Why we need fast flights to Mars.

Calliban · 2020-08-04 13:42:21

Kbd, this sounds like an excellent plan. I agree that for power levels of 6MWe, a space nuclear reactor makes little sense if we need to provide full shielding. Power to weight ratio generally increases for nuclear systems as power output increases. This is largely driven by self-shielding effects in the core and the simple fact that surface area to volume ratio decreases. Shielding mass is a function of core surface area. I did have a graphic that plotted specific power against reactor power output for a number of different reactor types. I will see if I can find it.

One stumbling block that I have mentioned before. The half-life of tritium is 12.7 years. If you are only fuelling once in Earth orbit then a significant fraction of tritium will have decayed to He3 by the time you leave Mars orbit a few years later. I don't know how that will effect the drive.

Back in the days when NASA still took seriously the idea of colonising other planets, it had a program to develop a very high power density space nuclear reactor. Project Neptune was an attempt to develop a boiling potassium reactor, with a closed rankine cycle.
https://ntrs.nasa.gov/archive/nasa/casi … 013609.pdf
https://archive.org/details/nasa_techdoc_19890067899

Power density would be enormously high. A direct cycle with heat rejected at relatively high temperature. Uranium nitride fuel that has high fissile density and remains stable at high temperatures.

I have always found fascinating the idea of a boiling potassium reactor connected with an MHD generator. Aside from the injection pump, there would be no moving parts.

Last edited by Calliban (2020-08-04 14:59:51)

kbd512 · 2020-08-05 09:58:25

Calliban,

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.

A pair of such systems, for redundancy, should have equivalent power system mass, but drastically reduce the overall complexity as compared to the original solution with its dependency on operating both a 1MW PV array and a pair of 10MW PEMFCs. The radiator system would be the hull of the ship's engineering section. This entire concept is the space-based equivalent of on ocean-going steam-powered cargo ship.

Sr90 just happens to produce heat in the same 700C to 800C range required by the US DOE / SWRI / GE 10MWe SCO2 gas turbine that they're developing as part of a $119M solar thermal power demonstrator plant project. The SCO2 turbine is actually named the "Apollo Compander Project" ("compander" for "compressor-expander"). The rotor for the 15MWt 4-stage compressor-expander turbine only weighs 82kg and produces 10MWe. The complete SCO2 turbine skid weighs 12,330kg, the 10MWe generator skid weighs 25,644kg, and the heat exchanger skid weighs 9,997kg, so 95,942kg for a complete solution.

The integrally geared SCO2 turbine uses CO2 at 250 bar, expands it to 80 bar, runs at 27,000rpm, and is geared down via the reduction drive to 1,800rpm. It's supposedly the highest horsepower-per-pound industrial gas turbine, second only to a rocket engine gas turbine. The mass flow through the 1MWe turbine was 8.41kg/s.

Human CO2 exhalation from 250 crew could provide ~47t of CO2 in 180 days, if required to top up the coolant loop for any reason, so some kind of less power intensive life support system might be feasible. I don't know how much CO2 needs to be in the coolant loop to make this work, though.

Development of a High-Efficiency Hot Gas Turbo-expander and Low-Cost Heat Exchangers for Optimized CSP Supercritical CO2 Operation

It's a nuclear decay heat and CO2 powered "steam ship". I like it.

Edit:
The boiling Potassium reactor sounds interesting. I've actually never heard of this one, so I'm going to do some more reading on it. However, it sounds like there were some technical challenges to overcome in operation.

Incidentally, the SCO2 turbine is designed for reliable 24/7/365 operation for multiples years at a time because it's intended to be used in a commercial concentrated solar or nuclear power plant.

Last edited by kbd512 (2020-08-05 17:08:10)

tahanson43206 · 2020-08-06 06:26:20

For lbd512 re Counter Rotating habitats in a saucer shaped form ...

The shape of the Enterprise came to mind, as I thought about the benefits of your proposal ...

https://i.imgur.com/B3fiVQr.jpg

This image shows the Enterprise (or something similar) under construction on Earth.

The design would (presumably) be modified in your version, with a thicker saucer to accommodate the habitat disks.

Still, the propulsion units mounted externally, with thrust transferred to the habitats via the central load bearing shaft seems appropriate for the hybrid fusion rockets you've described.

The propulsion pods would (presumably) contain chemical thrusters in addition to the hybrid fusion units.

(th)

Last edited by tahanson43206 (2020-08-06 06:27:53)

kbd512 · 2020-08-06 08:25:15

tahanson43206,

Something that looks like the Enterprise would be awesome, but I had a more aerodynamic design in mind that at least has the possibility of landing after appropriate stripping and outfitting. Maybe that capability is unnecessary. What I know for sure is that real ships inspire people, whereas giant gas tanks don't. If we built these ships, it would help our economy bounce back. At the end of the construction process, we'd also have real ships for Mars colonization to carry the many thousands of tons of consumables and equipment required to get a sizable colony up and running in a reasonable amount of time. I can't see how we could colonize Mars using giant gas tanks carrying tiny payloads.

tahanson43206 · 2020-08-06 08:49:52

For kbd512 re #29

The Enterprise ** was ** designed for flight through atmosphere.

I don't recall now if the first series included a flight through atmosphere, but later episodes (including the movies) certainly did.

Your idea of counter-rotating habitat disks lends itself to an aerodynamic configuration, as you suggested.

Do you need the assistance of an architect with vehicle training? Your expertise is wide and deep, but it can't include everything.

If you can break the design of this vehicle into portions you want to handle, and portions you want to farm out, it will give SpaceNut an opportunity to reach out beyond this tiny group to invite others to assist.

Edit#1: The gyroscopic effects of the counter-rotating habitat design would be interesting to work with, I would think. Thrust applied directly perpendicular to the axis of rotation should be free of consequences. The navigator would presumably rotate the vehicle so that the vector needed for thrust is perpendicular to the axis of rotation.

The engineering of counter-rotating masses has been going on since the first design for helicopters, and (it turns out) long before.

Edit#2: Engineering of counter (or contra) rotating propellers has been going on for a while:

From Wikipedia:

A contra-rotating propeller was patented by F. W. Lanchester in 1907.[9]

(th)

Last edited by tahanson43206 (2020-08-06 10:17:12)

RobertDyck · 2020-08-06 11:02:16

tahanson43206 wrote:

The Enterprise ** was ** designed for flight through atmosphere.
I don't recall now if the first series included a flight through atmosphere, but later episodes (including the movies) certainly did.

Star Trek distraction. Enterprise from The Original Series was built in space. It was never intended to land. Here's an image from the first movie showing the refit.
latest?cb=20090531185840&path-prefix=en

Season 1, Episode 14, "Tomorrow is Yesterday": Enterprise is sent back in time to the 1960s (current time when it was produced). Enterprise encounters a problem and enters upper atmosphere. A fighter jet (F-104A Starfighter) is scrambled to intercept and take pictures. That doesn't mean Enterprise is designed to do this.
MV5BNWEzODNhNTEtM2JkNC00MjA2LWJiYWQtMzU1YWI0ZGE0YzdhXkEyXkFqcGdeQXVyMTUyNjk1NDU@._V1_.jpg

Digitally remastered, with modern CGI

tahanson43206 · 2020-08-06 11:15:14

For RobertDyck re #31 and terrific images!

I bow to your knowledge of the initial series, but return with the observation that later versions of the Enterprise were built on Earth.

I offer as evidence a scene from the Startrek movie which featured a brash young recruit (who became Captain Kirk). I remember the scene in which the "citizen" was arrested by a very official looking officer ... in that same stretch there were scenes of the factory .

However, to the point I was trying to make ...

kbd512 seems (to me at least) to be developing a reasonable approach to offering artificial gravity.

Without going back to look, I am recalling a sketch from your Large Ship topic in which you showed a rotating habitat around a central vehicle body. Your design (as I remember it) is traditional, in that it assumes the axis of rotation of the habitat will be exactly the same as the axis of the centers of mass of the vehicle.

If I understand kbd512's design correctly, it would indeed look a lot like the Enterprise, and the engineering solutions will include dealing with the gyroscopic effects of a mass rotating at right angles to the direction of thrust.

In a search via Google, I found an Aircraft Owner's site, which included a mention of a Navy document:

https://blog.aopa.org/aopa/2012/01/29/g … recession/

One of the best explanations I’ve found for it is in the Navy’s Introduction to Helicopter Aerodynamics workbook (P-401) available here https://www.cnatra.navy.mil/pubs/folder5/TH57/P-401.pdf It starts on page 103 of the pdf, section 405 and 406. This is also a valuable resource for a lot of other rotor aero subjects.

The bulk of the useful comments around the core article were about how helicopter propellers are ** not ** pure gyroscopes, since the individual blades participate in the dynamic process at work. As an example, several authors pointed out that a factor of 90 degrees, which would be predicted for a pure gyroscope, is much less than 90 degrees, because of the aerodynamic forces that are involved.

However, getting back to kbd512's vision ... ** That ** WOULD be a pure gyroscope situation, because the habitats would be rotating inside an enclosure that would have vacuum as the surrounding.

Thus, an engineer planning an application of thrust while the habitat is in full operation will need to insure that the axis of rotation of the habitat is at precisely 90 degrees to the desired vector, so that the thrust can be applied equally to the rotor bearings at both ends of the habitat shaft.

The fact that (in kbd512's design) the habitats are counter-rotating will have a useful effect, but I suspect that the navigator will need to be conscious of the rotating mass inside the vessel while planning maneuvers.

I am hoping others active in the forum will give thought to this concern.

(th)

Last edited by tahanson43206 (2020-08-06 11:19:47)

kbd512 · 2020-08-06 11:18:06

tahanson43206,

I'm talking about gliding to a landing without invoking matter-antimatter reactors or impulse drives or other fictional technology that doesn't exist. To accomplish that using the airframe I posted a link to, it would be stripped of equipment and engines, landing gear would be installed, and a single-use disposable fabric heat shield would be wrapped around the Aluminum to prevent it from melting during reentry.

If anyone else is interested in the design of this vehicle, they should feel free to contribute. My personal interest is in establishing the basic feasibility of a realistic design that's many times larger than the giant gas tank designs we've been playing with. The primary purpose of this ship is as an interplanetary people and cargo transport. It's not intended to be an absolute speed machine and that makes the design infinitely more practical than these intellectual curiosity experiments NASA is constantly running without producing any flight qualified hardware. The speed and payload is intended to be an analog to that of the early WWII era ocean going cargo ships that we used to deliver supplies to our British allies across the pond. It's obviously going quite a bit faster than the "sizzling six knots" of our pre-WWII merchant fleet, but it's also crossing a much bigger "pond".

The general design goal is to produce a durable and maintainable open source interplanetary merchant ship design that most technologically advanced countries could actually build with appropriate tooling and a reasonably well trained labor pool. The defense-related hardware is already mostly shared amongst us, so the technology transfer is minimal. The advanced fiber lasers may not be distributed to everyone, but the rest of the technology is already pervasive. It's aerospace manufacturing at an industrial scale, similar to what happened in WWII, using many of the same materials and methods, with some changes to account for modern manufacturing technology. They don't need to launch it on their own, either. America can and should assist with that. Humanity won't become a space-faring civilization until we build the ships required to take us into the abyss.

Aluminum hull construction - cheap and easy to come by, doesn't require advanced composite molding know-how or associated costs, and serves as a gigantic radiator for dissipating thermal power
Sr90 RTG - cheap and easy to come by in massive quantities, provides adequate electrical power for the fusion engines
Linear implosion fusion engines - in use for plasma and fusion experiments in many countries since the 1980's, actually works for initiating fusion but not for producing 1 lousy watt of electrical power (thankfully, that's not the goal here), and we've had an unchecked 100% success rate with losing plasma containment (we're doing it intentionally this time around)
Deuterium / Tritium fusion fuel fuel - again, easy to come by since most technologically advanced countries have operating nuclear reactors
Aluminum foil fusion jet power fuel - cheap and easy to obtain in massive quantities, abundant on other planets like Mars
Super capacitors - a pervasive technology capable of storing adequate electrical charge to initiate fusion
SCO2 gas turbines - already in a very advanced state of development for commercial electric power for molten salt solar and nuclear reactors, very compact size and low weight thanks to the density of supercritical CO2
CO2 working fluid - again, cheap and easy to come by, both here on Earth and on other planets

There could always be reasonable technology changes to suit the needs / wants of partner nations, just as there's a bunch of specialized software running on F-35 fighter jets to interoperate with the weapon systems that other countries wish to employ from their fighters. We could have core computer and sensor technology that individual nations could then further develop specialized software for, for exploring their own targets of interest, such as hunting for resources on near-Earth objects or colonizing Venus vs Mars, etc. As long as fundamental things like hatches and communications and whatnot interoperate between ships produced by different nations, then tweaks and fixes are fine.

Mostly, there are reasonably well known costs for the selected materials and technologies because nearly all of them are readily available for commercial or governmental purchase if you have the money. The universities can produce the fusion engines to teach plasma physics to prospective doctoral candidates. Who knows, maybe one of them will figure out how to get electricity out of it. Stranger things have happened. In the interim, highly efficient interplanetary propulsion will be worth the money. It's about time we produced a practical working fusion device, even if it doesn't produce electrical power.

An aerospace engineering team would have to work on something like this, but I don't have the tens of millions of dollars to pay them. If I did, I would be spending it to keep them employed, with the goal of eventually putting a lot of people around the world to work, a respite from COVID-19, if you will. In aerospace engineering, if you want to make a small fortune, then you start with a large fortune.

Anyway, of course I'd love to exchange ideas to see if there's any broader interest in this concept.

tahanson43206 · 2020-08-06 11:28:53

For kbd512 re topic and your interesting idea in particular ...

I just tried to reply to RobertDyck when your long post arrived, so I've only had a chance to glance at it.

One point I would like to make is that the talent needed for a project like this is available in massive numbers of trained and experienced practitioners who earned enough while working to live long lives away from the daily grind.

In addition, there are (I am confident) thousands of young people with plenty of energy to learn, and not yet bogged down by the demands of a full time job.

It would be stupendous waste of all that talent and experience to fail to enlist at least ** some ** of it on behalf of an innovative idea such as you have offered.

RobertDyck would appear to be someone well qualified to help with living accommodations aboard ship.

If SpaceNut ever gets power back after the hurricane caused outage in his home state, I hope he will be interested in considering how talent can be attracted to support development of your idea.

In my opinion, the ability to land in an atmosphere is not a high priority for a design team for a large passenger vessel for the space trade.

(th)

RobertDyck · 2020-08-06 11:40:58

tahanson43206 wrote:

I bow to your knowledge of the initial series, but return with the observation that later versions of the Enterprise were built on Earth.
I offer as evidence a scene from the Startrek movie which featured a brash young recruit (who became Captain Kirk). I remember the scene in which the "citizen" was arrested by a very official looking officer ... in that same stretch there were scenes of the factory .

The first episode I saw was TOS season 1, episode 11, "The Menagerie". Live, first run.
Those of us who grew up with TOS choose to ignore the abomination which is the Kelvin timeline.
https://redshirtsalwaysdie.com/2018/04/ … lvin-kirk/

Prime Kirk
...TOS Kirk credited his father as his inspiration for joining Starfleet. He began and completed a five year officer training program. A long story short, he went through the ranks and at 32 was made a captain. Kirk was given command of the USS Enterprise and lead it on its famous five year mission.
Kelvin Kirk
Kelvin Kirk however, never knew his father. His father was killed on the day of Kirk’s birth. Kirk was dared into Starfleet. During his time at the academy, he was suspended from the academy. Within what seems like a day, he was promoted to first officer, then became acting captain, then he and the Enterprise crew saved earth from destruction.

It has often been asked, what soldiers would ever follow command of the spoiled brat that is the Kelvin timeline Kirk.

kbd512 · 2020-08-06 12:36:34

tahanson43206,

I likewise think that a lot of brainpower is being wasted on trivial pursuits that are unlikely to lead humanity to a better future. I'm primarily fighting against short-sighted and inward-focused thinking that hasn't generated better outcomes thus far. We seem to have a growing number of people who exhibit an unhealthy fascination with the otherwise meaningless superficial physical characteristics of other people and far too many plain old bad ideas that simply don't work whenever humans are involved. Right now we're stuck in a funk that isn't helping anyone and we need to break free of that so everyone can get back to living life and advancing our collective human cause.

We need to get off this planet so we can go forth and explore the unimaginably large universe we actually live in. There seems to be this objectively wrong idea that says you need to be or do "X" in order to contribute. It's funny how Elon Musk doesn't believe that and has accomplished so much without caring that he didn't "know how" when he started. Ultimately, we all learn by doing. Theory is good to know, but then there's actual practice. Medicine is actually called "a practice", because at least the doctors who came up with the term weren't so foolish as to believe that they actually understood everything that they were doing. The first people to engineer rocket engines didn't know a damn thing about rocket engines and sure, they made lots of mistakes along the way, but now we have reliable rocket engines as a result of them not believing that they had to know all the answers ahead of time. I'm not sure where the idea came from, but from talking to young people these days, that "I have to know everything about some task before I even try" idea seems to be very pervasive and it's disabling them. Maybe they have so much information now, yet not a clue as to what good it is to them, that they don't understand how to use their imaginations and basic logic. That was something I never thought would be a problem. Getting people to think about how to solve complex problems is like pulling teeth, but they still wonder why nobody wants to pay them what they think they're worth. We've never built interplanetary spaceships before, either, yet that seems like a poor excuse to not even try. Until we can get "the professionals" involved (that was only a joke, there aren't any), some amateur attempts will simply have to suffice.

I agree that landing is not a high priority, but it might be necessary from time to time (hopefully infrequently), which is why it should still be a fully functional ground-to-orbit machine. We presently lack ship yards in space or highly efficient orbital class launch propulsion systems, which is why I was thinking that the gutted hull would be launched first and then fitted out in orbit using assemblies that bolt-up to mounting brackets.

Calliban · 2020-08-06 14:02:09

Kbd, I have always liked the S-CO2 power generation cycle and I have never understood why it wasn't used more. Very compact after all. There would probably need to be a heat exchanger for the radiator, as the fluid it contains would ideally be low vapour pressure, like a liquid metal of some kind. That way, the space radiator can be thin aluminium and micrometeorites hitting the radiator will not give rise to high pressure coolant leaks.

Using a strontium-90 heat source is a good idea. The only stumbling block would be getting it out of spent fuel. The US has little in the way of reprocessing capability. Neither does Canada. The UK stopped a few years back. France has plenty. It would make sense in my opinion to restart reprocessing. But until that happens, Sr-90 will be in short supply. Stable strontium mined on Earth, is of course no use to us.

Although a pure beta emitter, Sr-90 will generate bremsstrahlung, so modules launched into orbit will require some shielding to be safe to handle. But as a power source, it should be very long lived. Half life is 29 years, so you have that long before heat production drops by half. One could maintain an almost constant heat generation rate throughout the life of the ship by starting out with a number of empty modules within the heat generating pile and adding a fresh module whenever the ship returns to Earth orbit. If you can maintain constant heat output for 30 years, then that is probably the fatigue life of the hull.

At end of life, the core could be placed in an orbit high enough that it will not collide with the Earth for several centuries. By which point, almost all strontium will have decayed.

I don't know how we got onto Star trek stuff. The future will be far more like 'The Expanse' than Star Trek. A much more believable sci-fi :-)

Last edited by Calliban (2020-08-06 14:15:34)

SpaceNut · 2020-08-06 14:12:43

Star trek = large ship

NewMars topic To RTG or not to RTG the size is the question

funny that we are again looking at the possible way to use the
Trains on Mars - Could a rail system provide martian need

tahanson43206 · 2020-08-07 04:43:23

For SpaceNut re kbd512's idea of counter-rotating habitat disks in Enterprise form factor ...

kbd512 came up with a vision for a deep space passenger transport I have not seen before. In the ordinary course of events it would flow under the bridge into the NewMars archive, never to be heard of again.

I'd like to see that idea developed on its own merits. In a recent post I tried to imagine how having a massive counter rotating saucer shaped personnel compartment would challenge the navigator of a space vehicle. It seems to me that the physics of attempting to accelerate a vehicle like this would be worth exploring.

A quick check with Google confirmed that counter rotating propellers have been "with us" since at least 1907. Applications include marine use as well as atmosphere, and fixed wing aircraft as well as helicopters.

The helicopter case is closest to kbd512's concept, because the disk of rotation is parallel to the flight path instead of perpendicular as is true for fixed wing aircraft.

Issues that could be explored in a separate topic would include:

1) How to orient the spacecraft before applying thrust
2) How to mount the disks so that they can be started into rotation, maintained at a desired rate of rotation, and slowed to a stop when necessary.
3) How to design the system so that humans can safely move from the non-moving part of the ship to either of the rotating disks and back
4) How to design gas and fluid seals between moving and non-moving parts of the ship.

The closest example of a large scale project along these lines is the restaurants at the top of towers in various cities.

I've been in the one in Toronto and the smaller one in Indianapolis. Those are aligned with the axis of rotation of the disk parallel to the axis of the towers.

kbd512's design is distinctively different, because the axis of rotation of the disk is perpendicular to the axis of the vehicle.

Engineering for kbd512's design would be quite challenging.

(th)

kbd512 · 2020-08-07 12:29:16

tahanson43206,

1. Spin-down the gravity wheels for the very brief periods of time when you're using the fusion engines. Exposing people to micro-G for a few days every couple of years won't hurt them.

2. I intended to use electric motors mounted between the wheels, but I'm sure there are lots of ways it could be done.

3. The two gravity wheels are connected to the rest of the ship by a micro-G center section that both stores consumables and provides access to the rest of the ship. If the humans have to leave the gravity wheel, the only other places to go on the ship are the escape pods or the unpressurized engineering section that contains the power and propulsion systems. The ship has a fair amount of "void space" for unpressurized cargo, but also because it's a giant gas tank when it's launched into space. The saucer section contains escape pods and cargo pods. The engineering section contains the RTG cores, gas turbines, propellants, radiators, and large cargo.

Rather than sending everyone to the surface using a single giant lander, the escape / cargo pods (Cygnus cargo capsules) are sent to the surface individually, in sequence (landings coordinated by the orbiting ship), where they become pressurized storage / "Mars homes" on the surface. They're collected and connected, for later burial, to a series of subsurface or partially buried tunnels / airlocks that form a larger settlement. The heavy equipment from the primary cargo bay is also sent to the surface using disposable fabric heat shields like ADEPT or HIAD. The propellant required for these low-dV landing operations is siphoned off from the upper atmosphere of Mars and collected in tanks aboard the ship, where it is split into CO / O2. By the time it's time to leave, all the passengers have been "kicked off" or brought back aboard for return to Earth.

Once we get molten salt concentrated solar and molten salt fission reactors and Aluminum smelters up and running on Mars, Mars will provide all or nearly all of the propellant, D-T and Aluminum and CO2 for landing operations. That way, none of the D-T will be older than 6 months upon first use. Only fresh food, new Cygnus capsules / Aluminum Mars homes (for burial on Mars) and of course new colonists, will be brought in from Earth. Earth will obviously provide the colonists, the ships, maintenance facilities and maintainers (some of this will eventually become the responsibility of our new Martians), and propulsion hardware for the ships until Mars makes their own.

We will use the same 10MWe CO2 gas turbines to power the ships and solar thermal (base power, with no restricted areas as with fission reactors) and fission reactors (relocatable industrial power for mining and refining, and to provide the fission products for the ships to use) present on the surface. First priority is to get the solar thermal power plant built to power the colony, then adding greenhouses and storage of working gases and fluids, next producing the propellants required for self-sustainment, and finally, adding more colonists.

4. Umm, maybe we don't do that and just have the power / water / air connections running through the spokes that tie the gravity wheel / torus to the micro-G center sections where most of the pressurized consumables are stored. I could be wrong, but this seems unnecessary.

SpaceNut · 2020-08-07 17:04:09

Rotating space habitat, usually consisting of a pair of counter-rotating cylinders. A pair of cylindrical orbital space colonies that rotate around their respective axis to produce simulated gravity (one rotates clockwise and the other counter clockwise to minimize torques).

https://www.orionsarm.com/eg-article/47 … torques%29.
https://en.wikipedia.org/wiki/O%27Neill_cylinder

typical orbiting station

the Et tanks reused

says that its a lifeboat
arki.medium.jpg

single ring rocket

this speakers to two rings rotating

Calliban · 2020-08-07 18:43:52

Whilst wheels like this are the optimum solution for minimising gravity gradients, they are non-optimum for cosmic ray shielding. We actually want stubby cylinders, with as much shielding per unit area as possible. The cylinders would be decked out inside, with different gravity on different decks. Shielding is expensive because of its enormous mass. We want solutions that get the most habitable volume for the minimum shielding mass. The best shape would be spherical. But it is difficult to fit decks into a sphere that make best use of internal volume. So a stubby cylinder may be better.

The rotating sections could in fact be entirely within the pressure hull. A centrifuge within it, capable of producing Martian levels of gravity. The lower the artificial gravity, the lower the gravity gradients and the less problem with motion sickness for any particular spin radius. What can we get away with? We actually don't need space based experiments to work this one out. We can build compact centrifugal habitats on Earth on thrust bearings. Decks would be tapered, as the effective gravity is the vector sum of Earth gravity and centrifugal gravity.

Last edited by Calliban (2020-08-07 18:50:05)

SpaceNut · 2020-08-07 19:27:46

https://www.ride-extravaganza.com/inter … gravitron/

When the automated door closes the fun really begins – riders barely realise they’re moving (everything they can see is moving with them). The ride quickly starts to rotate reaching its top speed of 24 rpm in less than 20 seconds. (fast given its wide diameter).
At this speed (turned with a 33kW 3-phase motor), the 45 panels that riders stand against rise upwards leaving people stuck to the wall with their feet off the ground. Regular riders will always move into position as the ride gains speed, sometimes turning sideways or upside down.

http://www.artificial-gravity.com/sw/Sp … inCalc.htm

RobertDyck · 2020-08-07 19:31:02

In the movie "2001: A Space Odyssey" the station was named Station 5. The hub was fixed, so both wheels rotate together.

tahanson43206 · 2020-08-07 20:03:11

For RobertDyck re #44

Thanks for that neat GIF from the movie! Out of curiosity, if you were designing that station now, all these years later, would you have recommended counterrotating habitat sections?

It seems to me that precession is a predictable consequence of the original design.

The vehicle that SpaceNut found would have a problem with seals leaking at the joints between the habitat sections and the central shaft.

(th)
The fixed hub certainly solved the problems that would arise from kbd512's concept of counter-rotating habitats in a space vehicle.

(th)

RobertDyck · 2020-08-07 22:01:53

The question is how much does the station move? If the station remains in fixed orbit, never moving, and always rotates in the same direction, then precession is not an issue. But there's another problem: if the rotating bit is asymmetrical, you can get the "Intermediate axis theorum". With Station 5 rotating while the second ring is only partially constructed, mass will not be perfectly balanced. This could result in the "flip" behaviour explained in this video. (14 minutes, 48 seconds)
The Bizarre Behavior of Rotating Bodies, Explained

RobertDyck · 2020-08-07 22:20:18

tahanson43206 wrote:

The vehicle that SpaceNut found would have a problem with seals leaking at the joints between the habitat sections and the central shaft.
The fixed hub certainly solved the problems that would arise from kbd512's concept of counter-rotating habitats in a space vehicle.

Yea. Now we have conflicting design criteria. This is where engineers have to get creative. I don't have a solution, but will describe the problems. Yes, a spin stabilization is an issue. But it's more than that. A spacecraft has to manoeuvre. How does the craft make turns? Yes, it can make turns easily within the plane of rotation, but to navigate you have to control trajectory in 3 dimensions. How do you turn against the axis of a large rotating body? And yes, SpaceNut's spacecraft with counter-rotating rings does negate the problem. But as you said, you then have a problem with seals at the joints which could leak. And friction at the joints will tend to slow rotation, with the two wheels rotating in opposite directions. Maintaining rotation would require an electric motor to actively overcome friction, maintaining rotation.

My design with the cargo hold, propellant tank and main engine all on one side of the wheel would create the "intermediate axis theorem" problem. :'( Solving my ship design would require the axle have an equal mass on either side of the wheel. But I wanted one face of the wheel to be covered in thermal fabric to form a heat shield for aerocapture and aerobraking. No way around it, mass will have to be distributed. Physics is physics.

But all designs with a single rotating mass have the problem of steering. How do you turn against the axis of rotation?

Oh! Another concern: radiation. My design was to have the aft end of the ship (rocket engine end) always pointed toward the Sun. That means one side wall of the ring will always face the Sun. That sunward wall would have radiation shielding.

RobertDyck · 2020-08-07 23:44:10

Steering. A ship with a spinning wheel for artificial gravity. Could we deliberately produce precession, cause it to precess 180°, then stop precession? Could we control it? Use precession to turn a spinning wheel against its axis? How do we do that?

tahanson43206 · 2020-08-08 06:21:06

For RobertDyck re recent posts about rotating habitats and design considerations ...

Thank you for your several helpful additions to this topic ... Your analysis so far is of the case of parallel axis designs.

Would you be willing to take a look at kbd512's concept, of habitats arranged like counter rotating helicopter propellers?

It is that design I found intriguing, perhaps because I've not seen it on offer before. The design would (it seems to me) help to address radiation protection, because the disk could be oriented edge on to the solar radiation, and thus the volume of mass needed for radiation protection would be reduced to the thickness of the habitat section, and to 1/2 of the circumference of the disk if the budget is tight.

It seems to me better to design for thrust to be applied while the habitats are in motion. Someone suggested recently that the habitats might be spun down before a thrust maneuver, but I think that would be a use of energy (and potentially fuel) that is not necessary. It should be possible to design the system from the outset to achieve all objectives, including accepting thrust while rotating.

Your suggestion of causing precession to orient a vehicle to the vector desired for a thrust maneuver reminded me immediately of the mechanical action needed to change the aspect of a helicopter propeller. The difference between a helicopter propeller and a rotating habitat is significant, of course, because a helicopter interacts with the atmosphere to assist itself with movement in plane of rotation, while a rotating habitat would be a pure gyroscope.

As a reminder, Dr. O'Neill and his students used the counter-rotating habitat design concept to plan for adjustment of the habitat orientation as it orbits the Sun. As I remember the technique, the two habitats are joined at both ends by girders which allow for adjustment of the spacing between the habitats. As one end is pinched, the complex will be caused to rotate in a direction resulting from the combination of the forces at work.

I ** think ** something similar is done in spacecraft with gyroscopic attitude control equipment. Working from memory, I believe that some major telescopes have been held to a desired orientation by gyroscopic systems, and those systems have been subject to failure which reduced the ability of ground controllers to maintain the desired orientation.

Google came up with this item about Hubble when I asked:

NASA's Hubble Space Telescope. NASA took great strides last week to press into service a Hubble Space Telescope backup gyroscope (gyro) that was incorrectly returning extremely high rotation rates. The backup gyro was turned on after the spacecraft entered safe mode due to a failed gyro on Friday, Oct. 5.Oct 27, 2018
NASA's Hubble Space Telescope Returns to Science ...www.nasa.gov › feature › goddard › update-on-the-hu...

(th)

Last edited by tahanson43206 (2020-08-08 06:49:53)

RobertDyck · 2020-08-08 15:16:30

This video gives very simple explanation of gyroscopic motion and precession. The part that concerns us is from 2:16 to 6:44. This explains that force applied to a rotating object will cause it to move 90° to the force applied. Torque arrows explain why. This shows us how to manoeuvre a simple rotating ring habitat. No need for counter rotating rings. No need for seals or bearings or electric motors to over come friction.
Gyroscopic Precession and Gyroscopes

The above allows a spacecraft such as the single ring rocket from SpaceNut to manoeuvre. You can orient the spacecraft very simply in any direction, without halting rotation. This can be done with thruster quads. Looking at the still image of the post above, visualize a single ring rocket facing up in the direction of the grey arrow. You want to rotate to the left, so it faces the direction of the green arrow before applying thrust from the main engine. This is while the entire spacecraft continues to spin about it's axis. Before the manoeuvre, the axis of ring rotation is the grey arrow. After manoeuvre, the axis is the green arrow. To accomplish the manoeuvre, apply thrust down on the side of the red arrow, and in the direction of that arrow. A thruster on the opposite side would apply thrust up. This may be counter intuitive, you've just applied thrust in what appears to be 90° from the direction you want to move. But that's how a gyroscope works.

This thrust could be applied by a series of thruster quads. Not just 4 thruster quads, like the Apollo service module, but many along the circumference of the ring. As a thrust quad passes the location where we must apply thrust, that thruster fires. Steady applied force would be accomplished by each thruster only pulsing briefly as it passes the location where we want thrust applied.

Alternatively, this could be accomplished by momentum wheels (aka reaction wheels) located in the hub. Advantage is momentum wheels do not require propellant, just electrical power. But experience has shown that momentum wheels can become saturated (rotating in one direction at their maximum speed). Desaturation is accomplished by thrusters applying thrust in one direction while momentum wheels apply thrust in the opposite direction. The craft does not experience any net movement, but the momentum wheels slow to near stop.

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