Comments on Rocketpunk Manifesto: Spaceship Design 101

Alas, the cosmic radiation is enough to be dangero...

2009-11-01T17:45:24.667-08:00

Alas, the cosmic radiation is enough to be dangerous without carrying much total energy to speak of.

Fun fact (for very specialized values of 'fun'): The fatal acute radiation dose has roughly the same total energy as a .45 slug.

What about the cosmic radiation being absorbed by ...

2009-11-01T10:18:19.701-08:00

What about the cosmic radiation being absorbed by the fuel?

Work out the extraction hydrogen from LiH crystals (which stay solid to almost 700C) and use that as your shielding material. Consider that, plus the habitat plus it's own power/drive system as the payload. Then stick that on a liquid hydrogen bus for long distance transport.

Another option is using Lithium Deuteride as fusion fuel and wrapping that around the habitat. I'm not sure about the proportions of remass to fuel to habitat size and radiation protection, but any time you can get double duty out of something that is better.

francisdrake - You wrote: There may be a lookou...

2009-11-01T10:17:04.279-08:00
francisdrake - You wrote: There may be a lookout post with some windows to see nearby objects with your own eyes, but most of the information would be take by cameras and other sensors and displayed on screens.

My solution, for story/cinematic purposes, is a position called the pilot tower. It would be manned for rendezvous and docking, or EVA activity, and for large ships has much the function and appearance of an airport control tower.

But there'd be no reason to have anyone posted there in cruise flight. In normal operation you are dealing almost entirely with life support and general engineering, so the center for those functions is where you'd probably keep your 'underway watch.'

To keep cryogenic propellant from boiling off on l...

2009-11-01T09:29:33.079-08:00
To keep cryogenic propellant from boiling off on long missions you will need active refrigeration, pumping heat out of the tank. Otherwise heat buildup will be inexorable.

The ship will be designed to keep the propellant tanks away from the main radiator fins and such, and generally minimize heat absorption from the rest of the ship, so the heat you mainly have to deal with is from sunlight.

The tanks will be painted white or silvery to reflect away most sunlight. I assume that you can reflect about 90 percent. For a spherical tank at 1 AU, that means about 35 W/m2 of absorbed solar radiation that you'll have to pump out of the tank.

A 20 meter diameter tank holds about 250 tons of hydrogen, or 1500 tons of methane. Surface area is 1250 m2, so at 1 AU you'd need 44 kW of refrigerating capacity, i.e. heat extraction, to keep propellant cold.

At Jupiter distance, 5 AU, solar flux is reduced by 96 percent, and you only need 2 kW of refrigeration for hydrogen - none for methane, which will stay liquid or even tend to freeze.

Suppose you put a 10 meter diameter hab inside the tank. (This only reduces tank capacity by 1/8.) But even with a vacuum layer you will have IR heating from the hab surface, at room temperature: 400 W/m2 * 314 m2 = ~125 kW.

So in this case putting the hab inside the fuel tank multiplies your propellant refrigeration bill by 4x. Which is a lot, but not horrible; the shielding might be worth it. But wrapping propellant around a spin hab is tougher.

Ammonia (NH3) is about 17% hydrogen by mass. It c...

2009-10-31T12:47:30.445-07:00
Ammonia (NH3) is about 17% hydrogen by mass. It could be converted to water by burning it with oxygen which yields:
4NH3 + 3O2 => 2N2 + 6H2O
Presumably that heat, in the form of steam would be used to generate electricity to perform electrolysis that turns the water to hydrogen and oxygen:
6H2O + e- => 3 O2 + 6 H2
That oxygen would then be returned to the burning chamber to convert more ammonia.

Remixing nitrogen into something else is a difficult power intensive process probably best reserved for production facilities.

It still requires significant cooling to around 40 degrees below zero. Further cooling (or applying pressure) could turn it to ice around -60C.

I think that there would be a point where it is too difficult to maintain the cryogenic temperatures due to heat build up from the sun. I'm not sure how close that would be, but that would be a definite design consideration.

This is a very interesting thread! Just a few thou...

2009-10-31T08:50:10.117-07:00
This is a very interesting thread! Just a few thoughts that came to my mind while reading:

- Mass is critical.
Even with future powerful engines the overall effort and cost will be dominated by the vehicle mass. Spaceship designers will always try to achieve their goals with as little mass a possible.

- Size matters.
All spaceship components have to be transported to space, coming from Earth for quite a long foreseeable future. They have to fit into some kind of telephone-pole shaped rocket. I assume the aerodynamic shrouds will not exceed a diameter of 10 -12 m, with more contemporary designs like the Ares V being in the 8 m range.
So all modules (or building blocks) would not exceed this size, with their longitudinal axis maybe being twice that value.

- Crew protection:
I fully agree on putting the most precious cargo (the crew) well protected inside the ship. While exposed fly-bridges look cool in Hollywood movies, you would not expose the crew more than necessary. There may be a lookout post with some windows to see nearby objects with your own eyes, but most of the information would be take by cameras and other sensors and displayed on screens.

- Placement of tanks:
While it seems obvious to place them near the engine, it might make more sense to pack them (or at least some of them) around the crew quarters as radiation protection shields. We have very little flight experience outside the Van-Allen belt, which protects us from a lot of stellar radiation. Only Apollo went out further, but just for days and they probably had delayed the launches in case of a major solar event coming up.

But on a month long interplanetary journey you may not be able to avoid solar flares. Fuel (or reaction mass) is always present. It would provide shielding without extra weight penalty, also for non-magnetic radiation like gamma rays. And if fuel runs out, you are either short before reentry into Earths atmosphere, or you are having really serious troubles, with radiation bring one of your least :-)

I don' think the heat transfer is much of a problem with this arrangement. There would be heat radiators anyhow, it should be no problem to orient them up front of or behind, edge-on to the tanks. And maybe NH3 would be a better storeable reaction mass than H2, providing nearly the same ISP at much lower requirements for tank insulation or active cooling. (CH4 is not so nice, as the C tends to deposit as soot inside the engine.)

I was just thinking that the complex and possibly ...

2009-10-31T05:54:52.405-07:00
I was just thinking that the complex and possibly quite fragile nature of the 'hot rod' system may lend itself more heavily to the modular construction. The engines are going to be finicky and the fuel supply is going to be hell on the plumbing. Ejected fuel pods may also make the 'hot rod' expendable. In the end, that set up becomes a one way trip into the service station where it then undergoes a total overhaul. People won't want to wait a year for their power bus to be deemed space worthy, which leads to the payload pod sort of arrangement.

Payloads may be cargo, habitats, or even fully operational ships. Many settings use the carrier model (particularly with FTL jumps), which then deploy the smaller ships while the carrier 'recharges'. The hydrogen hot rot model would seem to move that concept from interstellar to simple interplanetary scales while the more durable short range vessels become orbital platforms.

Now the question becomes what would the models look like for 'reactionless' drives. I think I saw some concept for some spinning dohicky thing that somehow pushes against the rest of the universe. And then there is the idea of compressing space itself (warping). I'm sure both methods are power hogs but they don't use reaction mass (or they minimize it greatly). I think that in both instances the cycling rate will determine top speed whereas reaction drives are limited to top acceleration (well also their delta V and the speed of light).

UmbralRaptor - Welcome to the comments thread! You...

2009-10-30T16:52:19.174-07:00
UmbralRaptor - Welcome to the comments thread! You caught me in a quick and dirty calculation. But the real lesson here is that a small difference in radiative energy balance can make a big difference for human comfort.

Citizen Joe - Yes; white is for reflecting visible sunlight.

You have a point regarding hot rods v utility trucks. I concentrate on the hot rods because they are the human carrying ships. Most space traffic is the trucks, but they are robotic and sort of in the background. (But pervasively so!)

I think that there are still some issues with the ...

2009-10-29T05:47:22.229-07:00
I think that there are still some issues with the tankage relating to whether or not the tank is ejected after the fuel is spent. From the stand point of mass to surface area exposed, a sphere is the best tank design. That is also useful for pressure reasons as well. But that means hanging on to your tank until it is completely dry before you can eject it. A more likely scenario is a grapevine style system with many smaller tanks. That however increases surface area and thus heat gain. Liquid hydrogen also has pumping concerns. Moving cryogenic fluids around puts a huge thermal shock load on materials.

I think what we might be looking at is the difference between a performance race car and a simple utility truck. The liquid hydrogen spacecraft would be fast and efficient, but subject to a lot of maintenance and finicky adjustments. It would also rely heavily on supplemental systems (like the cryogenic systems to make the fuel). Meanwhile the more utility space craft, which probably operates in orbit of a planet, would use hydrogen locked up in a more stable form, like LiH or H2O. The race car model might use fusion power while the utility model would use something simpler like a fission core or RTG.

White reflects light not heat. The white surface ...

2009-10-29T04:33:01.034-07:00
White reflects light not heat. The white surface trick tries to get rid of the visible light frequency photons before they turn into heat.

At the risk of nitpicking, I get 400 W/m^2 for a 2...

2009-10-29T00:03:09.764-07:00
At the risk of nitpicking, I get 400 W/m^2 for a 290 K blackbody (350 is for a 280 K one, which will probably make the crew unhappy). I wonder if painting tank-facing parts of the hull white would help.

I'll take the fish, too.

2009-10-28T20:28:05.296-07:00
I'll take the fish, too.

"Incidentally, it has been shown that vita...

2009-10-28T16:45:18.335-07:00
"Incidentally, it has been shown that vitamin D helps with chronic radiation exposure" - Luke

From what I have skimmed from the article,
some payload of the spacecraft's supplies might consist mostly of sea foods that are listed in the article.

That or install tanning salons on board or some way to dilute and channel sunlight into the ship into safe levels without allowing the nastier forms of radiation to enter as well for crew usage in addition to general or personal centrifuges to help with overall crew health.

Personally, and in disregard to Airplane lore, I'll take the fish.

- Sabersonic
Gmail Address

Rick - According to the simulations and experiment...

2009-10-28T14:52:50.148-07:00
Rick - According to the simulations and experiments I have seen, your summary is correct. However, there is one possible additional method of mitigating the GCR dose - medication. As we learn more about cellular repair and cell "suicide", new treatments may become possible for both chronic and acute radiation poisoning (and oddly, you are likely to want the opposite reaction in these two cases - for chronic exposure, you want the damaged cells to destroy themselves to prevent cancer; for acute exposure you want the damaged cells to repair themselves to prevent anemia, hemophilia, a compromised immune system, and digestive difficulties). Incidentally, it has been shown that vitamin D helps with chronic radiation exposure, although the mechanism is not clear.

Luke

I kind of figured, but I thought the image of a sp...

2009-10-28T13:19:14.838-07:00
I kind of figured, but I thought the image of a space ship running on fossil fuel too funny not to mention.

But in all honesty, it looks like all the heavy cryogen equipment for hydrogen stays at the production facility, and the insulation to keep it cold doesn't mass much, (and even less in space where hard vacuum is easy to come by).

Hmm...that seems to go back to the cost of the har...

2009-10-28T10:12:34.782-07:00
Hmm...that seems to go back to the cost of the hardware and that of the highly trained crew...just because some designer doesn't think it's worth it to provide armor for crew protection, doesn't mean that the people who actually man those ships share that same opinions...

Ferrell

Mark & Citizen Joe - As best I can tell, the e...

2009-10-28T07:43:49.036-07:00
Mark & Citizen Joe - As best I can tell, the extra tankage for bulky hydrogen is still a lot less mass penalty than using any other propellant. Mass is a double consideration, because it also reduces acceleration, and electric ships spend a good fraction of their trip under thrust.

Ferrell & Qwert - True, the disappearance of naval armor has not meant the abandonment of protection, only of one layer.

That said, I pretty much agree with Qwert's point. The protection of a constellation is its armament. If that is defeated the constellation must withdraw, be abandoned (and perhaps 'scuttled'), or surrender.

There is also lithium hydride which stores hydroge...

2009-10-27T12:47:12.216-07:00
There is also lithium hydride which stores hydrogen at about 12.5% by mass, but forms into salt crystals at standard temperature and pressures.

That yields about 100 kg of hydrogen per cubic meter of LiH compared to about 70 kg of hydrogen per cubic meter in liquid hydrogen form.

Cracking it out is easy too, just add water. In the meanwhile, you can use the LiH as coolant and shielding for your reactors without worrying about your habitat module heating it up.

Now you get to dump that shadow shield mass in favor of fuel. No huge hydrolysis processor. No cryo hardware. It's win win... so long as you keep it away from oxygen or water ;)

Thats exactly the kind of scenario i was thinking ...

2009-10-27T11:42:51.392-07:00
Thats exactly the kind of scenario i was thinking of. The crewed command ship is actively protected by the rest of your fleet.
It doesn´t make any sense to try to protect the crew if the enemy manages to defeat your active protection.

Rick-"A curious exception to your earlier gen...

2009-10-27T11:07:11.302-07:00
Rick-"A curious exception to your earlier general point about military thinking is modern era navies, which have pretty much totally abandoned armor protection even for 10,000 ton ships."
I'm sure by this you mean that naval planners have shifted away from passive protection (armor) to active protection (guns, missiles, ect). AWACS planes also (usually) have escort fighters to protect them during missions. Combat Air Patrols (CAP) are meant to protect surface ships and direct combat support planes. I don't see that changing in Qwert's scenerio of fleet-type space warefare.

Ferrell

@Citizen Joe: Looking around at LH2 storage on the...

2009-10-27T10:56:34.192-07:00
@Citizen Joe: Looking around at LH2 storage on the web it seems that the major component in most cryogenic storage is insulation, and vacuum makes great insulation. the big problem with LH2 is the volume, necessitating such large (& bigger means heavier) tanks. I read somewhere that hydrocarbons have more hydrogen by volume than LH2, but the questions are, is the mass of the carbon less than the mass of the extra tank? how do you separate the H to get the high isp? Can you just put the hydrocarbon through the engine? But aside from all that, how funny is it to imagine spacecraft running on gasoline?

Jean - I don't think many parts of a laser wea...

2009-10-27T10:10:44.328-07:00
Jean - I don't think many parts of a laser weapon are field repairable. On the other hand, if lasers are dominant and fight by eyeball frying, the risk to an onboard crew may not be that great.

A curious exception to your earlier general point about military thinking is modern era navies, which have pretty much totally abandoned armor protection even for 10,000 ton ships.

Citizen Joe - Whatever you carry as propellant, a high specific impulse drive will reduce it to plasma and break any chemical bonds. An exhaust velocity of 50 km/s is equivalent to about 250,000 K for hydrogen, and more like a million K if heavier nuclei are involved.

So hydrogen not only delivers more oomph than methane, or anything else, but does it at a much lower core temperature.

(To play along at home, think of atoms as billiards balls of varying mass, flying around and banging into each other.

If two hydrogen atoms collide, the distribution of energy and momentum is not changed. But if a hydrogen atom bounces off a carbon or oxygen atom, the hydrogen atom flies off much faster, carrying most of the kinetic energy but only half the momentum. The upshot is that you waste energy with plasmas of mixed atomic weight.

My opinion, worth what you paid, is that hydrogen is so much better than any alternatives that it is worth carrying a cryo plant to keep the stuff cold.

Luke - In other words, you're saying that an electric ship probably doesn't need a storm shelter, though there is still a CGR background dose for which the only protection is a) armor, or b) fast travel reducing exposure time.

I'm certainly happy to get rid of storm shelters, which are very inconvenient to do if your hab uses spin gravity. But deep space ships (or stations), far from rescue, should have some sort of life support backup, whether an onboard 'lifeboat' or a dual hab.

Rick: That was me, and yes I was trying to stop GC...

2009-10-27T08:46:37.509-07:00
Rick: That was me, and yes I was trying to stop GCRs. The plasma magnets wouldn't stop the solar wind protons either, when considered as individual particles - you need the plasma effects of the electrons to stop the solar wind. This lets you get by with a much smaller magnet.

Luke

All I'm saying is compare the mass of the oxyg...

2009-10-27T08:12:33.904-07:00
All I'm saying is compare the mass of the oxygen (usable as remass) and hydrolysis equipment to the mass of the cryogenic equipment to keep H2 liquid.

I wonder if the compromise might be a mixed load of ice and methane where the byproducts get recombined into CO2 and that gets frozen into dry ice. Is dry ice easier to make than liquid hydrogen?

On a molecular level we're looking at 2 water molecules (H2O) and 1 methane (CH4) to produce 8H + C02. That masses out at 8 hydrogen per 44 byproduct... about 18% hydrogen by mass with no particularly special requirements for storing the dry ice. This is compared to water at 12.5% and methane at 33%. I suspect that a water/methane mixture could be frozen making it easier for storage as well.

By mass, straight up LHyd is the way to go. However, LHyd has so many handling issues that it may simply not be feasible.

However, those command ships will become the prima...

2009-10-26T21:45:55.001-07:00
However, those command ships will become the primary targets. Even if they are not armed, they'd better be well-armored and well-protected. They would be the equivalent of a general on a horse standing on a hill, broadcasting "hello, kill me." Not only that but the more complex the weapon system, the most likely it will need maintenance. Roughly a fifth to a quarter of a carrier crew are the mechanics necessary to keep the air wing operational. In case of a Laserstar, I can see a crew of "window cleaners" in charge of mirror maintenance. In the likely event sectioned mirrors are used, they would replace those sections damaged by accident (a micrometeorite hitting it). The laser itself might need replacement (once again, in case of modular solid-state lasers, the switching out of a module) Even if done by robotic means, I think you'll want someone to monitor the process. I agree there will be extensive automation, and even the largest of Laserstars massing in the tens of thousands of tons might just require 20 to 40 crew members, but I don't really see the human element being entirely removed.