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Upside-down Aerospike... ...Aerohole?


Astroturf
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I think I mentioned this back in 2013, but there wasn't much science behind the concept.

Basically I propose an inline rocket engine (nozzle). It's an upside-down inline aerospike (only noticed that after functionally modeling it) that let's you stack tanks or other engines directly behind it.

Back in the day IIRC we didn't have radial thrusters yet, so I realize that three years later the utility is a little lost, BUT, I still think it's interesting that it may actually work, and may potentially be used to reduce surface drag. In any case, it would have helped with regular staging in lieu of the abominable asparagus staging that is so common nowadays. This idea was motivated by the concept of aerodynamic skin effects (IDK what they're actually called, I'm just a pretend engineer)

Well anyways, let me share with you my findings, and you can let me know what y'all think:

the nozzle:

6ydyypL.jpg

now to validate this thing a little I ran a couple sims. here's a 2d prototype:

and here's the thing in 3d. although I think the 2d cross section  is more useful.

The boundry conditions here are all the walls at 1atm pressure, and the spigots in the engine shovel 1kg/s of air through the engine. Color is temperature. 

Now in the 2d pic we almost see an airstream that stays under sqrt(x). that would mean that the efficiency on these things (given the right geometry) could be really good! 

What I really didn't expect to find was this low pressure area/hourglass air density around the nozzle. that surprised me, but it's possible that I set up the boundaries wrong. (not a real scientist).

So I was wondering if y'all had any opinions on the concept, or if someone was willing to independently verify this. 

you think this type of engine would be a nice addon for ksp? 

PS: a name for this thing would also be nice, if it doesn't already exist.

Edited by Astroturf
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Surely if that exhaust is actually impacting on a part of the engine you're going to lose a huge amount of efficiency?

I.e. isn't this a bit like firing a second stage engine with the first stage still attached?

Edited by Steel
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16 minutes ago, Steel said:

Surely if that exhaust is actually impacting on a part of the engine you're going to lose a huge amount of efficiency?

I.e. isn't this a bit like firing a second stage engine with the first stage still attached?

Yes and no! With internal reflection (similar to an x-ray telescope) and the interaction with the outside atmosphere, the hope is that there isn't actually that much efficiency lost. 

 

look at the 2d pic: while initially it looks like half of the propellant is going straight out, it surely enough is redirected downward, and some of it even back towards the rocket!

Yes, you could say the rocket is fired while the stage is still attached. But if the stage decoupler handled the rocket exhaust like this, it wouldn't be that big of a deal!

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*Also a pretend engineer*

Although that looks like an efficient design, I think it is mitigated by the fact that your are blasting your lower stage with high-temperature rocket exhaust. It's a really neat idea, nonetheless.

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55 minutes ago, Astroturf said:

Yes and no! With internal reflection (similar to an x-ray telescope) and the interaction with the outside atmosphere, the hope is that there isn't actually that much efficiency lost. 

 

look at the 2d pic: while initially it looks like half of the propellant is going straight out, it surely enough is redirected downward, and some of it even back towards the rocket!

Yes, you could say the rocket is fired while the stage is still attached. But if the stage decoupler handled the rocket exhaust like this, it wouldn't be that big of a deal!

From the look of your 2D picture the exhaust is significantly slower after leaving the side of the rocket than when it initially hits the top if the inverted spike part. That suggests to me that you're losing a lot of the exhaust energy.

Edited by Steel
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Looks like an air-augmented rocket engine. Except with less working mass (that's what the air going in is called, right?), worse Isp and exhaust velocity being reduced by the friction of the lower tank part (+obvious heat from that). Certainly not suited for cryogenic fuel.

I don't mean to criticize the concept, but I'm pretty sure there already were people who thought of this.

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I am not really an expert on fluid dynamics, but that low pressure area looks believable and seems like bad news for the efficiency of the rocket. Although, it might subside dramatically as rocket builds up speed relative to air flow around it, in which case it wouldn't be a big deal.

There are several things you might want to look into.

1) Are the input temperature and velocity reasonable? An engine concept isn't much use if there is no fuel that can possibly generate it, or if it's incompatible with much better fuel options.

2) Is temperature at boundaries sane? As in, can any materials actually withstand it?

3) Once your flow hits the expanding portion of the nozzle, speed of the flow should always be right around the speed of sound. If that isn't the case, consider adjusting geometry.

4) Given that all of the above is tuned, what's the efficiency of this rocket? How does it change with changes in ambient pressure and and air speed? The numbers you're looking for are TWR and ISP, just like you would in the game. TWR is a big hard to estimate without doing a crap load of additional material science footwork, but ISP you should be able to get. Just run a boundary around the whole rocket, closer to the outer boundary, but not quite touching it, and look at momentum in vs momentum out through that boundary. Net will give you net thrust, and knowing that and mass flow of your fuel you can get the ISP.

P.S. See that vortex on the 3D view? That's the result of the low-pressure and that's what's going to generate a ton of drag. When considering efficiency, make sure the boundary you draw includes the entire vortex system. Move outer boundary further out if you have to to accommodate that. Otherwise, your estimates will be unreasonably optimistic. But like I said, this might not be a problem at higher air speeds or lower pressure, and at low speeds and pressure, you might be getting enough air-augmented effect going to set it off.

P.P.S See how flow starts to expand, then contracts back in and hugs the fuselage? That tells me that the expansion isn't quite right. This happens to nozzles designed for vacuum when operated at sea level, for example. And it eats a LOT of efficiency of a rocket. If you look at Space Shuttle nozzle, you'll see how its expansion kind of slows down near the end. that's exactly to avoid this sort of a problem. Aerospikes do, typically, manage to avoid this problem. So I tend to think it's something you can tune out of the design by playing with the size of the opening gap.

Edited by K^2
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a lot of good feedback, thanks guys

i'll get back later to it but for y'alls (and my) references here's a lavalish (eyeballed geometry) nozzle with ~3km/s output (6km/s in chamber) @ 2200K

tObknnG.png animated: https://gfycat.com/SnappyPortlyDegus

 

now this kinda shows you how a dyson fan/vacuum works! breddy cool if I dare say so myself. sorry for potato quality but I'm still new to this stuff.

bottom boundary surface with engine on:

Local Parameter Minimum Maximum Average Bulk Average Surface Area [m^2]
Pressure [Pa] 100281,2453 101497,2783 101321,0811 101270,2163 655,7328423
Density (Fluid) [kg/m^3] 0,074390084 1,205116759 1,160990033 0,554836263 655,7328423
Velocity [m/s] 0 1414,377315 22,15909103 449,8063345 655,7328423
Velocity (X) [m/s] -19,37759152 26,30267251 -0,060125633 -1,216657183 655,7328423
Velocity (Y) [m/s] -1414,12439 4,627844849 -19,86051311 -447,9727883 655,7328423
Velocity (Z) [m/s] -27,89804162 31,35409891 -0,019769064 0,440334467 655,7328423
Mach Number [ ] 0 1,155443032 0,024356665 0,418744205 655,7328423
Temperature (Fluid) [K] 293,1999933 4695,396954 369,594359 1831,531807 655,7328423
Relative Pressure [Pa] -1043,754687 172,2782917 -3,918892942 -54,7836935 655,7328423

 

 

Integral Parameter Value X-component Y-component Z-component Surface Area [m^2]
Mass Flow Rate [kg/s] -969,4566667       655,7328423
Volume Flow Rate [m^3/s] -12895,09482       655,7328423
Surface Area [m^2] 655,7328423 -5,15261E-14 655,7328423 -4,26089E-16 655,7328423
Total Enthalpy Rate [W] -8459255428       655,7328423
Uniformity Index [ ] 0,094985986       655,7328423
Area (Fluid) [m^2] 655,9346166       655,9346166

 

bottom boundary with engine off:

Local Parameter Minimum Maximum Average Bulk Average Surface Area [m^2]
Pressure [Pa] 101325 101325 101325 101325 655,7328423
Density (Fluid) [kg/m^3] 0,609022284 1,203010699 1,179019768 0,92837813 655,7328423
Velocity [m/s] 0 0,017849372 0,00041043 0,006948162 655,7328423
Velocity (X) [m/s] -0,00051274 0,000203903 -3,94771E-06 -8,27855E-05 655,7328423
Velocity (Y) [m/s] -0,01784011 6,24284E-05 -0,00034236 -0,006933303 655,7328423
Velocity (Z) [m/s] -0,000338888 0,000133815 -2,05226E-06 -3,11429E-05 655,7328423
Mach Number [ ] 0 3,72843E-05 1,01455E-06 1,57724E-05 655,7328423
Temperature (Fluid) [K] 293,2 579,49684 300,0733601 403,0617121 655,7328423
Relative Pressure [Pa] -2,35741E-09 8,0734E-08 -6,43164E-10 -1,44566E-10 655,7328423

 

 

Integral Parameter Value X-component Y-component Z-component Surface Area [m^2]
Mass Flow Rate [kg/s] -0,182005847       655,7328423
Volume Flow Rate [m^3/s] -0,224813941       655,7328423
Surface Area [m^2] 655,7328423 -5,15261E-14 655,7328423 -4,26089E-16 655,7328423
Total Enthalpy Rate [W] -80684,23777       655,7328423
Uniformity Index [ ] 0,204215928       655,7328423
Area (Fluid) [m^2] 655,9346166       655,9346166

-----

Now question about methodology:

to calculate efficiency of the rocket, I would multiply Mass Flow Rate [kg/s] * average Velocity (Y) [m/s] to get a force in Newtons.

I'll divide the value from the other nozzle by the laval nozzle value to get an efficiency factor. 

does that sound sound?

-19.86051311 kg/s *-969.4566667 m/s ≈ 1.1 × estimated force of a bite from a great white shark (Carcharodon carcharias) (≈ 18 kN ) (19.2539068 kilonewtons)

no it's not. I'll try and see if I can get an integral of air momentum or something.

-----

couldn't find it, went ahead and remodeled the thing anyways with #realistic boundry conditions.

initially, the solver failed so I ramped up the mesh resolution. 

Initially I assumed that the eddies at the edge were just artifacts because of sharp corners in the boundry. But aaa... ... IDK what is going on. these actually gigantic torrents are taking up a huge chunk of the airflow, and everything is just messed up. But I'm glad the exhaust sticks to the body. Buuuut, that doesn't mean a whole lot:

@Steel you might find this one interesting:

(these last two were done under the exact identical conditions.) It almost looks like the airflow here is better...

sigh... this is hard...

And I should work on my actual thesis which has absolutely nothing to do with this :V

Edited by Astroturf
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Ok, I stand corrected! Putting my more constructive hat on, have you considered trying to model it with non-zero boundary velocities to see how it behaves then? i.e to simulate the rocket moving through atmosphere

Edited by Steel
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This is really cool.  I agreee with the other posts here regarding exhaust gas energy.  I would like to see what you come up with as you keep working on your idea, though. 

I seem to remember something similar to this as part of a supercavitating torpedo.  Obviously it was designed for different pressures and fluids.  Let me see if I can't find what I am looking for.

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10 hours ago, Steel said:

Ok, I stand corrected! Putting my more constructive hat on, have you considered trying to model it with non-zero boundary velocities to see how it behaves then? i.e to simulate the rocket moving through atmosphere

( @K^2 , reason for not producing your results ) I have terrible version control and I wrecked one of my models. I'll have to get back to this later, sadly. :( I'll try to use a more systematic approach (I'm not an engineer! :o )  

2 hours ago, Jonfliesgoats said:

Here it is.  This popscience article talks about a gas ejector using a tank of stored gas.  In reality, I think some of the rocket exhaust is tapped or a separate rocket motor is used as the cavitator on the nose of the torpedo.  It looks a lot like your idea.

http://www.popsci.com/scitech/article/2004-06/supercavitating-torpedo

I remember that. this might be useful if it turns out that a multistage approach may be better (eddies caused due to sudden change in diameter, ie, the exhaust makes it unaerodynamic)

48 minutes ago, Veeltch said:

Why blast the lower fuel tank with exhaust though? Wouldn't a plug nozzle or an air-augmented nozzle be better? Why not try aerospike+air augmentation combo and see how that works?

well, a) it's no fun if it exists already, and b) the intent with this is that you could hypothetically make an infinitely long rocket, maybe even bootstrap a space elevator by having a bazillion stages that fire simultaneously. unless I'm misunderstanding you. could you provide a drawing on how to attach the lower stages to your breathing shroud?

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Yeah.  In airplanes active boundary layer control was experimented with the control drag by blowing gasses at specific locations to energize lower layers of flow and prevent flow separation.  

Still not sure about exhaust gas velocity, which is important though.  Why do you want to have a rocket motor thrusting on top of or in the middle of your rocket?  What's the application you are considering?

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