Register    Login    Forum    Search    FAQ

Board index » Discussion » Mecha and Technology




Post new topic Reply to topic  [ 18 posts ] 
Author Message
 Post Posted: Mon Jan 23, 2012 9:14 pm 
Offline
Creepy Bishonen
User avatar

Joined: Mon Mar 06, 2006 12:01 am
Posts: 190
Location: Classified Location
This question just popped into my head just recently and it concerns with the UC branch of the Gundam franchise. From what I can recall from the technical explanations from various source such as the Power Crazy article, the main rocket engines of mobile suits, mobile armor, and most combat spacecraft use the thermal energy of the onboard fusion reactor to heat up the remass ("Propellant" for the uninitiated) to generate the thrust required for orbital maneuvers. From the description alone, it reminded me of the Nuclear Lightbulb concept. It was also suggested that the rocket engine design was able to have two modes of operation: Low-Thrust, High Efficiency for cruises and High-Thrust, Low Efficiency for combat maneuvers. It also reminded me of how the VASIMIR operated in theory.

Then my mind began to wander due to the fact that as far as I know, the Nuclear Lightbulb or Closed Gas Core Nuclear Rocket design derives its thermal energy from nuclear fusion whereas the thermonuclear rockets of the Universal Century from the original series forward were fusion.

Anyway, to conclude my ramble before it goes on forever, I am curious as to the performance of the Nuclear Fusion Lightbulb for lack of a better term compared to its fission design. That is to say what is its acceleration and thrust power in comparison to other rocket engine design of the thermonuclear class. I know that there are statistics listed in the mobile suit section of the website but I have also been warned to take the numbers with a grain of salt if only due to the multitude of contradictory published material to which said information was gathered.

_________________
Though he may have his flaws and faults, he was a husband and a father without equal. May the Angels welcome and accept him with open arms.

Rest in Peace, Dad

"If I had seen farther than others, it has been by standing on the shoulders of giants." - Sir Issac Newton


Top 
 Profile  
 
 Post Posted: Mon Jan 23, 2012 9:50 pm 
Offline
Master of Mecha

Joined: Mon Mar 06, 2006 12:24 pm
Posts: 2961
Location: San Francisco, CA
Most real-world fusion engine designs seem to involve releasing material directly from the reactor, rather than using the reactor heat to warm bulk propellant, "lightbulb" style. As a result, when you look at specs for real-world fusion engine concepts, they tend to have really high specific impulse (i.e. fuel efficiency) and really low thrust. I don't think I've really seen numbers for the precise combo used in U.C. Gundam, the "fusion lightbulb" as you put it.

As you'll see from my notes on Gundam Century, that book does hint at some ballpark figures. But the Gundam Century specs have long since been discarded, and based on the animation it seems like the average mobile suit can run its engines at full throttle for much longer than 30 seconds, so the actual specific impulse is probably way higher than Gundam Century suggests. For example, the MSV books claim that the MS-06R-1A carries enough propellant to run its engines at full throttle for 15 minutes during combat.

-- Mark


Top 
 Profile  
 
 Post Posted: Wed Jan 25, 2012 12:16 pm 
Offline
Okawara x Katoki Love Child
User avatar

Joined: Tue Jan 30, 2007 3:14 pm
Posts: 1234
As far as I understand the concept, the source of the heat doesn't actually matter. You just heat up the propellant and toss it out the back, and that generates your thrust. As far as exhaust velocity and specific impulse and the like... well, we don't really have the numbers to calculate that. We'd need to know both the reactor operating temperature and the type of propellant being used; as far as I'm aware, we've never been given either.

_________________
Fighting evil so you don't have to!


Top 
 Profile  
 
 Post Posted: Thu Jan 26, 2012 12:12 am 
Offline
Creepy Bishonen
User avatar

Joined: Mon Mar 06, 2006 12:01 am
Posts: 190
Location: Classified Location
Brave Fencer Kirby wrote:
As far as exhaust velocity and specific impulse and the like... well, we don't really have the numbers to calculate that. We'd need to know both the reactor operating temperature and the type of propellant being used; as far as I'm aware, we've never been given either.


And you can see my dilemma, though Toysdream/Mark's article suggests that the propellant might be Helium. Not sure if it's the Helium-3 for the onboard reactor or Helium-4 to be honest, though the amount of naturally occurring Helium-4 compared to Helium-3 would suggest the latter.

As for the thermal output of the Ultracompact/Minovsky-Ionesco Fusion reactor, I can only assume that it's exponentially larger than the equivalent fission reactor. One could derive the output by the theoretical specific impulse of thermonuclear fusion rocket engines, the published equation of UC's nuclear fusion, and Helium, but such math is far beyond my paygrade.

I could simply use the data collected on the website for acceleration and thrust force and extrapolate from that, but I have been warned time and time again to use extreme caution when one uses such stats. Especially if it's to compare the animated performances on screen.

_________________
Though he may have his flaws and faults, he was a husband and a father without equal. May the Angels welcome and accept him with open arms.

Rest in Peace, Dad

"If I had seen farther than others, it has been by standing on the shoulders of giants." - Sir Issac Newton


Top 
 Profile  
 
 Post Posted: Thu Jan 26, 2012 3:04 am 
Offline
Okawara x Katoki Love Child
User avatar

Joined: Tue Mar 07, 2006 6:43 am
Posts: 1229
Sabersonic wrote:
And you can see my dilemma, though Toysdream/Mark's article suggests that the propellant might be Helium. Not sure if it's the Helium-3 for the onboard reactor or Helium-4 to be honest, though the amount of naturally occurring Helium-4 compared to Helium-3 would suggest the latter.


Although it's conspicuously absent in the description of 'propellant' in various IMIDAMs & other sources, Gundam Century does mention that liquid hydrogen was the propellant used by the MS-09 Dom, where it doubled as reactor coolant. The text also heavily implies that this was the propellant of choice for Zeon's space warships as well. Of course, much of the info in G-Cen has been superceded over the years, but one takes what one can get from the stuff that falls in between the cracks. :|

It's definitely Deuterium + Helium-3 for the fusion reactor though, as stated in various official sources.

_________________
RINRIN NANO DA!!


Top 
 Profile  
 
 Post Posted: Thu Jan 26, 2012 3:35 am 
Offline
Master of Mecha

Joined: Mon Mar 06, 2006 12:24 pm
Posts: 2961
Location: San Francisco, CA
I'm inclined not to suggest hydrogen simply because liquid hydrogen has extremely low density. The hydrogen compartment in the space shuttle's external tank, for example, is about 30 meters long and 8 meters in diameter, but only holds about 100 tons of propellant.

Aside from helium, regular water is actually one of the better propellant candidates. It has a relatively low molecular weight (which yields a higher average molecular velocity, and thus higher specific impulse, at any given temperature) and its density is pretty close to what we can infer from the existing specs. In particular, the EWAC Zack has exactly 13 tons more propellant than the standard Hizack (we can compute this from the published mass ratio specs), and the two big propellant tanks on its backpack seem to have a volume of 15-20 cubic meters each. A cubic meter of water weighs one ton; a cubic meter of liquid hydrogen weighs about 70 kg; a cubic meter of liquid helium weighs about 150 kg. So water is really about the only practical propellant that these tanks could hold 13 tons of.

(Just FYI, the Model Graphix "Gundam Wars II: Mission ZZ" book claims that the Ewac Zack's propellant tanks each have a capacity of 1200 gallons. That's 4.5 cubic meters, which seems a bit low given the size and weight of the tanks, but we can take it as a low-end estimate.)

As for reactor temperature, this isn't all that relevant to the kind of engine we're discussing. It's the temperature of the propellant that matters, not that of the reactor core, and the amount of fuel inside the reactor is far smaller than the amount of propellant that's constantly flowing past the reactor on its way to the engines. So the propellant will only be heated to a tiny fraction of the core temperature, and there are so many engineering factors that affect the amount of heat that gets transferred that the actual core temperature would probably be almost irrelevant. I think the specs would probably end up being pretty close to the fission-based design studied in the real world, even though the fusion reactor has a far higher core temperature.

-- Mark


Top 
 Profile  
 
 Post Posted: Thu Jan 26, 2012 11:20 am 
Offline
Creepy Bishonen
User avatar

Joined: Mon Mar 06, 2006 12:01 am
Posts: 190
Location: Classified Location
toysdream wrote:
It's the temperature of the propellant that matters, not that of the reactor core


Though considering the lightly detailed description of how the thermonuclear fusion rocket drives operate in the UC branch, the thermal output of the reactor would actually matter to net acceleration and thrust force.

Still, the amount of thermal output generated by the fusion reactor of any design would probably have a small portion of said heat for either secondary power generation or remass expansion. The rest is simply waste heat that needs to be radiated away due to the conductive thermodynamic nature of interplanetary/interstellar vacuum (aka it sucks horribly).

_________________
Though he may have his flaws and faults, he was a husband and a father without equal. May the Angels welcome and accept him with open arms.

Rest in Peace, Dad

"If I had seen farther than others, it has been by standing on the shoulders of giants." - Sir Issac Newton


Top 
 Profile  
 
 Post Posted: Thu Jan 26, 2012 3:22 pm 
Offline
Okawara x Katoki Love Child
User avatar

Joined: Tue Jan 30, 2007 3:14 pm
Posts: 1234
The output of the reactor only matters as an upper bound -- I'm used to thinking of NTRs as heating their propellant directly, which means that the propellant gets as-close-as-possible to the reactor temp. Gundam doesn't work that way, though, so toysdream is right in saying that there are other more important considerations than reactor core temperature in determining the propellant's temperature as it's being exhausted.

Water is a sort of weird propellant. As toysdream points out, it's not as high-efficiency as the lowest-weight elements like hydrogen and helium but it's a lot easier to handle, which might make it a good fit for mobile suit propellant. I would suspect, though, that ships -- less concerned with volume restrictions in particular -- would use something more efficient. Water also takes a lot of energy to heat, which makes it not-great as a propellant but is good for a coolant (or a heat sink).

Actually, now that I think about it (in other words, rampant speculation incoming) it might be possible to get the best of both worlds. Store your propellant as H2O, but electrolyze it into H2 and O2 before you actually burn it. That way you get nice, efficient H2 propellant, with O2 left over to dump in and increase thrust (as with the LANTR) when necessary. It might also be possible to dump that extra O2 into the cockpit for all those times when pesky teenage pilots keep jumping in and out of their mecha mid-battle, but the overhead costs of the plumbing required might not be worth it (not to mention the fact that pure O2 is ugly stuff to work with, especially in a combat situation). Of course, given that power constraints are one of a mobile suit's main performance limiters, it's probably not worth the bother of doing all that when water works well enough. Mobile suits are probably more concerned with thrust than specific impulse in any case.

_________________
Fighting evil so you don't have to!


Top 
 Profile  
 
 Post Posted: Thu Jan 26, 2012 11:10 pm 
Offline
Creepy Bishonen
User avatar

Joined: Mon Mar 06, 2006 12:01 am
Posts: 190
Location: Classified Location
Brave Fencer Kirby wrote:
Gundam doesn't work that way, though, so toysdream is right in saying that there are other more important considerations than reactor core temperature in determining the propellant's temperature as it's being exhausted.


Well it still kinda does considering the description of how thermonuclear rockets work in the UC branch since the thermal output of the reactor is used to heat the propellant, though small the amount that is used. And considering that the Ultracompact Reactor is supposedly have a self-sustained catalyst reaction, I can only assume that the open cycle version of Fusion NTRs aren't the chosen design for Mobile Weapons. Then again, I am splitting hairs on the subject when the question is asking about the acceleration of the UC "Fusion Lightbulb" rocket and its thrust force in either kilograms or newtons.

As for water being a propellant/coolant medium, I've always found it to be a rather questionable choice. I'm not sure how great a mass difference is between water and hydrogen or Helium in terms of equal volume, but I can assume that it would cut rather deeply into the DeltaV budget.

_________________
Though he may have his flaws and faults, he was a husband and a father without equal. May the Angels welcome and accept him with open arms.

Rest in Peace, Dad

"If I had seen farther than others, it has been by standing on the shoulders of giants." - Sir Issac Newton


Top 
 Profile  
 
 Post Posted: Fri Jan 27, 2012 12:40 am 
Offline
Master of Mecha

Joined: Mon Mar 06, 2006 12:24 pm
Posts: 2961
Location: San Francisco, CA
Water is way denser than liquid hydrogen or helium - I gave you the figures above. If you have limited space, you can fit in way more propellant mass if you fill those tanks with water. Water's higher molecular weight does give you a lower specific impulse - so you ultimately get less delta-v for a given propellant payload - but it's still one of the less bad choices. :-)

As for reactor temperature, this really isn't directly relevant here. Once again, the mass of superheated fuel inside the reactor is pretty small compared to the mass of propellant that is briefly circulated past the reactor to absorb some of its heat output, so the propellant will only be heated to a tiny fraction of the reactor core temperature. If you put a kettle of water on a lit stove burner, the entire contents of the kettle won't be instantly heated to the exact same temperature as the burner flame, right?

-- Mark


Top 
 Profile  
 
 Post Posted: Fri Jan 27, 2012 9:50 am 
Offline
Creepy Bishonen
User avatar

Joined: Mon Mar 06, 2006 12:01 am
Posts: 190
Location: Classified Location
toysdream wrote:
If you put a kettle of water on a lit stove burner, the entire contents of the kettle won't be instantly heated to the exact same temperature as the burner flame, right?


No, but I can only assume that the higher the burner, the quicker the water heats up to a boil more or less. Granted, it also includes pressure, volume, contact area, and all that in that particular equation but like I said, I'm just splitting hairs on an issue that still doesn't really answer my questions. Kinda helps in my mental imaging of the operation, but doesn't really put the numbers into the equation.

toysdream wrote:
Water's higher molecular weight does give you a lower specific impulse - so you ultimately get less delta-v for a given propellant payload - but it's still one of the less bad choices.


And I can only imagine that the cost in transport, remass-wise and such, would also increase the- uh..... pump cost for lack of a better terminology. Even if said water isn't lifted off the planet mostly to serve as remass, it'll take alot of regolith and slag to get it out of the moon and asteroids. Water may be common in space, but it ain't exactly concentrated enough in the local aerospace orbit as far as I figure to really warrant its use as Remass. Then again, the same could be said of Helium-3 and yet the Jupiter Energy Fleet sends enough to feed the multitude of fusion reactors with little worry in the UC Branch so I guess it's a moot point for both resources.

Still, it does make one curious if the UC "Fusion Lightbulb" design would allow for possible Torchship or Demi-Torchship levels at best if it had a high enough DeltaV budget. The posted statistics of the main rocket engine found in some technical data of mobile suits would suggest it, but then again I've been advised time and time again that such data (especially if one is comparing the on-screen animation action) is as contradictory as any career politician.

_________________
Though he may have his flaws and faults, he was a husband and a father without equal. May the Angels welcome and accept him with open arms.

Rest in Peace, Dad

"If I had seen farther than others, it has been by standing on the shoulders of giants." - Sir Issac Newton


Top 
 Profile  
 
 Post Posted: Fri Jan 27, 2012 12:31 pm 
Offline
Master of Mecha

Joined: Mon Mar 06, 2006 12:24 pm
Posts: 2961
Location: San Francisco, CA
Sabersonic wrote:
No, but I can only assume that the higher the burner, the quicker the water heats up to a boil more or less.

Sure sure, just wanted to address the common misconception that reactor core temperature = exhaust temperature. That only applies to the "leaky reactor" type of engine that vents reactor fuel directly; if you're just using the reactor to heat a bulk propellant, they're not even remotely close.

And really, water shouldn't be that hard to come by. You'd need huge amounts of it to support your colony populations anyway, and its constituent elements - hydrogen and oxygen - are very standard chemical rocket fuels. If you don't have a way of obtaining water, you probably wouldn't be able to sustain a space empire in the first place. :-)

-- Mark


Top 
 Profile  
 
 Post Posted: Sat Jan 28, 2012 2:03 am 
Offline
Creepy Bishonen
User avatar

Joined: Mon Mar 06, 2006 12:01 am
Posts: 190
Location: Classified Location
toysdream wrote:
That only applies to the "leaky reactor" type of engine that vents reactor fuel directly; if you're just using the reactor to heat a bulk propellant, they're not even remotely close.


Though has a higher ISP compared to closed-cycle TNR engines, open-cycle Nuclear rockets don't really seem safe radiation-wise, especially if one's mobile suit is launched from an enclosed catapult with a full burn. Granted, such a problem could be alleviated with such techniques and technologies such as Cascade Vanes, though several of them are counterproductive.

toysdream wrote:
If you don't have a way of obtaining water, you probably wouldn't be able to sustain a space empire in the first place. :-)


I could imagine that water recycling and reclamation of such settlements to be advanced enough to require an import of the liquid to match the amount lost from said systems. With water as a rocket remass, it seemed like kind of a waste compared to the infrastructure used to extract it in the first place to me. I guess it's just something about space/orbital resource collection that escapes me.

_________________
Though he may have his flaws and faults, he was a husband and a father without equal. May the Angels welcome and accept him with open arms.

Rest in Peace, Dad

"If I had seen farther than others, it has been by standing on the shoulders of giants." - Sir Issac Newton


Top 
 Profile  
 
 Post Posted: Sun Jan 29, 2012 8:59 pm 
Offline
Okawara x Katoki Love Child
User avatar

Joined: Tue Jan 30, 2007 3:14 pm
Posts: 1234
Well, part of the problem is that we don't see much of UC Gundam's surface-to-orbit infrastructure, unlike CE's mass drivers or 00's space elevators. Though thinking about it, I can't recall much heavy lifting they do anyway -- with the notable exception of the Revil's fleet near the end of Mobile Suit Gundam, I don't think we see much of anything bigger than an HLV-load launched into space.

Though there was a mass driver early in Victory, now that I think about it...

_________________
Fighting evil so you don't have to!


Top 
 Profile  
 
 Post Posted: Wed Feb 22, 2012 9:18 pm 
Offline
Lackey GM Pilot

Joined: Sun Sep 24, 2006 6:53 pm
Posts: 78
Well the operating times for mobile suits is one area that I have been thinking of.

Well we do know a few things, Mobile suits generally do not have the thrust or fuel loads to reach low earth orbit from the ground or even at altitude. Also mobile suits do not seem to have the endurance to reach the earth from the moon (or Vice versa) in a reasonably short time frame (note if fuel was not an issue it would take a unit roughly 3.55 hours to reach the moon from earth orbit at 1G in a acell - decell profile).

In the MS-01 to 04 history thread it seems that some of these early MS prototypes only had 10 to 20 minutes of endurance, so later MS probably have around 20 to 60 minutes of fuel at combat ratings (the Zaku II R1A in one source is stated to have 15 min at max thrust which is supposedly considered short).

---------------
Gundam Seed has a MS reaching earth Orbit from the moon, supposedly in a matter of hours, though the timeline indicates 3 days (if under constant acceleration that's about .0025G), which would indicate either low acceleration at this cruising rate or they used a lengthy coasting phase (a likely option).

IIRC Gundam Unicorn also has a MS reach earth from a ways off but I do not recall the specifics, though I do recall that they had to use a "catapult" assist due to the lack of fuel due to their previous combat engagement (this takes place in EP 3). -Is their a time line of this?


Top 
 Profile  
 
 Post Posted: Wed Feb 22, 2012 9:45 pm 
Offline
Hot-blooded Gattai
User avatar

Joined: Sun Mar 05, 2006 7:44 pm
Posts: 8070
Location: Detroit, MI
Nebfer wrote:
IIRC Gundam Unicorn also has a MS reach earth from a ways off but I do not recall the specifics, though I do recall that they had to use a "catapult" assist due to the lack of fuel due to their previous combat engagement (this takes place in EP 3). -Is their a time line of this?

The Delta Plus plugs into the same sub-flight system (sled) that was used by Char's Neo Zeon and gets a kick start from Palo's mass driver then uses the sled's boosting power to help head to Earth. IIRC, Riddie said it would take about 2 days to get from Palo to Earth orbit.

Presumably the Delta Plus coasted the rest of the way in between as the original reason for using the mass driver was because the MS was low on fuel from being in combat.

_________________
The Gundam wiki

"Reality makes a crappy special effects crew." - Adam Savage


Top 
 Profile  
 
 Post Posted: Thu Feb 23, 2012 1:33 am 
Offline
Master of Mecha

Joined: Mon Mar 06, 2006 12:24 pm
Posts: 2961
Location: San Francisco, CA
The travel times referenced in timelines and episode dialogue tend to track those of real-world moonshots - about 3 days from Earth to the moon, about 2 to 2.5 days for the gravity-assisted return trip. I believe these are the most efficient trajectories, and anything quicker would require a lot of extra acceleration and deceleration to avoid overshooting the mark. Or so I recall...

-- Mark


Top 
 Profile  
 
 Post Posted: Thu Feb 23, 2012 6:18 pm 
Offline
Okawara x Katoki Love Child
User avatar

Joined: Tue Jan 30, 2007 3:14 pm
Posts: 1234
Good old Hohmann orbits. I find myself wondering if mobile suits can actually handle Hohmann transfers, above and beyond the ability of the pilot to survive a few days in a mobile suit. They require a pretty good kick to get going, and I'm not sure that MS thrusters are powerful enough to do it. That said, I haven't actually been able to find any hard numbers on what sort of accelerations are required for Hohmann transfers -- plenty of info on delta-V needed, but unfortunately we lack that info on the mobile suits.

Generally speaking, I'd tend to assume that MS aren't capable of making transfers on their own. Between the amount of thrust, amount of propellant, several days worth of water and air, and the problem of waste heat, it seems unlikely that your average MS is going to be cruising around the Earth Sphere solo (though occasional jaunts by high-performance units that get a boost somehow, like the Nu or the Freedom, isn't entirely out of the question).

_________________
Fighting evil so you don't have to!


Top 
 Profile  
 
Display posts from previous:  Sort by  
 
Post new topic Reply to topic  [ 18 posts ] 

Board index » Discussion » Mecha and Technology


Who is online

Users browsing this forum: Bing [Bot], Google [Bot] and 5 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: