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Hydrazine Monopropellant Reciprocating Engine Development and Game Balance


USB4

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I decided to get into modding, and decided to make some parts I wanted that didnt exist. 

One such part was an engine that did not need oxyg.en, but also used air to gain far greater efficiency on planets with atmospheres but no oxygen. Yes, electric engines exist, and there are some decent ones out there, however due to the weirdness of electricity in ksp, I wanted to make something without that. After searching for a while, what I discovered and later modeled for ksp

inY87Pq.png

which was this, a mono propellant powered single cylinder piston engine.

 

b0Y8y7s.png

 

Why Im making this post, is Im kind of lost in terms of how to balance this in KSP with the model ive made being a 0.625m 2 cylinder version of this.

 

For power, I think my model looks just about the size of a cessna engine and the description here makes me think it wouldnt be far from a regular prop, so Ive put the power at 15kn stationary dropping to almost nothing by about 0.5 Mach. 

 

For efficiency however, I have no clue what my curve should look like  and what the isp should be so I was hoping someone who could make sense of the values in that pdf might be able to help me figure out a relatively reasonable balance.

Edited by USB4
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Low pressure in an engine of that would be 200 PSI or so and outlet pressure above 14 PSI in the atmosphere (deepending of exhaust manifold design and the size of the exhaust valve) .. .. . . .It would not change much until you got into really dense atmospheres.

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For balance within KSP, I'd put its Isp relative to other engines. A turbofan produces most of its thrust from its fan, and a fan is basically just a propellor.

For the same amount of energy, one can accelerate 4x the mass of air at 1/2 the velocity, for double the thrust. So pushing more air, at a slower speed = higher effective specific impulse.

A prop like that should get a higher specific impulse than a turbofan, all else being equal, so I'd say assume 1.5x the turbofan standard for an air breathing LF fueled piston engine.

Now IRL, the oxidizer to fuel ratio for hydrocarbon fuels is more like 2.6:1, in KSP its 1.1 : 0.9 so the 10500 Isp of the turbofan would be 10500* (0.9/2) = 4,725 . These are the sort of calculations that I use in my mods for "air augmented rockets" and "turborockets". So for a piston prop using LF+Ox I'd say...  7087 Isp. This one uses monoprop instead of LF+Ox, so that should be less energetic per unit mass. I'd use Isp ratios of Monoprop vs comparable LF+Ox engines to balance that. The Puff gets 250 Isp, LF+Ox engines get around 320 (more or less depending on engines from 290 to 350 vacuum Isp.)

So I'd say 7087* 250/320 = 5,537  then I'd round it down to 5,500 Isp.

Anyway... that's how I'd do it with a lot of guestimating

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15 minutes ago, KerikBalm said:

For balance within KSP, I'd put its Isp relative to other engines. A turbofan produces most of its thrust from its fan, and a fan is basically just a propellor.

But Turbofans are quite a bit less efficient than props arent they. a quick google says not terribly so, but somewhere between 10-50% ish (very quick google)

15 minutes ago, KerikBalm said:

For the same amount of energy, one can accelerate 4x the mass of air at 1/2 the velocity, for double the thrust. So pushing more air, at a slower speed = higher effective specific impulse.

A prop like that should get a higher specific impulse than a turbofan, all else being equal, so I'd say assume 1.5x the turbofan standard for an air breathing LF fueled piston engine.

Now IRL, the oxidizer to fuel ratio for hydrocarbon fuels is more like 2.6:1, in KSP its 1.1 : 0.9 so the 10500 Isp of the turbofan would be 10500* (0.9/2) = 4,725 . These are the sort of calculations that I use in my mods for "air augmented rockets" and "turborockets". So for a piston prop using LF+Ox I'd say...  7087 Isp. This one uses monoprop instead of LF+Ox, so that should be less energetic per unit mass. I'd use Isp ratios of Monoprop vs comparable LF+Ox engines to balance that. The Puff gets 250 Isp, LF+Ox engines get around 320 (more or less depending on engines from 290 to 350 vacuum Isp.)

So I'd say 7087* 250/320 = 5,537  then I'd round it down to 5,500 Isp.

Anyway... that's how I'd do it with a lot of guestimating

Seems pretty reasonable for game balance I think.

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4 hours ago, KerikBalm said:

For the same amount of energy, one can accelerate 4x the mass of air at 1/2 the velocity, for double the thrust. So pushing more air, at a slower speed = higher effective specific impulse.

Most of the economy in Jet aircraft is not in the engine efficiency, but the ability to fly in air that is 1/3 to 1/5th as thin as air on the surface just below 275 m/s

If you are just going to be using the plane to scout along the surface, model the cub, thats all you need.

 

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28 minutes ago, PB666 said:

Most of the economy in Jet aircraft is not in the engine efficiency,

In real life? In KSP? Relative to rockets? Relative to Props?

In real life, and sort of in KSP, relative to rockets, the economy of the engine comes from 2 factors.

1) is that they don't need to carry oxidizer, so 2.6 out of 3.6 units of combined Fuel+Ox mass are gathered from the atmosphere. In KSP this would be 1.1 of 2... I guess.

2) The 2nd thing that increases efficiency is the use of air as reaction mass. An SR-71 burns  what is basically just Kerosene (from the energetic and fuel to ox ratio perspective). If we take a Kerlox rocket, and "magically" spawn lO2 in the combustion chamber, we'd get an increase in Isp of ~3.6. This would take a rocket that gets ~330 Isp and it would increase to ~1,180 in a vacuum... much less in the atmosphere, yet the SR-71 manages about double that in the atmosphere, because of inert air that gets heated, expands, and is expelled for a higher mass flow at lower velocities. Its a big effect, but not as big as #1, particularly if the fuel being used is liquid hydrogen (because then the fuel to oxidizer ratio is more like 9:1)

Anyway, those are the causes of extra engine efficiency, and for the purposes here, it is engine efficiency that we care about for scouting in the lower atmosphere. Its also what we care about for SSTOs and long distance high altitude (but suborbital flight) in KSP. There's a reason that rocket only SSTOs manage barely above a 10% fraction, and airbreathers can get over 50% (when increasing kerbin's size to 3x, these figures drop to something like 0.1% and 13%, at least as far as my designs have managed).

IRL these increases in engine efficiency result in about 5-10x the Isp of a conventional rocket for hydrocarbon fuels, and an even better for Lh2 fuel, which is why the Sabre engine in theory could enable something like the skylon.

Those sort of fuel economy increases are not to be ignored.

Edited by KerikBalm
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1 minute ago, KerikBalm said:

In real life? In KSP? Relative to rockets? Relative to Props?

In real life, and sort of in KSP, relative to rockets, the economy of the engine comes from 2 factors.

1) is that they don't need to carry oxidizer, so 2.6 out of 3.6 units of combined Fuel+Ox mass are gathered from the atmosphere. In KSP this would be 1.1 of 2... I guess.

2) The 2nd thing that increases efficiency is the use of air as reaction mass. An SR-71 burns  what is basically just Kerosene (from the energetic and fuel to ox ratio perspective). If we take a Kerlox rocket, and "magically" spawn lO2 in the combustion chamber, we'd get an increase in Isp of ~3.6. This would take a rocket that gets ~330 Isp and it would increase to ~1,180 in a vacuum... much less in the atmosphere, yet the SR-71 manages about double that in the atmosphere, because of inert air that gets heated, expands, and is expelled for a higher mass flow at lower velocities. Its a big effect, but not as big as #1, particularly if the fuel being used is liquid hydrogen (because then the fuel to oxidizer ratio is more like 9:1)

Anyway, those are the causes of extra engine efficiency, and for the purposes here, it is engine efficiency that we care about for scouting in the lower atmosphere. Its also what we care about for SSTOs and long distance high altitude (but suborbital flight) in KSP. There's a reason that rocket only SSTOs manage barely above a 10% fraction, and airbreathers can get over 50% (when increasing kerbin's size to 3x, these figures drop to something like 0.1% and 13%, at least as far as my designs have managed).

IRL these increases in engine efficiency result in about 5-10x the Isp of a conventional rocket for hydrocarbon fuels, and an even better for Lh2 fuel, which is why the Sabre engine in theory could enable something like the skylon.

Those sort of fuel economy increases are not to be ignored.

Just in comparison of piston propelled versus turbine propelled.

But if you want an economic analysis Airlines make money flying people, the more people they can put on one craft and take between two far off points the more profit they can make. From that perspective you talk about wing loading, If you were to fly at 125 MPH you would need pretty big wings, if you fly at 250 you can reduce the wing size by a quarter, but you need to increase the power output and lower the efficiency, but you can compensate for that by moving at 250 IAS where GS = 400 kts. In addition Jet engines work most efficiently in cold air, but thrust falls, so you can push the speed into coffin's corner.

You cannot simply look at ISP, you have to look at all the parameters. For the OPs design, I would posit that the cub is the craft he wants to model, it makes no sense to fly at 16km if you are doing ground observations, 3km will do.

 

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2 hours ago, PB666 said:

For the OPs design, I would posit that the cub is the craft he wants to model, it makes no sense to fly at 16km if you are doing ground observations, 3km will do.

I didn't get the sense that this is what he wants to model. I got the sense that he wants "an engine that did not need oxygen, but also used air to gain far greater efficiency on planets with atmospheres but no oxygen"

Maybe he wants it for a biome hopper on Eve. Maybe he wants it to ferry kerbals from a mountaintop (where an ascent vehicle waits) to a shoreline for science. Maybe he wants it to easter egg hunt in a certain areas.

As for real world airliners... I don't see how its relevant to this game, since those sort of things completely fall out of the economic model of KSP.

He asked "For efficiency however, I have no clue what my curve should look like  and what the isp should be" so I don't really understand why you bring up these other things.

 

So... back to the OP, the document states the SFC would be about 0.99 kg of monopropellent per MJ of output power of the engine.

So... how much thrust that prop can generate with an engine power of 1 MJ depends on a lot of factors.

1 MJ is enough to accelerate 200 kg of air to 100 m/s each second, which would provide about 20kN of thrust, and would yield a specific impulse of about 2,000 (consider 1 kg of propellant in a NERVA expels 1 kg of propellant at about 8,000 m/s for 8 kN)

That is of course assuming 100% efficiency of the prop, although with prop blades generating a reasonaby high L/D (lets say 10:1 or better), I don't see why this wouldn't be close to a good value).

* of course, its not a 1 megawatt engine, its an 11.2 kW engine, so I'd guess under those assumptions its more like a 0.2 kN thrust.

The slower it moves the air, the better its Isp can be, but keep in mind, there has to be enough air there to move... so you'd need a huge prop diameter on Duna to move any significant air mass, for instance

Edited by KerikBalm
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8 hours ago, KerikBalm said:

That is of course assuming 100% efficiency of the prop, although with prop blades generating a reasonaby high L/D (lets say 10:1 or better), I don't see why this wouldn't be close to a good value).
 

 

Not even close to 100% efficiency, your talking about a prototype engine. You can look at the efficiencies of all prototypical engines. This is what I am saying the cub was 1930s powerplant and could fly on 40 HP. Even there its a powerplant that would be more like the efficiency of a two stroke engine given the energy released relative to an O2 burn. Its not a 787 by any means, but OTOH 80% of WWII pilots trained on the little buggers. As far as model airplanes are concerned you also have to deal with form drag, if the boundary layer your craft is a measurable proportion of the crafts diameter you are paying a heavy drag cost. Its better to flap your wings like an insect or bird than have a prop. :P

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4 hours ago, MatterBeam said:

For 150m altitude, a 2-blade 30.5cm (12x7) wide propeller at 15000 RPM produces 7kN of thrust while consuming 11.2kW of power.

The RPM for a reciprocating engine is going to be under 3,000. The aforementioned 152 will max out at about 2,500. In practice, it means you'll fly with larger prop, with smaller pitch, and lower max speed.

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For the purposes of game balance, I think @KerikBalm 's answer should work fine. The efficiency and performance don't match the jet engines, so no worry about them supplanting jets in air breathing roles. Despite that limitation, they offer new opportunities for exploration of atmospheric planets.

For the thrust curve, I'd say that depends on whether you want it to model a fixed blade or variable pitch prop. If it's treated as variable pitch, you can extend the usable airspeed out to nearly Mach 1. The thrust would remain mostly level through the envelope, then fall off dramatically approaching Mach 1. The density/ Isp curve would be linear.

Best,
-Slashy

Edited by GoSlash27
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13 hours ago, PB666 said:

Not even close to 100% efficiency, your talking about a prototype engine. You can look at the efficiencies of all prototypical engines. This is what I am saying the cub was 1930s powerplant and could fly on 40 HP. Even there its a powerplant that would be more like the efficiency of a two stroke engine given the energy released relative to an O2 burn.

Prop efficiency != engine efficiency. The engine efficiency was already given in the .pdf file (in terms of kg of fuel consumed to actual power output). The L/D of the prop blades should determine what percent of the power output of the engine actually is used to accelerate air rearward, and that was the efficiency that I was referring to. A L/D of even 5:1 still gives 83.3% efficiency at this step, and 5:1 is pretty easy to get.

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Max Power = Max Torque * Max RPM. Max torque is a function of Engine volume. 100 Nm per liter is a good estimate. So the larger the volume the lower the Maximum RPM since power is limited by the capacity of the engine to dissipate heat and pressure. maximum head speed should not exceed 25 m/s per ring friction and inertial pressure on the conn-rod end.

 

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Propeller efficiency for a variable pitch prop can be approximated as about 80% out to around Mach 0.8, where things start getting funny with wave drag.

I searched around a bit and found this with google. The theory is beyond my realm of understanding, but it features the propeller efficiency (η) of a Cessna 172's prop plotted against its advance ratio (J).

e1low.gif (12691 bytes)

Even if you were to model your prop based on this, you'd still need to find static thrust, which I have no idea how to calculate.

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4 hours ago, Silavite said:

Propeller efficiency for a variable pitch prop can be approximated as about 80% out to around Mach 0.8, where things start getting funny with wave drag.

I searched around a bit and found this with google. The theory is beyond my realm of understanding, but it features the propeller efficiency (η) of a Cessna 172's prop plotted against its advance ratio (J).

e1low.gif (12691 bytes)

Even if you were to model your prop based on this, you'd still need to find static thrust, which I have no idea how to calculate.

 

The aspect needs to change reflecting the need for thrust versus the airspeed.

Therefore the range of Advance ratio needs to be between 20 (Ground) and 80 percent (High speed). The efficiency to get those ratios at a given speed will be  determined by the propeller pitch, RPM and airspeed.
We have to remember that thrust off a propeller is like lift off a wing, if the AoA (see image below) is too high or too low the propeller will fail to generate thrust. As airspeed increases the pitch needs to be adjusted so as to produce an optimal AoA to the wind, not the wind the aircraft experiences, but the composite vector of windspeed and moment of motion of the propeller.  At low speed you don't need alot of efficiency you want to convert power to thrust and there is alot of engine to spar since drag is near zero the wheels are doing the lifting, consequently the propeller needs to transfer alot of mass that is not moving particularly fast in front of the blade but increases speed as it crosses the blade (essentially creating a low pressure in front of the blade). At high speed the pressure builds in front of the blade, but you need to move a larger volume of air (but because you are traveling at high altitude a lower mass of air per volume) the pitch of the blade increases due to the speed of the air.

blade relative speed = SQRT(AS^2 + (wr)^2) along arctan (As/(wr)) relative to the axis of rotation. If the blade speed is held constant and AS increase the blade relative angle of the wind increases. The force vector generated by the blade is the coefficient of lift * the area * the relative speed, however the lift vector is not always point the same direction. The optimal angle of attach is less than 17 degrees relative to arctan(As/wr) and more than 8 degrees; direction vector of the blade relative speed is typical in the teens at the start of the flight, thus the pitch is in the degree 20s. As AS increases the direction vector of the blade increases, and thus the pitch also needs to increase. Remember that energy is force over distance and power is energy/time. So over a unit time we want force applied with as large a volume passing between the blades as possible. Its the blades travel through the wind per unit time as well as the pitch of the blade so that while it appears that AoA is everything, if you can get higher blade travel relative to the columm of air moving across the propeller, then AoA needs to come down, it will create less drag anyway.  That blade needs to advance to increase that volume up to the point where the motion becomes turbulent, then the blades are turning the air and not pushing it around in the blades plane of rotation.

450px-Lift_curve.svg.png

To prevent this from happening you need to keep the blade below a certain RPM and the Angle of Attack (relative to blade relative flow direction vector) in the linear range (away from the top of the curve). Turbulence around the blade and vibration one of the most feared enemies of high flying propeller type aircraft and so the closer to mach the more AoA and other factors (RPM, engine imbalances, firing rotation, etc) add up. Since the blade is always at an AoA to the wind, except when feathered, the blade tip will always encounter the Mach effects before the blade relative airspeed reaches Mach speed. Blade tips, wing tips and other peripheral structures of aircraft do not like Mach Effects.

How does this differ with Turbine, in turbines the air flow is pressurized and slowed down, because the air is moving slower and under pressure the Mach effects are postponed to much higher speeds, finally after pressurization the air is fed out a restrictive outlet increasing its speed markedly and creating a reaction momentum capable of moving the craft closer to the speed of sound or beyond.

Now, one way to think about the curve above is to note at the highest charted AoA above change to non-linearity, that change is something most pilots don't think about and want to avoid, but what it is, is one of the most effective airbrakes on the aircraft, that deviation is pretty much drag friction. Inertia converted to vertical lift is returned when the aircraft falls, the horizontal component of lift vector is deceleration as an airfoil AoA tilts up past 13 degrees both drag and lift in the plane of motion increase, this can be used with great caution to rapidly slow a moving aircraft to a speed such that as long as the aircrafts vertical motion is negative (otherwise a full stall will occur). And it surprising that you can actually put an aircraft going down into a semi-stall, and it will actually slow down and fall faster. Its one of those situations where the set of airflow aprioris are more determinant than the average analysis would predict, it works well for fast moving aircraft bacause there is alot of energy to be transformed by slowing down and speeding up wind. 

One of the tricks I used to do on FS is to slow a bit roll up an aircrafts nose to just about 15 and roll very slowly up' and wait for the vertical motion to lay the aircrafts VS to negative, then begin to drop the nose as ascent angle decreases as the ascent angle increases you can get closer to a full stall (its impossible to be in a stall with the nose pointing strait down while the plane is falling so the closer you are to that fall vector the easier it is to pull out of a stall, but the faster you will need to do so), at some point you basically are in a controlled fall and throw out flaps as IAS falls, gear, spoilers .  . . .its more or less like an albatross trying to land. But once you are a certain elevation, say 3000 feet, spoilers come in nose drops speed increases lift increases VS stabilize. No need for ILS on this approach you can pick your picnic spot on the runway waiting for the MGH to burn off. If it were a real plane I can only imagine the puke flying around in the passenger compartment. On a typical jet aircraft you can increase the loss of inertial energy by 10 fold over a normal descent.

But the point is that unless your are planning to exploit stall turbulence, its best to stay clear of it,  alot of power lost in the stall.

 

 

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I'm actually liking the idea of monoprop powered atmospheric engines. It would finally give us a use for those big monoprop tanks. I'm thinking that I may change my ramrocket/airaugmented rocket/turborocket "mods" to run on monoprop instead of hydrazine.

Spoiler

They aren't much of a mod, just a duplication of a stock part, text editing the part .cfg file, and changing the texture color.

Ramrockets on 3x Duna:

MMRgmMq.png

Ramrockets, electric fans for the atmosphere, electric props for water, and turborockets (the panther model)... I toned down the color on the blue and green parts:

LPX4hJA.png

I also had "Rocket-fans", which I haven't actually heard real proposals for, but I don't see why they wouldn't work if turborockets could work.

Kx9sDss.png

I think a prop would be better, maybe I'll use this mod

 

 

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On 12/28/2017 at 6:54 AM, GoSlash27 said:

For the purposes of game balance, I think @KerikBalm 's answer should work fine. The efficiency and performance don't match the jet engines, so no worry about them supplanting jets in air breathing roles. Despite that limitation, they offer new opportunities for exploration of atmospheric planets.

The elephant in the room is monoprollant vs. air breathers.  KSP gives kerolox rockets and Isp of 250 while using the same fuels with free air and jet engines have an Isp of 6k-12k.

I also suspect that such a system would be "early" on the tech tree and that later kerbals would be using fuel cells (or nukes) and electric motors for non-oxidizing atmospheric flight.

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the only real world monopropellant piston engines i know of are torpedo motors

https://www.youtube.com/watch?v=eRCUqcwqu5w

they are external combustion axial piston engines. the monopropellant is burned in a combustion chamber and the gasses are routed through the engine. the motor itself is really more of a high speed, high torque compressed air motor. only the compressed air is the high pressure combustion products rather than stored air.

on a side note, the fuel (and exhaust) for these things is not something you want to have anywhere near anything biological. nasty stuff.

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8 hours ago, wumpus said:

The elephant in the room is monoprollant vs. air breathers.  KSP gives kerolox rockets and Isp of 250 while using the same fuels with free air and jet engines have an Isp of 6k-12k.

Wumpus,
 I'm not seeing how this is a problem. Could you elaborate?

8 hours ago, wumpus said:

I also suspect that such a system would be "early" on the tech tree and that later kerbals would be using fuel cells (or nukes) and electric motors for non-oxidizing atmospheric flight.

The OP's original intention was to avoid electric props due to potential game balance issues. Hence the monoprop.

Best,
-Slashy

 

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