Thrust to weight ratio

[Zolar V]

What is the Thrust to Weight ratio?

 

im planning to build a scale model of a floating island. im going to make it out of toothpicks(building materials), a mini thermonuclear reactor(energy) and fans(propulsion). for the purpose of having a floating, controllable runway with the ability to land other model planes on it.

 

i wana see if it is even possible to make such a thing.

Edited December 21, 2009 by Zolar V

[npts2020]

What is the Thrust to Weight ratio?

 

im planning to build a scale model of a floating island. im going to make it out of toothpicks(building materials), a mini thermonuclear reactor(energy) and fans(propulsion). for the purpose of having a floating, controllable runway with the ability to land other model planes on it.

 

i wana see if it is even possible to make such a think.

 

You mean like an aircraft carrier?

[Zolar V]

i mean an aircraft carrier.. IN AIR BAM!

[Moontanman]

Actually zolar i once had some fantasies around a floating island type environment, you can generate power by using the difference in temps between the deep ocean the surface. I thought of a hollow ferro-cement body full of foam with ballast tanks and even a freshwater lake in the center. Not very practical, probably be millions of dollars an acer to build, possibly much of the living areas could be underground, but still not impossible.

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OOOhh, I was thinking of floating in the ocean, not the air, my bad

[Sisyphus]

So by "floating" you mean flying? So you want to build a flying nuclear reactor out of toothpicks? That you can land model planes on? I'm just trying to understand the project, here.

[Moontanman]

There have been some ideas floating around considering the possibility of a floating type structure in the atmosphere of Jupiter kept aloft by heating a huge balloon full of Jupiter's atmosphere via a nuclear, fusion I think, reactor. I am pretty sure it wasn't meant to be manned so radiation shielding wasn't as much of a problem for this application as it would be if humans were on board.

[Zolar V]

So by "floating" you mean flying? So you want to build a flying nuclear reactor out of toothpicks? That you can land model planes on? I'm just trying to understand the project, here.

 

you can float on air considering air is considered a fluid

but essentially yes i want to make an island that is flying. made out of toothpicks.. powered by a thermonuclear reactor(not made of toothpicks). the island contains enough space to land planes.. and such

[Sisyphus]

I see. So a flying aircraft carrier for model planes. That's the core of the idea, right? Is there any particular reason you need toothpicks? Or that particular, wildly impractical fuel source?

[Zolar V]

I see. So a flying aircraft carrier for model planes. That's the core of the idea, right? Is there any particular reason you need toothpicks? Or that particular, wildly impractical fuel source?

 

actually yes, toothpicks are cheap and light. and the fuel source is a representation of the fuel source if the model was to be made real.. aka giant floating island that acts as a halfway point between earth/space launching and an airport of some proportion. i am envisioning this as a 3-5 deck island.. (think aircraft carrier decks) that houses a energy system (the thermonuclear energy) a propulsion system, fuel storage, and .. stuff...

i got the idea while reading the new popsci on the article about Virgin Galactic and how it launches a rocket plane from a plane in the air.My thought was to bring the whole damned airport into the air.. instead of the plane

[Cap'n Refsmmat]

Then how do you get to the airport?

[Zolar V]

fly to it. DUUHHH>

 

it would be interesting if you could launch into space from a mid-air platform, i would imagine that it would require less energy due to it taking of horizontally and also being quite high in the atmosphere.

 

of course it goes without saying that it may be possible to just have the whole island just fly up into space itself without being a mobile platform.

 

but if it was its only capacity you would loose the ability to land and refuel or whatever on a mobile platform.

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(edit) im not in anyway eliminating ground based airports, just making a new one somewhere else

[Mokele]

One word: blimp

[Sisyphus]

http://en.wikipedia.org/wiki/Airborne_aircraft_carrier

 

May as well move on to actual examples. It's pretty clear that if you want something that could conceivably be in the air on a semi-permanent basis, it has to be lighter than air. There is one exception I can think of: the NASA Helios, but that couldn't really carry anything but itself, let alone act as an aircraft carrier.

 

Nothing in the air has an actual "runway," because obviously runways are huge, heavy, awkward, and relatively immobile: not feasible.

 

So, the "carrier" itself either has to be able to match speeds with the docking aircraft (probably not feasible for a lighter than air craft, so it would just be an airplane), or the docking craft has to have hovering capability to match speeds with the mothership. For the former possibility, that's pretty much what they do already. For the latter, hovering capability takes extra weight, so it's unlikely to be practical for a space ship.

 

The question remains of why not just piggyback up on a mothership aircraft, like they do already. What does an in-air rendezvous add to the mission?

Edited December 22, 2009 by Sisyphus

[npts2020]

Also, it seems to me that any scheme like you are suggesting would use more energy than simply launching a craft from the ground.

[Mr Skeptic]

I've had this idea too, and as Sisyphus showed, so have others. In practice, however, it may be more useful to just dock on earth and then take off.

 

My idea was a hot hydrogen lighter-than-air craft. This would be extremely light so it could go very high.

 

The most practical use for it would have been to lift spaceships to save on rocket fuel, much like Virgin Galactic does with their mothership craft.

[Edtharan]

Toothpicks area very poor material for building a light object out of. Sure an individual toothpick is light, but that is because it is small.

 

Also, the big problems with toothpicks being small is that they aren't big. So if you are going to build a big object out of them, then you need to some how stick them together.

 

This creates two main problems:

 

1) The glue adds weight, and you need a lot of glue so the mass of the thing is going to go up fast.

 

2) It will be fragile.

 

If you have ever built a model out of toothpicks you will know that they can break easily. So for structural strength to weight ratio, toothpicks are an extremely bad idea.

 

What you probably want is some kind of framework made from a material that is light and strong (and not pliable). Aluminium is a good start here (they make planes out of it for a reason). Composite materials are another good idea, but can be expensive to make.

 

If it is a scale model you are making, then plastic has a good weight/strength ratio (if the model is not too big).

 

However some type of Carbon Fibre embedded composite material would be your best bet (if cost is not an option).

 

Nuclear reactors are not small enough or light enough to fit into a model. Nuclear reactors need shielding in the form of a thick, dense material. this means that any nuclear reactor would be far too heavy to allow the model to fly.

 

But to answer your question:

What is the Thrust to Weight ratio?

Thrust to weight ratio is the comparison of how much thrust a vehicle can produce as compared to its weight.

 

Mathematically, if you divide the thrust of the vehicle by its weight you will get this ratio.

 

This is useful because if the aircraft can not produce enough thrust to lift its weight, it will not fly.

 

This means you need to make the model as light as possible and give it as much thrust as possible.

 

This is not an easy thing to do.

 

Nuclear Reactors are heavy. This will increase your weight and reduce the thrust to weight ratio.

 

Toothpicks glued together are heavy. This will reduce your thrust to weight ratio.

 

In other words, the initial plan you ahve is a very bad plan. You will need to start again from the beginning and remember that you need to keep the weigh down, you need to increase the amount of thrust and you will need to make it strong (which is a balancing act between this and the weight - strong materials tend to be heavy).

 

The only thing the Nuclear reactor could bring to this is that it can provide a large amount of power for a long time (but there are actually lighter ways of doing this).

 

So there are 3 ratios you need to look at:

 

1) Energy to Weight ratio: This is basically how much energy you can get form a given amount of weight (in the energy generation system you use). You want to minimise the weight and maximise the energy.

 

2) Strength to Weight ratio: This is how strong your design is for a given weight. You want to use materials that are light and yet strong. The problem is that many materials that are strong are also heavy.

 

3) Thrust to weight ratio: This is your initial question. The solution is that it relies on the amount of energy you can produce and the efficiency of your engines. If your engines can't turn your energy into enough thrust then it can not work. Also, remember each engine you add to the design increase the amount of weight. So you want to minimise the amount of weight and maximise the amount of thrust your engines can produce this means getting the most efficient engines as possible.

[Zolar V]

thanks for the tips, those are what i was looking for

but why does it have to be a lighter than air vehical?

i imagined mine as a massive heavy object being held up by 4-8-16-20 some giant turbo engines, situated vertically in the island, powered by a nuclear reactor.

The nuclear reactor takes care of any energy concerns and allowes for flight longevity.

i would think using this, the craft could infact stay afloat for quite a while.

why would any boarding aircraft have have the ability to hover to land?

if the island went stationairy and had a 15000 foot runway then a plane could easily land on it using the same types of procedures already designed to land on ground based landing strips.

[CaptainPanic]

Using nuclear power to keep something up in the air indefinitely is a tried concept... or at least, it was investigated by both the Americans and the Russians.

 

In another thread that I opened once I also linked to a documentary about Project Pluto - a nuclear ramjet cruise missile.

 

Concluding: it could be done... in more than one way... but practical problems are huge but I assume that's not unexpected.

[Zolar V]

Using nuclear power to keep something up in the air indefinitely is a tried concept... or at least, it was investigated by both the Americans and the Russians.

 

In another thread that I opened once I also linked to a documentary about Project Pluto - a nuclear ramjet cruise missile.

 

Concluding: it could be done... in more than one way... but practical problems are huge but I assume that's not unexpected.

 

heh, i read thos same articles a few years back. i never was able to find the reason why those projects were cancled in the late 60's. the most i could come up with was the end of the cold war and the nuclear bill signed by ?kennedy? prohibiting and inhibiting much if not all nuclear research for quite a while.

honestly i really dont see any reason as to why we have not made nuclear powered aircraft yet. or used nuclear power in any sort of vehicle. it seems to me that despite the radiation it is quite usefull. and if we were to have industries allowed to build/research/maintian/operate such things then our level of understanding on nuclear engines would increase, along with our nuclear technology.

from what i have seen in the past, it usually takes industry to advance tehcnology as opposed to research doen by non-enterprising people.

[Sisyphus]

But again, keeping a plane in the air is different than hovering a massive object with brute force. Aircraft have wings for lift and aren't directly opposing gravity, and every aspect of their design is geared towards minimizing weight, but it still takes huge amounts of fuel.

 

And nuclear power is used in vehicles. Most naval vessels are nuclear powered. It works great, but it's not enough power to get them to hover in the air!

 

In fact, why not use that as an example for some rough calculations. Compare to a real-life nuclear-powered "floating airport:" an aircraft carrier. Wikipedia says the Nimitz class are about 101,000 long tons, or 102620738kg. Let's round down to an even 100000000kg. To directly oppose gravity, you need enough force to accelerate 100000000kg at 9.8m/s/s. Eh, round up to ten. So, the gravitational force on an aircraft carrier is about 1 billion newtons.

 

For comparison, the Saturn V engine, the most powerful rocket engine ever (that powered the Apollo rockets), had a maximum thrust of 34 million newtons, and fired for six minutes. So what you want is the thrust of about 30 Saturn V rockets, firing permanently.

 

And that is why we don't have huge, permanently hovering, heavier than air structures.

[Mr Skeptic]

Nuclear powered spacecraft, on the other hand, would be a very good idea. Chemical rockets have a huge difficulty: they have to carry their own oxidizer, which generally weighs more than their fuel. Also there is a limited amount of energy that can be stored in the fuel. A nuclear powered rocket on the other hand, can heat up hydrogen and expel it at very high velocities using the reactor as a heat source rather than combustion with oxygen. The faster it is expelled, the more efficient the craft. In space, this (called specific impulse) is more important than the thrust to weight ratio. For example, ion engines are used despite the ridiculously small thrust they have, because of their higher specific impulse.

[npts2020]

The reason nuclear reactors are not generally used for any type of aircraft (besides safety considerations) is that all reactors big enough to produce large amounts of power for a long period of time, are very large and heavy. In addition to size and weight of the reactor, you also have the related steam power plant (which is really where you get the usable energy from) and the moderation medium in the reactor (usually water). The reactors on board a submarine are approaching the limits for small size that are still useful. The scenario you are talking about would require one (or more) many times the size of the ones on board an aircraft carrier. I can tell you from personal experience that those reactors and their associated equipment are HUGE and take up substantial space on board even such a large ship (on submarines it is about 1/3 of the sub). It may be one day that somebody figures out how to scale them down substantially but for now the known laws of physics do not allow it.

[Edtharan]

As an off the wall idea, what about Radioisotope Thermoelectric Generators (http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator), Beta Voltaics (http://en.wikipedia.org/wiki/Betavoltaics) or Atomic Batteries (http://en.wikipedia.org/wiki/Atomic_battery).

 

These need less shielding than fission reactors and so are much lighter, although the power output is not as great (so more would be needed, and this might mean that the power to weight ratio is not good enough).

[ydoaPs]

The scenario you are talking about would require one (or more) many times the size of the ones on board an aircraft carrier.

Each of an aircraft carrier's rectors(they all have 2, except for the Enterprise which has several submarine reactors) are indeed quite large. The reactor itself is 3 stories tall, then you have the primary system whose pumps are as big as submarine reactors. Then, you have the steam system(which is quite large-it's the rest of the heat cycle).

 

I can tell you from personal experience that those reactors and their associated equipment are HUGE and take up substantial space on board even such a large ship (on submarines it is about 1/3 of the sub).

On a carrier, they take up the space directly under the mess decks. Although, if we were just going to use them for just electrical power(or, in the case of the spacecraft, heating a gas), they could be considerably smaller.

 

Also, submarines aren't terribly large. A submarine reactor is only like 12 feet across(but then you have the rest of the systems).

[Zolar V]

huh, it seems to me that the potential for smaller reactors and increased energy output is there but the technology is not.

if we were to try to get this type of thing off the ground (no pun intended) then we would have an industry supporting nuclear research. if we had an industry then the technology would exponentially get better, supposedly.

of course i do not know all of the physics and equations that coencide with nuclear engins, but i have memorized several different working reactor designs and analyzed them. and after analyzing there does seem to be the potential to increase power output and decrease size, as stated above.

but this is all just hypothesis, cuz im just a highschool grad bidding my time for college.