For Questions That Don't Merit Their Own Thread

[DerekL1963]

2 hours ago, NoobTool said:

Kinda spit-balling here, I've seen hybrid solid/liquid fuel rocket engines using a solid fuel and a liquid oxidizer, but what about using a liquid fuel and a solid oxidizer? This could conceivably allow for jet and rocket engines to use the same fuel on-board the same craft. Is there anything that immediately jumps out that would preclude the viability of such a set-up?

AFAIK, there is no solid oxidizer.  There are mixtures that burn oxygen rich (that is, their combustion products are enriched with oxygen) though.

[NoobTool]

@DerekL1963

I'm not aware of any either, I sort of presumed their existence based on solid fuel boosters containing both fuel and oxidizer.

[NoobTool]

@DerekL1963

Actually, it appears that there are plenty of solid oxidizer options:

Composites typically consist of a mixture of granules of solid oxidizer (examples: ammonium nitrate, ammonium perchlorate, potassium nitrate) in a polymer binder (binding agent) with flakes or powders of: energetic compounds (examples: RDX, HMX), metallic additives (examples: Aluminium, Beryllium), plasticizers, stabilizers, and/or burn rate modifiers (iron oxide, copper oxide). 

(From Wikipedia)

It appears that this question might warrant a bit of discussion, maybe I shouldn't have posted it in this thread.

[K^2]

The biggest problem with solid oxidizers that I'm aware of is that all of the good solid oxidizer options I know are brittle. This is why solid rocket boosters have granules of oxidizer in poly matrix. Hybrid rockets tend to have a very similar matrix, but with no oxidizer embedded. This still works, because you still have a material resistant to cracking. But if you were to take a typical oxidizer and try to build a hybrid with a liquid fuel, under typical rocket engine mechanical stress, the solid portion is going to develop cracks, at which point, the whole thing goes kablamo.

In contrast, everything you'd use as a matrix to resolve this is either a fuel or wasted weight. Making a hybrid with oxidizer in poly matrix would probably work, but then it's going to combust even if you turn off the fuel flow, which defeats the purpose. Even just throttling back a little is going to cost you performance. At this point, you might as well just go pure solid.

On the other hand, non-fuel matrix would be extra weight you have to carry. I'm pretty sure you could make a silicone matrix solid oxy that you could pipe fuel through and get combustion. It would technically be a solid oxy hybrid. But I_SP is going to be absolutely terrible because of how much extra weight you are carrying that you aren't using as either fuel or oxidizer.

[shynung]

@K^2 That, or the matrix that has the desired properties have other properties that are undesirable, e.g. Teflon/PTFE can be used as a matrix, but the resulting rocket motor spits out fluorine compounds.

[K^2]

With Teflon, I don't even know if you'd qualify it as a fuel or oxidizer. I think it's going to depend on what you use as your liquid component. But good point. I was thinking of substances that are typically used as an oxidizer, while there are plenty of options that are simply too unstable or too hazardous to be considered.

I think, by now, I can mention Sand Won't Save You This Time without linking to it. Not that it'd work as solid in a hybrid, just as illustration to things that would make a good oxidizer, but just no.

[shynung]

I think Teflon is somewhere in between, as it has carbon (fuel) and fluorine (oxidizer) in it. Leaning towards oxidizer, because having light molecules in the exhaust stream would help Isp, and HF is lighter than CO or H2O. Yes, there's the fact that HF is terribly corrosive and toxic (especially if it touches any hint of water), but if you don't care about what's left of the motor, or anything around it, it'll work.

And yes, I am familiar with the story you mentioned. I've also read Ignition! several times over. Goes to show how much some people are willing to go in search of the ultimate rocket propellant.

[Wjolcz]

What are retrograde orbits good for? And I mean for spacecraft. Moons have a tendency of being torn apart by tidal forces.

[Kryten]

 Radar EO sats tend to be put into retrograde orbits as it increases the Doppler shift from ground objects, and most sun-synchronous orbits used by optical EO sats happen to be slightly retrograde.

[RainDreamer]

What is the lowest speed possible to achieve a single orbit around earth?

[peadar1987]

5 hours ago, RainDreamer said:

What is the lowest speed possible to achieve a single orbit around earth?

Depends how much you want to cheat...

If you start at the very edge of the Earth's SOI (well, its Hill Sphere) which extends to 0.01AU from the earth, more or less, and just kiss the edge of the atmosphere at perigee, you only need to start off at 24 m/s: http://www.wolframalpha.com/input/?i=SQRT(Gravitational+Constant+*+Mass+of+the+Earth+*((3250km%2F(0.01AU*(0.01AU%2B3250km))))

By the time you hit perigee, at 100km, you will now be doing 11.1km/s, and it will have taken you nearly 38 days to get there from apogee. You'll lose a little bit of speed when you hit the upper atmosphere, and the orbit certainly won't be stable over any sort of long timescale, but you'll get your one orbit out of it.

 

[RainDreamer]

 

3 hours ago, peadar1987 said:

Depends how much you want to cheat...

If you start at the very edge of the Earth's SOI (well, its Hill Sphere) which extends to 0.01AU from the earth, more or less, and just kiss the edge of the atmosphere at perigee, you only need to start off at 24 m/s: http://www.wolframalpha.com/input/?i=SQRT(Gravitational+Constant+*+Mass+of+the+Earth+*((3250km%2F(0.01AU*(0.01AU%2B3250km))))

By the time you hit perigee, at 100km, you will now be doing 11.1km/s, and it will have taken you nearly 38 days to get there from apogee. You'll lose a little bit of speed when you hit the upper atmosphere, and the orbit certainly won't be stable over any sort of long timescale, but you'll get your one orbit out of it.

 

Alright thanks, so technically I wasn't making a mistake in one of my poem, saying that things stay in orbit with just Mach 7, lol. Sometimes making scientifically accurate poems are hard.

[K^2]

The actual problem with that statement is referring to the Mach number in vacuum. It depends on atmospheric pressure. But there are certainly stable orbits around Earth with orbital speed equal to Mach 7 at sea level under standard conditions.

[Green Baron]

On 27.10.2016 at 2:07 PM, RainDreamer said:

 

Alright thanks, so technically I wasn't making a mistake in one of my poem, saying that things stay in orbit with just Mach 7, lol. Sometimes making scientifically accurate poems are hard.

Earth has no SOI, SOI is a game concept to keep calculations easy.

As K^2 said, for a Mach number you need a medium that transports sound at a certain speed. No medium (vacuum), no speed of sound. Mach number becomes a division by 0 sotosay. Calculating the Mach Number from speed of sound and true air speed is not as trivial as one might think.

The earth atmosphere has no abrupt border, it just gets thinner until the breaking force can be neglected (which is pretty high, far beyond low earth orbit). So in principle nothing stays in orbit forever or it must be very high. Even the ISS in 400km has to be lifted every few weeks.

The question how fast must i go (in a certain altitude) so that i get around once before atmospheric drag slows me too much to stay above a certain altitude then maybe Space Shuttle documentation could help, they had an abort modus "Abort once around". That should state the conditions somewhere.

Cheers

 

[peadar1987]

2 hours ago, Green Baron said:

Earth has no SOI, SOI is a game concept to keep calculations easy.

 

 

True, but it does have a Hill Sphere in which it is gravitationally dominant, which acts quite a lot like an SOI over short timescales, and away from the edge

[TheDestroyer111]

Why do many helicopters have their tail rotors above center of mass? This could explain why it is so hard to hover a typical helicopter, but it seems like nothing can explain why the rotor is placed more upwards.

Edited October 29, 2016 by TheDestroyer111

[shynung]

@TheDestroyer111 I'm guessing it's to reduce tail-rotor-groundstrike incidents.

[Green Baron]

I guess the same :-)

I never flew helicopters, but from what i heard It's "difficult" to hover a helicopter stationary just above ground because it partly "floats" on compressed air. To illustrate this, imagine there is only one point on top where it is stable and the pilot has to counteract any tendencies to "slide off" from this point.

Furthermore, no part must touch the ground when the helicopter floats. When the gear touches the ground and is held (e. g. by gras) while the helicopter floats (moves) the gyro forces will cause it to flip.

 

[TheDestroyer111]

5 hours ago, Green Baron said:

I guess the same :-)

I never flew helicopters, but from what i heard It's "difficult" to hover a helicopter stationary just above ground because it partly "floats" on compressed air. To illustrate this, imagine there is only one point on top where it is stable and the pilot has to counteract any tendencies to "slide off" from this point.

Furthermore, no part must touch the ground when the helicopter floats. When the gear touches the ground and is held (e. g. by gras) while the helicopter floats (moves) the gyro forces will cause it to flip.

 

Actually it's easier to hover near the ground due to ground effect - if any part of the rotor gets closer to the ground, it generates more lift, and thus stops going down. But hovering out of ground effect is less so (at least for conventional main+tail rotor helicopters)

And for ground strike incidents, this could be true, but it doesn't just sound enough satisfactory - for some helicopters the rotor could be moved downwards a bit without causing more ground strike incidents, while at the same time significantly lowering the probability of the main rotor hitting the tail rotor (which is more common and dangerous).

[shynung]

1 hour ago, TheDestroyer111 said:

And for ground strike incidents, this could be true, but it doesn't just sound enough satisfactory - for some helicopters the rotor could be moved downwards a bit without causing more ground strike incidents, while at the same time significantly lowering the probability of the main rotor hitting the tail rotor (which is more common and dangerous).

The same goal can be achieved by lengthening the boom so that the tail and main rotors never intersect each other's path. Makes for a bulkier airframe, though.

[Green Baron]

4 hours ago, TheDestroyer111 said:

Actually it's easier to hover near the ground due to ground effect - if any part of the rotor gets closer to the ground, it generates more lift, and thus stops going down. But hovering out of ground effect is less so (at least for conventional main+tail rotor helicopters)

And for ground strike incidents, this could be true, but it doesn't just sound enough satisfactory - for some helicopters the rotor could be moved downwards a bit without causing more ground strike incidents, while at the same time significantly lowering the probability of the main rotor hitting the tail rotor (which is more common and dangerous).

It needs less energy, but it's more difficult to hold the helicopter on the spot near the ground. The compressed air cushion is like a capsized bowl and the helicopter on top. Once it starts to drift it will continue until counteracted. And since the system has some inertia when using cyclic the pilot must anticipate the movements in order to keep it stable on the spot. Once it starts to flip it can't be stopped, torque forces are by far stronger than ground effects. See accident reports by usa-en ntsb or german lba (bfu), very thrilling reading :-)

But again, i never flew helicopters myself.

I would think that the engineers weighed all possible consequences of tail rotor positions and did it right, complying with physics and construction demands, environment (sand that abrades the surface, cold that influences gears etc.), safety and whatever applies. As far as i know the main rotor of any single rotor helicopter cannot hit the tail rotor, construction has to insure this (needs verification). What it can hit is the boom, in extreme conditions.

And, btw. tail strikes are among the most frequent helicopter accident causes ... (no 5 in this list, i'm sure the ntsb has similar info on this).

 

[K^2]

The reason why ground-effect doesn't add to stability the way you think is because of gyroscopic effect.

Lets start with hover at high altitude. Suppose, you pitched slightly forward. There is no immediate change to lift, but you start accelerating forward. That results in advancing blade having a slightly higher lift due to higher relative wind velocity. If advancing blade gets more lift, gyroscopic effect causes the helicopter to start pitching backwards. So once you have a bit of altitude, hover is actually self-stabilizing. If the flight stick is slightly displaced from center, this will result in a constant air speed in that direction. This is very easy to get used to, so hover at altitude tends to be straight forward.

Now lets look at the same situation in ground effect. Again, we start with a small tilt forward. Before helicopter even started picking up speed, the forward blade already has more lift. And due to gyroscopic effect, this will cause a tilt to a side. Which side, depends on the rotor rotation direction. As you are tilting to the side, you are also picking up forward speed. That will result in maximum lift shifting a bit towards advancing side, and this will still correct the forward tilt, but by this point, you are tilted to the side, picking up lift on that side, and are starting to accelerate in that direction. Now you are tilting to the back, and the cycle repeats. This is inherently unstable.

Because the self-righting mechanism from high hover is still there, the level attitude point is still nominally stable, but it is not dynamically stable, and that's what you really care about. Instead of stabilizing, the helicopter starts to gyrate, building up more and more deflection from center. Worst part is, the inexperienced pilot will try to correct for the immediate tilt, which is going to be corrected by increased lift on the advancing blade anyways, instead of correcting for the ground-effect. So there is a 90° phase lag between what's happening to helicopter and pilot's corrections, making it very difficult to keep helicopter put and could actually make the situation worse. This is why an inexperienced pilot should never try to hover in the first place except for learning how to do it under instruction. If you have to fly a rotor in a simulation or in case of emergency, do takeoffs vertically up, then build up forward speed. Once moving, helicopter isn't that different from fixed wing. For landings, come in with forward speed, as if you were landing an airplane. You can come down with much greater slope, though. If you flare to a stop, you'll have much better chances of putting the machine down gently than if you tried to come in vertically and land from hover. You'll also be that much less likely to encounter the other great enemy of rotor, settling with power. I would wager that settling with power and hovering errors account for majority of helicopter accidents.

 

As to the reason why tail rotor is placed high, yeah, I think the chief reason is tail strikes, as shynung suggests. Although, helicopters have fairly high CoM, since the engine is typically up top, and so I doubt any significant torque comes from tail rotor that isn't in line with the main rotor's torque.

[Green Baron]

25 minutes ago, K^2 said:

I would wager that settling with power and hovering errors account for majority of helicopter accidents.
 

If that belongs to the category "pilot input errors" then you are right :-)

 

[p1t1o]

The tail rotor may be out of line with the CoM, but perhaps the way that this puts it more in line with the plane of the main rotors (which are generating significant gyroscopic stability) takes precedence? Perhaps a force out of line with the gyroscopic centre generates a worse defect than a force out of line with the CoM? Due to their speed the rotors having a larger...something-or-other...than the stationary fuselage?

[K^2]

@p1t1o The only thing that matters are torques. In steady hover, the torque generated by main rotor always passes through the axis of its rotation. So the tail rotor needs to produce torque exactly opposite to that, meaning still along the line of the axis of rotation. To achieve that perfectly, rotor would need to sit directly behind CoM. Whether that's close or far to the plane of main rotor is absolutely irrelevant. Of course, if tail rotor doesn't sit directly behind CoM, you'll also be generating a slight amount of rolling torque, which you'll have to compensate for with the stick. And of course, depending on how much rudder you give it, you'll have to adjust compensation. But that's nothing new for heli piloting. Pretty much any input you give will require you to compensate with something else, then compensate for that compensation, and so on. Unlike a fixed wing, where we can at least pretend like inputs are independent (it's not true, but it's more subtle), with rotors, everything influences everything else in very obvious ways.

@Green Baron Yeah, but there are many different ways a heli pilot can screw up. My claim is that most frequent mistakes leading to accident are these two. It's a claim based entirely on stories I've heard, and not any kind of statistics, however.