can.

Anyway, they're using ~60t of fuel to produce more than 10 times as much dV as we're going to use per mission, whereas we're using 600t of fuel on minimum energy trajectories (for a 5,000t ship). I need to go back and look at the fusion gain using Aluminum foil to see if my fuel mass figure actually checks out. The higher the gain, the more thrust and the less fuel required.

Here's a document describing MSNW's FDR research:

Summary of Research - John Slough - 9/10/2012 - 3/10/2015 - NNX12AQ61G

tahanson43206 · 2020-08-12 06:12:55

For SpaceNut re #59

SpaceNut wrote:

list post number to split out to create new gyro ship topic with respect what remains making sense to fast flights to mars....
nice human pedal powered "bigwheel"

Thanks for your encouragement!

As I understand your guidance, you'd be interested in seeing a list of posts to be copied to a new "Big Wheel Gyroscope Space Transport" topic.

I'll start working on that! My first inclination is to look for the post where kbd512 first tossed out the counter-rotating flat disk concept.

However, I'd like to start with a post that I can edit, so the top of the new topic can be updated from time to time to help future readers to understand what the topic is about, and where to find specific contributions by specific members.

***
I'm not convinced this subtopic belongs in Fast flights to Mars, because it is about a vehicle design that in itself does not necessarily contribute to speed.

What the "Big Wheel" design does do is to contribute to (1) artificial gravity for passengers and crew, (2) radiation protection for both, (3) other aspects of space flight between planets as the need becomes apparent.

Edit#1: Thanks for noting the image of an "adult" Big Wheel << grin >>
(th)

Last edited by tahanson43206 (2020-08-12 06:14:07)

Calliban · 2020-08-12 07:13:22

kbd512 wrote:

I have an even better idea that doesn't require covering the ship with photovoltaic panels that will be exposed to ionized gas or storing 260t of LOX/LH2 reactants for the fuel cells or space-based fission reactors, which so many people lose their minds over. If we install a very large direct thermal power Sr90 RTG using a 50% efficient SCO2 gas turbine, then we need ~32,609kg of Sr90 to produce 15MWt. World production of Strontium ore is ~140,000t per year, ignoring the 3.7% Sr90 yield from depleted Uranium fuel rods removed from reactor cores, so even 100t is not a significant quantity in the realm of yearly production.

I ran a few numbers, assuming that we can build a 1GWe (3GWth) natural uranium fuelled nuclear reactor on Mars. We could in principle, surround the core with a Li-6 reflector to capture leakage neutrons and breed tritium for our ships. We could also extract Sr-90 from the spent fuel via nitric acid leaching, after a sufficient cooling period.

For Sr-90, about 5% of all fission products are Sr-90 by number, from 235U thermal fission and ignoring terniary fission, there are two fission products per fission event. A 3GWth reactor will yield 2E21 fission products per second, of which ~1E20 will be Sr-90. That is 5240mols (314.4kg) of Sr-90 per year. It would therefore take some 311GWth-years of nuclear heat production to generate enough Sr-90 for one ship. That is equivalent to all the reactor in the US operating for 1 year. Quite a lot of reprocessing would be needed to make this work. It could be done, but it wouldn't be cheap. I don't think there will be enough nuclear power for a long time to come on Mars to support this.

For tritium, the situation may be a little better. I have assumed that a single tritium atom can be produced for every 10 fissions, assuming that leakage of neutrons from the core is 0.3 per fission and that one in three of these is absorbed by Li-6. That would generate 15.72kg of tritium per year from a 3GWth natural uranium reactor. The problem here is that only Li-6 is useful in this way, which is only 7.5% of naturally occurring lithium. If we are forced to rely on natural lithium occurring on Mars, then tritium yields would be reduced significantly. But if we receive only a few kg of tritium per year, that is still sufficient for a lot of ships.

If we can generate enough plutonium to build fast reactors on Mars, then tritium breeding can be accelerated dramatically. Suddenly, there are a lot more spare neutrons to play with. We can also build very compact nuclear cores to power interplanetary ships, which can be cooled and shielded with lithium. The ships will then generate their own tritium, which can be fabricated into fresh fuel in a facility on Phobos, using aluminium manufactured on Mars.

Last edited by Calliban (2020-08-12 07:25:57)

kbd512 · 2020-08-12 16:00:34

Calliban,

With respect to Sr90, there's always a fly in the ointment somewhere. Earth should be able to supply a sufficient quantity of Deuterium and Tritium. The quantities required are well within our production capacity and larger quantities will be required for future fusion reactors.

If the more numerous 5,000t supply frigates can be powered by orbital or onboard solar thermal and flywheels that are "charged" from an orbital solar power station just prior to departure, then the production of 25,000t colonization cruisers would be far lower, so onboard fission reactors would be an acceptable risk. Reactor cores could be built to survive reentry intact, if required. Nobody bats an eye at the 100+ nuclear powered aircraft carriers and submarines in our fleet.

Have you ever considered the use of aqueous homogeneous reactors?

The coolant, fuel, and moderator are all mixed together. The Uranium or Thorium fuel salts can be topped up and neutron poisons removed, as required, and they can handle extreme reactivity changes without issue. We could keep fissile inventory to a minimum using this method and burn-up rates should be far higher than rod-based reactors since cracked fuel rods aren't an issue. The cores are also user-serviceable if you have a small facility that can filter out the waste products.

Back in the day they experimented with spring-loaded control rods to eject them from the reactor in less than the blink of an eye and the reactor suffered no damage as a result of the power excursion. This could be ideal for our purposes since the reactor needs to operate at very high output for brief periods of time, followed by far lower output levels for many months.

Calliban · 2020-08-13 04:13:05

kbd512 wrote:

Calliban,
With respect to Sr90, there's always a fly in the ointment somewhere. Earth should be able to supply a sufficient quantity of Deuterium and Tritium. The quantities required are well within our production capacity and larger quantities will be required for future fusion reactors.
If the more numerous 5,000t supply frigates can be powered by orbital or onboard solar thermal and flywheels that are "charged" from an orbital solar power station just prior to departure, then the production of 25,000t colonization cruisers would be far lower, so onboard fission reactors would be an acceptable risk. Reactor cores could be built to survive reentry intact, if required. Nobody bats an eye at the 100+ nuclear powered aircraft carriers and submarines in our fleet.
Have you ever considered the use of aqueous homogeneous reactors?
The coolant, fuel, and moderator are all mixed together. The Uranium or Thorium fuel salts can be topped up and neutron poisons removed, as required, and they can handle extreme reactivity changes without issue. We could keep fissile inventory to a minimum using this method and burn-up rates should be far higher than rod-based reactors since cracked fuel rods aren't an issue. The cores are also user-serviceable if you have a small facility that can filter out the waste products.
Back in the day they experimented with spring-loaded control rods to eject them from the reactor in less than the blink of an eye and the reactor suffered no damage as a result of the power excursion. This could be ideal for our purposes since the reactor needs to operate at very high output for brief periods of time, followed by far lower output levels for many months.

You might be able to buy strontium-90 from the French and Russians. They both have reprocessing programmes. You would need to import the material into the US for launch. It could work in principle, but god knows how much it would cost. Solar would probably end up being cheaper, even if it fails to provide the same performance on a mass basis. Another option might be to get the Russians to launch the material for you. As the material is leaving the Earth, this could be viewed as a form of waste disposal. Any form of nuclear solution ends up being expensive in real life, because of the mountain of red tape that you have to go through. The Russians are generally more pragmatic about these things than the west.

The aqueous homogeneous reactor sounds like a very promising concept. It achieves high power output and high core power density using what would appear to be tiny quantities of fissile material. It can function as a breeder with respectable breeding ratio and is cheap to build compared to other reactors of the same power. I know it has been used for medical isotope production in several countries and there is now about 70 years of operational experience.

The low operating temperature would make this less than ideal for a spacecraft. It is hard to say without a detailed engineering study. But it would work very well as power reactor on Mars, if we could dump waste heat into an ice sheet or use it to heat habitats using radiators. Reactors like this could be made small enough to be portable for water mining and as mobile power supplies.

To run an AHR on natural uranium would require at least 100 tonnes of D2O. That much D2O would cost $30million on Earth. We could ship it to Mars for not much more, if Elon can really launch materialsfor as little as $10/kg. It would be much better if we could buy some low enriched uranium. A few hundred kg of that would be enough for a city sized power reactor. And Martian thorium could breed enough 233U to fuel the reactor after the initial fuel load.

A good reference, albeit old:
https://fluidfueledreactors.com/1958-ff … opment.php

It is more or less inevitable that a Mars based settlement would need to produce a lot of electrolytic hydrogen to produce synthetic fuels, to generate process heat and to reduce metal ores. I wonder if we could attach some sort of gaseous diffusion plant to a hydrogen stream to separate D2 from the H2?

Last edited by Calliban (2020-08-13 04:20:58)

kbd512 · 2020-08-13 22:14:58

Calliban,

I'm primarily concerned with fabrication cost and the overall complexity of the device, along with the cost and complexity of refueling or de-fueling. The AHR is, for all intents and purposes, a steel ball filled with an aqueous fissile solution. Instead of finely machined rods with finely graded UO2 pellets, there's a "pre-mix" of Uranium and Thorium salts in a D2O solution. There's no fuel rod headaches to deal with. For a surface reactor, we can leave the Uranium in the core until nearly every last Watt of power has been extracted. If it's loaded with Th232 / U233 as you suggested doing, then a significant portion of the long-lived fission products would also be absent. Mixing partially spent fuel with fresh fuel or removing neutron poisons is likewise much easier to accomplish and the reactor can remain online during refueling and fission products removal. I agree that it'd be better to have a higher temperature reactor for a ship to reduce the radiator area and mass, but maybe the increased cost and complexity of doing that outweighs the benefits.

The US has many thousands of tons of HEU and Thorium sitting around collecting dust, so unless these 25,000t interplanetary transports are hijacked by space pirates (presumably, anybody with the money required to operate interplanetary spaceships wouldn't need to steal Uranium) or turned into nuclear weapons by someone onboard with the skill and equipment to take fissile material out of an operating nuclear reactor, I don't think there's much practical reason for concern. Put another way, it'd be a hell of a lot more difficult for a terrorist to take over a spaceship that's typically tens of millions of miles from Earth than it would be to steal something sitting in a bunker here on Earth.

There's supposedly quite a bit of D2 present in Martian ice, so it would be to our benefit to separate it for use in reactors. I think it's better to obtain D2 from Mars, since virtually every plan calls for producing massive quantities of hydrocarbon fuel from local water resources. Although humans can tolerate quite a bit of D2O before it starts interfering with cellular processes, it would be beneficial to remove it from the potable water as well.

tahanson43206 · 2020-08-14 05:10:06

Re GW Johnson post on Artificial Gravity [56 meter radius baseline - 4 rpm - 1 g]

http://newmars.com/forums/viewtopic.php … 70#p170670

SearchTerm:ArtificialGravity

For Big Wheel Passenger Transport model study

Edit#1: Artificial Gravity calculation examples: https://www.school-for-champions.com/sc … zZxa1RDsuo

A Big Wheel habitat built according to the 56 meter radius example would be 112 meters in diameter, or 367 feet and change.

For RobertDyck (when you get back from your trip ...) How thick would you want a habitat to be constructed for your comfortable accommodations concept?

I'm asking because I am restarting the Big Wheel model from scratch.

(th)

Last edited by tahanson43206 (2020-08-14 05:20:38)

GW Johnson · 2020-08-15 10:41:53

If all you want is 0.38 gee, then all you need is 0.38*56m = 21.2 m radius at the same known-to-be-tolerable 4 rpm spin rate. That's fine for folks going from Earth to Mars, because 0.38 gee is what they will have when they get there. They will be well-acclimatized to it.

Might ought to think about folks returning from Mars back to Earth. They will face 1 full gee when they get there, and we should all have doubts about 0.38 gee being enough for them to accommodate it upon arrival.

Now 4 rpm is about the max that most folks seem to tolerate for long periods of time. A few can be trained (at great expense) to tolerate higher spin rates, but usually for shorter times. Nobody really knows whether higher rates are tolerable for timelines measured in months.

What that says is you want a 1 gee design at 4 rpm, and just spin it slower for 0.38 gee on the outbound voyage to Mars. Spin it at 1 gee for the return voyage to Earth.

Maybe you don't want a wheel because 56 m is too big. There is end-over-end, which is also stable. That geometry is more restrictive, but 2*56 m = 112m lengths are far easier to reach than are 2*56m = 112m diameters.

Although, if you believe the old Project Orion explosion-drive designs, those ships are more efficient if they are around 100 m diameter and 300-400 m long. 15-20,000 tons corresponds to 15-20,000 sec effective Isp, at ship accelerations in the 2-4 gee range. Short burn, then coast and spin-up for artificial gravity. Simple.

GW

SpaceNut · 2020-08-15 13:27:57

new topic contains post ArtificialGravity where total ship does not spin or tumble

For Big Wheel Passenger Transport model study

davidmark12 · 2020-10-16 22:57:13

Thanks for sharing us.

tahanson43206 · 2020-10-17 06:20:15

For davidmark12 ...

India is developing a strong space program.

There is already an orbiter at Mars, and an impressive attempt was made to land on the Moon.

This forum contains several topics with India in the topic title, and it would be terrific if we could find someone from India to update them.

To see the topics we have with India in the title, use the Search tool.

Enter "india*" into the Search window.

In the middle of the page, where the heading says: "Search in" select topic subject only

Then click on Submit, with "Show results as" set to the default of Topics.

(th)

Mars_B4_Moon · 2021-06-11 13:43:58

Can a 3-D printer generate human organs ... in outer space?
https://www.heraldsun.com/news/business … 06188.html
Bone and Muscle Loss in Microgravity
https://www.nasa.gov/mission_pages/stat … rogravity/

Mars_B4_Moon · 2021-06-25 06:29:17

A New Era of Spaceflight? Exciting Advances in Rocket Propulsion
https://scitechdaily.com/a-new-era-of-s … ropulsion/
Space nuclear power is nearing critical mass as the final frontier’s next frontier
https://www.geekwire.com/2021/space-nuc … -frontier/
China plans first crew to Mars in 2033
https://www.denipt.com.au/world/2021/06 … rs-in-2033

Mars_B4_Moon · 2021-12-18 07:32:17

China’s new rocket for crewed moon missions to launch around 2026
https://twitter.com/AJ_FI/status/1471821549003055109

An even larger rocket, the Long March 9, is also being developed. Its role will be for launching large infrastructure such as elements of the planned ILRS. That launcher is currently slated for a first launch in 2028, with previous reports stating 2030.

https://spacenews.com/chinas-new-rocket … ound-2026/

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#76 2020-08-11 13:03:00

Re: Why we need fast flights to Mars.

#77 2020-08-11 16:05:17

Re: Why we need fast flights to Mars.

#78 2020-08-11 18:14:32

Re: Why we need fast flights to Mars.

#79 2020-08-11 19:49:14

Re: Why we need fast flights to Mars.

#80 2020-08-11 21:48:26

Re: Why we need fast flights to Mars.

#81 2020-08-12 06:12:55

Re: Why we need fast flights to Mars.

#82 2020-08-12 07:13:22

Re: Why we need fast flights to Mars.

#83 2020-08-12 16:00:34

Re: Why we need fast flights to Mars.

#84 2020-08-13 04:13:05

Re: Why we need fast flights to Mars.

#85 2020-08-13 22:14:58

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#86 2020-08-14 05:10:06

Re: Why we need fast flights to Mars.

#87 2020-08-15 10:41:53

Re: Why we need fast flights to Mars.

#88 2020-08-15 13:27:57

Re: Why we need fast flights to Mars.

#89 2020-10-16 22:57:13

Re: Why we need fast flights to Mars.

#90 2020-10-17 06:20:15

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#91 2021-06-11 13:43:58

Re: Why we need fast flights to Mars.

#92 2021-06-25 06:29:17

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#93 2021-12-18 07:32:17

Re: Why we need fast flights to Mars.

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