Nuclear Rockets [WIP]

[Kommitz]

LOL whoa you said it was big but I didn't realize it was that big omg. Yeah we're rescaling that monster down a fair bit.

It's mostly the nozzle that contributes to the size, and I was thinking of changing the design a little and adding a partially extendable nozzle.

The reactor itself isn't any larger than the FTmN 280.

Also I'm just deciding how to unwrap it at the moment:

Edited May 9, 2014 by Kommitz

[Tommygun]

Nice looking, I wish it had room for some of the upper parts, but it's so long already.

Unless you do those parts for your Octagonal Truss Set.

[Kommitz]

Nice looking, I wish it had room for some of the upper parts, but it's so long already.

Unless you do those parts for your Octagonal Truss Set.

It's actually based on these drawings for the NERVA II that I found with some googling, they came with dimensions and it looks good so I went with it:

(The only downside is that I'm not entirely sure of the propellant cycle (and what particular pipes do what), but that's just me worrying about details that don't matter in game )

As for the height I'm considering altering the design a bit for the sake of practicality and making the nozzle partially retractable. Will have to see how that would work with all the pipes.

[YANFRET]

Nice looking, I wish it had room for some of the upper parts, but it's so long already.

Unless you do those parts for your Octagonal Truss Set.

http://www.astronautix.com/graphics/z/znerva.jpg

Oooh look! I like those items!!

[Yilmas]

As for the height I'm considering altering the design a bit for the sake of practicality and making the nozzle partially retractable. Will have to see how that would work with all the pipes.

Eh, might be a silly question. But what do you mean by retractable ?

[Tommygun]

Eh, might be a silly question. But what do you mean by retractable ?

I think Kosmos Spacecraft Design Bureau has some in there parts pack.

It's a bit like a telescoping antenna or funnel made with two parts and the lower larger funnel slides up over the upper smaller funnel shortening the length.

[LameLefty]

Eh, might be a silly question. But what do you mean by retractable ?

Basically in the real world, a number of upper-stage engines have been optimized for use in vacuum with 2-piece nozzles. The basic nozzle is attached to the combustion chamber throat as normal, while a much longer, larger-diameter extension skirt is mounted around it. Upon staging, extension slides downward (generally mounted on rails) and the top of the skirt matches up with the edge of the main fixed nozzle. Variants of the American RL-10 LH2/LOX engine have used this for years.

Google the term "extendable rocket nozzle" for images and the combustion thermodynamics reasons for wanting a very high expansion ratio in a vacuum ...

[YANFRET]

Basically in the real world, a number of upper-stage engines have been optimized for use in vacuum with 2-piece nozzles. The basic nozzle is attached to the combustion chamber throat as normal, while a much longer, larger-diameter extension skirt is mounted around it. Upon staging, extension slides downward (generally mounted on rails) and the top of the skirt matches up with the edge of the main fixed nozzle. Variants of the American RL-10 LH2/LOX engine have used this for years.

Google the term "extendable rocket nozzle" for images and the combustion thermodynamics reasons for wanting a very high expansion ratio in a vacuum ...

Another great reason is that assuming you want the benefits of a larger nozzle for vacuum performance, the mechanism that extends the nozzle has less mass than a longer interstage fairing or other structural extension method to accommodate the greater length.

Edit;

Kommitz, strictly speaking however for Chaka Monkey operations we don't need an extensible nozzle because the unit will be launched inverted for orbital assembly and there is plenty of extra center line space at the top of the payload fairing

[phoenix_ca]

As long as it can fit in a procedural fairing, I don't care how big it is.

Okay, I do. That thing looks like it might need to be launched as its own payload. O.O

[Yilmas]

Kommitz, strictly speaking however for Chaka Monkey operations we don't need an extensible nozzle because the unit will be launched inverted for orbital assembly and there is plenty of extra center line space at the top of the payload fairing

Yes, to the last part, it will be hauled to space for orbital assembly. But in a way, it would be kinda cool if the nozzle would extend depending on the amount of thrust.

So it is retracted while docking maneuvers or rather cruising speed. But becomes extended when interplanetary/max throttle is issued.

[LameLefty]

Yes, to the last part, it will be hauled to space for orbital assembly. But in a way, it would be kinda cool if the nozzle would extend depending on the amount of thrust.

So it is retracted while docking maneuvers or rather cruising speed. But becomes extended when interplanetary/max throttle is issued.

If it gets implemented that way, it should have substantially lower vacuum ISP with the nozzle retracted. The extension is there to prevent under-expansion of the exhaust in vacuum.

[Yilmas]

The extension is there to prevent under-expansion of the exhaust in vacuum.

Not entirely sure what you mean about this ?

And whether it is thrust or isp that is decreased, doesn't matter for me, however I'm guessing that the ISP would be the correct(scientifically) choice.

[LameLefty]

Not entirely sure what you mean about this ?

It's hard to explain without math (it involves compressible fluid mechanics and thermodynamics). This Wiki article is pretty basic and light on the mathematics; it's not very technically specific in spots but it explains the basics fairly well.

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

[Yilmas]

It's hard to explain without math (it involves compressible fluid mechanics and thermodynamics). This Wiki article is pretty basic and light on the mathematics; it's not very technically specific in spots but it explains the basics fairly well.

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

Without reading through it all. I'm gonna assume that it is some kind of relation to area of expansion or perhaps rather the area that it can affect.

But regarding the ISP, shouldn't it still be rather large(compared to regular engines), considering it being a nuclear engine.

[Streetwind]

Not really, no. A rocket engine without a nozzle is like a car without rubber tires on its wheels. The car can be a Ferrari, but it's not going to give you any sort of performance when driving on blank rims because they have no grip whatsoever. Similarly, even the best rocket engine is going to suck if its exhaust isn't properly shaped.

This goes as far as having specific nozzle designs for atmospheric and vacuum use. When SpaceX launches a Falcon 9 v1.1 nowadays, then the first stage booster has nine Merlin 1D engines with atmospheric nozzles delivering 282s Isp at sea level and 311s in vacuum. Meanwhile the second stage has a single Merlin 1D Vacuum, which is the exact same engine except that it is fitted with a vacuum optimized nozzle, now suddenly yielding 340s vacuum Isp. The atmospheric Isp of this nozzle is probably below 240, but it doesn't really matter because it's not used in that regime.

Edited May 12, 2014 by Streetwind

[Yilmas]

Not really, no. A rocket engine without a nozzle is like a car without rubber tires on its wheels. The car can be a Ferrari, but it's not going to give you any sort of performance when driving on blank rims because they have no grip whatsoever. Similarly, even the best rocket engine is going to suck if its exhaust isn't properly shaped.

Alright, I can understand this part. And in a way it really is logical, perhaps it was the time of day, that made me not think about it.

Then a question pops in mind. Is the nozzle design build for the atmospheric pressure only, or partway between that and the rocket it connects to ? Reason for the question, comes after it has been answered.

Also, would it be better to make a thread in general chat about this? This feels a bit , ninja'ish.

[Streetwind]

You obviously engineer for maximum efficiency - that is, you model your first stage's flight path and time, and then build a nozzle that gives you the best result for the projected burn time. The longer a first stage burns, the more the nozzle will tend towards low-pressure/vacuum configuration.

Of course, there are such things as altitude compensating nozzles - aerospikes are an exotic member of this group. By directing the exhaust in a specific way along the central "spike", which forms the inner nozzle wall, complex rules of flow dynamics let you actually use ambient atmospheric pressure as the outer nozzle wall. Sounds funny, but it's true - aerospike Isp varies very little between sea level and vacuum, because the expansion ratio scales with the exact thing that requires the scaling of the expansion ratio in the first place! Sadly, aerospikes pay for this behavior with a lower base performance and higher engine complexity, and you're going to stage your rocket anyway (for dV reasons), so they can never fully realize their potential. Which is why conventional nozzles are still heavily used.

[wasmic]

Ooooh, oooooh, I've got it! a nuclear engine with an aerospike nozzle! It'd only be the most horribly overpowered thing ever!

[Yilmas]

You obviously engineer for maximum efficiency.

Ya, that part was self-explanatory.

But what I meant was that you generally see nuclear engines in KSP that comes out at 800'ish ISP(vacuum), is this some kind of general boundary or is it simply the optimal ISP no matter thrust/size (for nuclear) ?

Which also leads me to question the ISP of all ION engines, 4200'ish. Something so small, with very little thrust giving out more ISP than any other engine. Which regarding to IONs just dosn't seem/feel right, considering the usually very small nozzle(some of them dosn't even have one :S)

[Streetwind]

Ooooh, oooooh, I've got it! a nuclear engine with an aerospike nozzle! It'd only be the most horribly overpowered thing ever!

But that implies that you want to use said nuclear engine inside the atmosphere. That sure is going to make you popular...

(In all seriousness, do not know anything about whether or not this has a chance of working. A chemical rocket exhaust is on fire, a nuclear engine exhaust is just a stream of gas. This may or may not change the flow properties.)

[Streetwind]

But what I meant was that you generally see nuclear engines in KSP that comes out at 800'ish ISP(vacuum), is this some kind of general boundary or is it simply the optimal ISP no matter thrust/size (for nuclear) ?

Which also leads me to question the ISP of all ION engines, 4200'ish. Something so small, with very little thrust giving out more ISP than any other engine. Which regarding to IONs just dosn't seem/feel right, considering the usually very small nozzle(some of them dosn't even have one :S)

I think you have a number of definitions a little wrong, which may make it difficult for you to understand how things work.

- Specific Impulse (Isp) is not entirely a design target, but also a design result of your engineering approach. The Isp an engine can develop in an ideal condition (i.e. vacuum with a proper nozzle) is directly related to the energy put into the propellant, as well as the propellant itself. A kerosene/oxygen reaction might do 310s to 320s, whereas a hydrogen/oxygen reaction might do 450s to 460s, and a nuclear thermal rocket might achieve 800s to 2000s or more... completely dependant on how hot it runs, because there is no chemical reaction, only hydrogen gas being superheated by cooling the reactor core. You can use other, heavier gases aside from hydrogen in a NTR, which results in less Isp (but more thrust, see below).

- The LV-N atomic rocket motor is a nod to the NERVA XE, the only nuclear thermal rocket engine in human history that has ever been assembled in full flight configuration. It happened to have an Isp of around 800 and a TWR around 2.75, which is what the LV-N replicates fairly exactly. Now, the NERVA XE was a first generation NTR used for proof of concept; theoretically there exist ways to make NTRs with much higher Isp, simply by using a reactor core that runs much, much hotter. The so-called gas-core nuclear lightbulb could theoretically achieve over 2000s Isp with hydrogen. Of course, nobody has actually tried building one because nobody wants to be caught dead tinkering with nuclear fission tech in today's political climate, so we don't know if the concept would actually work.

- Isp is measured in seconds, which sounds odd at first for a figure that's effectively a fuel efficiency indicator. It works out like this: an engine burns for X seconds at Y thrust using Z propellant. If you have an engine developing 1 N of thrust, and this engine then burns for 800 seconds on 1 kg of propellant, then that engine has 800 seconds of Isp. It is quite literally "my fuel lasts this many seconds long".

- From this relationship, you can immediately see that in order to have a higher Isp, the engine must take longer to expel 1 kg of propellant. However, a rocket is defined by the act of expelling propellant. Doing so is the very thing that makes something a rocket. By expelling mass, it gains forward momentum, much like jumping off a boat causes the boat to float away from you. And if it expels mass more slowly, it moves more slowly: it has lower thrust. Higher Isp directly begets lower thrust, and there is no way around it. If you want to have the same thrust at higher Isp, you must not only choose the right fuel (see above), but also put an enormous amount of energy into the fuel, to offset that natural limitation by sheer brute force. And there are physical limits to how much energy you can impart - for example, limits to how hot you can run a reactor before your engine simply melts.

- Ion thrusters have gigantic Isp because their thrust is so low. Sounds silly at first, but it's in the way they are designed: they don't expel much mass at all. No, they work by taking a really really small mass (single gas atoms) and pumping that tiny mass full of gigantic amounts of energy. That way, expelling this tiny mass gives a lot more forward momentum than the same tiny mass used in a different engine that imparts less energy. You're shooting it out at screaming velocity instead of just lobbing it casually out the back, so to speak. Of course, as said above, you are still limited by the amount of energy your engine can handle at once. So if your finite amount of energy goes into a very small amount of mass, you only have a very small amount of thrust (again, rockets are all about the mass they expel). The beautiful part is, energy weighs nothing*, but propellant does. The ion thruster is simply the result of a human saying "what if I can trade using less heavy fuel for using more weightless energy". So merely by throwing gobs of energy at the problem, the ion thruster gets away with taking much longer to go through its fuel. 4200 seconds - the time it takes an ion thruster to go through 1 kg of propellant while producing 1 N of thrust. It burns more than five times as long as a LV-N would, at the same thrust, with the same amount of fuel. Absolute thrust is lower, unavoidably - but so long as you have time to make that slow, efficient burn, you're going to be accelerating at the same rate for more than five times as long, making your spaceship that much faster in the end and being able to go that much more distance.

- Ion engines do have nozzles: their exhaust stream is shaped by a magnetic field. Just because you cannot see it doesn't mean it's not there It works because the ionized gas atoms are charged particles and respond perfectly and predictably to magnetic forces.

* Energy production equipment, however, can get quite heavy...

Edited May 12, 2014 by Streetwind

[jfull]

Edit: NVM, I've found your release thread

Edited May 12, 2014 by jfull

[Yilmas]

- Isp is measured in seconds, which sounds odd at first for a figure that's effectively a fuel efficiency indicator. It works out like this: an engine burns for X seconds at Y thrust using Z propellant. If you have an engine developing 1 N of thrust, and this engine then burns for 800 seconds on 1 kg of propellant, then that engine has 800 seconds of Isp. It is quite literally "my fuel lasts this many seconds long".

First of all, thanks for spending the time clarifying these things for me.

After having read your post, I ventured out into cyberspace looking for more information and equations. And that have brought some weird results.

I found this equation for calculating ISP: F = I * m * g (Thrust(kN) = Isp * mass flow rate(kg/s) * earths gravity(m/s²)).

Using the stats from ksp wiki for the LV-N, I found the ISP to be 900 (60kN*1,53kg/s*9,807m/s²). What did I do wrong or perhaps, what am I forgetting ? (And, if I did it for the atmosphere, the numbers would be even more screwed up.)

OT: This is what I love about these types of games, (KSP, EVE and the like), there is so much more in it than the game.

[Streetwind]

The 1.53 figure you picked off the wiki is "liters per second", which is a funny thing because KSP doesn't actually give the fuel volume in tanks a unit and we don't know if it should be liters or not (likely not, as the numbers would be way too small for the dimensions of the tanks). But regardless of the unit, you put the wrong number into the equation. If you mouse over the 1.53 l/s (it is subtly underlined), you get an alternative rating in the form of 7.64 kg/s. That's the number you should be using.

Also, solving the equation F = I * m * g for I does not equal I = F * m * g. First, you divide by F, getting 1 = (I * m * g) / F. Then, you divide by I, which gives you 1 / I = (m * g) / F. Then you flip the fractions, giving you I = F / (m * g). Inputting the numbers, you get I = 60 / (7.64 * 9.81) = ~0.8.

That's an interesting number, because it's exactly three orders of magnitude off. Are you sure thrust should be in kN? I'm inclined to say it should be in N, because after all, the definition of Isp with the "time to burn 1 kg fuel at 1 N thrust) also uses just N, not kN. It also fits with unit analysis; you resolve N into kg*m/s² and use that to cancel out most of the units from gravity acceleration and mass flow rate, leaving only s as the one remaining unit for Isp.

If we take that, then we get: I = 60,000 / (7.64 * 9.81) = 800.55

That's reasonably accurate, considering we're working with rounded figures.

[Yilmas]

That's reasonably accurate, considering we're working with rounded figures.

Sigh

Wanna know the sad part, I work as a programmer... Must be a sign of sleep deprivation, or something along those lines...

Anyway, thx for the help. Gonna try and find a bed, now that I've been beaten by middle school math...