Author Topic: Basic Rocket Science Q & A  (Read 270967 times)

Offline MP99

Re: Basic Rocket Science Q & A
« Reply #880 on: 01/20/2014 09:35 AM »
I've read in quite a few places now, that the Isp of different propellant mixes scales with the density of the fuel. So, for example, RP-1 offers more ISP than methane etc. (All other things being equal.. consider some hypothetical engine that burns Metholox as well as it burns Kerolox).

Why is this?

Specific impulse scales with density?  That's a new one on me.  Hydrolox has a notoriously low density yet a very high specific impulse.

Specific impulses are usually quoted for a particular expansion ratio.  In effect, specific impulses given for propellant combinations are actually for particular idealized engines.

More complex molecules (in the exhaust products) will have more non-translational degrees of freedom.  In principle this does make complex exhaust products less attractive, but for a reasonably large expansion ratio, the effect is pretty small.

Dredging this up...

I wonder if this is simply a question of pragmatics.

Certain prop combinations have standout Isp vs density (vs usability).

Imagine a prop combination with the Isp of kerolox, but the density of hydrolox. Given it has the disadvantages of both, you'd expect a designer to choose either kero- or hydro- instead. The commonly used prop combo's are just those that stand out from the crowd somewhere on the Isp/density relationship.

Of course, sometimes usability gets in the way as well, and biases the decision - dangers of hypergolics or  hydrogen/fluorine, hassles of deep cryogens/LH, etc.

cheers, Martin

Offline mheney

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Re: Basic Rocket Science Q & A
« Reply #881 on: 01/20/2014 03:50 PM »
ISp and fuel density impulse are only one of a number of considerations when designing a vehicle.  The Lunar Modules used hypergolics, for example, because of reliability - you want to make as sure as possible that the Ascent Module stage lights when you push the button.  You'd look at different prop combos if/when the infrastructure is there - if you have a base that you can walk back to if you have a launch abort  with in-situ propellant production, yu'd use a different vehicle than you would an "expedition" mission like Apollo.


Offline Hop_David

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Re: Basic Rocket Science Q & A
« Reply #882 on: 02/26/2014 02:51 PM »
However earth's orbit about the sun is a much more leisurely degree per day. (360 degrees/365 days). In this case [of a spacecraft in heliocentric orbit] we'd remain in a 60 degree neighborhood of perihelion for two months. 100 micro-gees * 2 months = 5 km/s.

For heliocentric orbits, low thrust, high ISP engines might be able to enjoy some Oberth benefit. Once out of planetary wells, it seems to me we can have our ion cake and eat it too.

That's very well expressed. Thanks! And just to check the reverse works too? I.e. a spacecraft that has just managed to escape from Mars (but is still in essentially the same orbit around the Sun as Mars) has plenty of time at aphelion to perform an ion "burn" that drops its perihelion down to intersect Earth?

Even better for Mars. Angular velocity slows farther from the sun. Mars is about half a degree per day.

Jupiter's about .1 degree per day. Saturn .03 degrees. The more relaxed pace of the outer bodies can give an ion engine a lot more time to do her slow burn in the neighborhood of an peri or aphelion. SEP probably wouldn't work in the outer system though, ion engines would need nukes.

I think the Main Belt might be a sweet spot for SEP. A slow enough pace that SEP can enjoy the Oberth benefit and enough sunlight to power them.

Offline Hop_David

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Re: Basic Rocket Science Q & A
« Reply #883 on: 02/26/2014 03:37 PM »
Certain prop combinations have standout Isp vs density (vs usability).

Martin's observation is for chemical engines. I'm wondering if similar considerations can be applied to ion engines.

Here's some of my assumptions on ion engines:

1) Ion engines often use noble gases because the plasma is less corrosive than chemically active elements.

2) Xenon is often used because of higher atomic weight. Increasing molar weight decreases ISP but increases thrust. Ion engines already have plenty of ISP but miniscule thrust. Heavier xenon partially mitigates an ion engine's tiny thrust.

I'm hoping those more knowledgeable than I can tell me if I'm wrong or on the mark.

Mars atmosphere is about 96% carbon dioxide, 2% argon and 2% nitrogen. One of my daydreams is exporting Martian argon to the Main Belt for use as reaction mass.

« Last Edit: 02/26/2014 03:54 PM by Hop_David »

Offline Adaptation

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Re: Basic Rocket Science Q & A
« Reply #884 on: 03/30/2014 02:55 AM »
So rocket segments are almost always cylinders and the tanks are cylindrical with domed ends.  Spheres have the optimum volume to surface area ratio.  But just about every design chooses cylindrical rockets so it must have substantial benefits.  I want to understand which reasons are the most important.  I came up with a few please let me know if my order of importance is correct or if I missed any important reasons.

A. Cylinders (pillars) are stronger at holding things up than spherical tanks and the tanks form a major structural component of the rocket.
B. Rockets need to be aerodynamic to reduce stress at max q.
C. Rockets need to be aerodynamic to reduce atmospheric drag.
D. Manufacturing large cylinders is easier than manufacturing large spheres. 
D. Shipping large cylinders around earth is easier to do than large spheres. 
« Last Edit: 03/30/2014 03:05 AM by Adaptation »

Offline Robotbeat

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Re: Basic Rocket Science Q & A
« Reply #885 on: 03/30/2014 04:23 AM »
N-1 used spheres for the first 3 stages. Also, spheres are used for things like helium tanks and for some upper stage engines, I think (on Soviet lunar probes, at least).

Cylinders are nice for the reasons you mentioned.
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Offline Adaptation

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Re: Basic Rocket Science Q & A
« Reply #886 on: 03/30/2014 04:59 AM »
Approximately what portion of delta v goes into atmospheric drag?

Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #887 on: 03/31/2014 10:37 PM »

Approximately what portion of delta v goes into atmospheric drag?
I think it's around 150m/s.
But ink of the other considerations. You need both fuel and oxidizer. As the N-1 showed, spherical tanks need a lot of support mass, and piping is a mess. And then all they things you said did played a role. Transportation is one of the biggies. But aerodynamic drag is very important for smaller rockets. Drag is proportional to the front surface. But rocket weight is a cubic measure. Since drag losses are drag/thrust, as your rocket grows (and thus thrust), the aspect ratio is less important. Look how thin sounding rockets are vs Shuttle or Saturn V, for example.

Offline Robotbeat

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Re: Basic Rocket Science Q & A
« Reply #888 on: 04/01/2014 07:06 AM »
Actually, thrust also goes as area, too. That places an upper limit on rocket height given a certain chamber pressure and propellant bulk density.
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Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #889 on: 04/01/2014 01:18 PM »

Actually, thrust also goes as area, too. That places an upper limit on rocket height given a certain chamber pressure and propellant bulk density.
I've calculated at around 1000MN for a sphere at RD-191 MCC pressure / expansion ratio.

Offline pagheca

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Re: Basic Rocket Science Q & A
« Reply #890 on: 04/15/2014 10:29 PM »
I know how the specific impulse of a rocket engine is defined.

What I'm asking here is if any of you knows a way to "think" at the specific impulse of a rocket engine. What actually means 282 sec?

For example, one can express the speed of an object in sec, by dividing its length by his speed: m / (m/s) = sec. This number tells how many sec are required for the vehicle to make a single move as long as its own length.

Does anyone knows something like that for specific impulse?
« Last Edit: 04/16/2014 01:00 AM by pagheca »

Offline ddunham

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Re: Basic Rocket Science Q & A
« Reply #891 on: 04/15/2014 10:42 PM »
From wikipedia...
http://en.wikipedia.org/wiki/Specific_impulse

Quote
This Isp expressed in seconds is somewhat physically meaningful—if an engine's thrust could be adjusted to equal the initial weight of its propellant (measured at one standard gravity), then Isp is the duration the propellant would last.

Offline pagheca

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Re: Basic Rocket Science Q & A
« Reply #892 on: 04/16/2014 01:27 AM »
Approximately what portion of delta v goes into atmospheric drag?

I do not know if you are referring to something in particular. However, from another post and, in turn, from this book:

Ariane A-44L: Gravity Loss: 1576 m/s Drag Loss: 135 m/s
Atlas I: Gravity Loss: 1395 m/s Drag Loss: 110 m/s
Delta 7925: Gravity Loss: 1150 m/s Drag Loss: 136 m/s
Shuttle: Gravity Loss: 1222 m/s Drag Loss: 107 m/s
Saturn V: Gravity Loss: 1534 m/s Drag Loss: 40 m/s (!!)
Titan IV/Centaur: Gravity Loss: 1442 m/s Drag Loss: 156 m/s


Offline aero

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Re: Basic Rocket Science Q & A
« Reply #893 on: 04/16/2014 02:05 AM »
Approximately what portion of delta v goes into atmospheric drag?

I do not know if you are referring to something in particular. However, from another post and, in turn, from this book:

Ariane A-44L: Gravity Loss: 1576 m/s Drag Loss: 135 m/s
Atlas I: Gravity Loss: 1395 m/s Drag Loss: 110 m/s
Delta 7925: Gravity Loss: 1150 m/s Drag Loss: 136 m/s
Shuttle: Gravity Loss: 1222 m/s Drag Loss: 107 m/s
Saturn V: Gravity Loss: 1534 m/s Drag Loss: 40 m/s (!!)
Titan IV/Centaur: Gravity Loss: 1442 m/s Drag Loss: 156 m/s

I wonder if gravity loss correlates with the rocket lift-off T/W ratio, that is, Gross thrust/GLOW.  It seems that it might.
« Last Edit: 04/16/2014 02:11 AM by aero »
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Offline pagheca

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Re: Basic Rocket Science Q & A
« Reply #894 on: 04/16/2014 03:10 AM »
I got a few questions about gravitational drag.

Frankly speaking, I haven't been able to fully understand how this drag depends on the vertical and horizontal components of the speed. Why GD (gravitational drag) isn't described by an analytic function of the trajectory(x,y), weight(t) and speed(t)?

GD is shown for other rockets in the table above. However, how to extrapolate the approximate value for the 1st stage only of a Falcon 9? Staging happens at a relatively low altitude for the Falcon (~100 km), respect to other rockets. On the other side, I guess DG is a function of rocket mass, so that most of it is "spent" at a low altitude. So, for the 1st stage only, I would say GD is much less than the one relative to a Titan, for example. Something like... 1/2? Is that assumption correct?

Moreover, one should add or not the GD while the rocket is re-entering the atmosphere?

Last but not least, why is the Saturn V value of the Drag Loss so drammatically low??? What made it so efficient - assuming this is not just a typo?
« Last Edit: 04/16/2014 03:15 AM by pagheca »

Offline aero

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Re: Basic Rocket Science Q & A
« Reply #895 on: 04/16/2014 06:19 AM »
I got a few questions about gravitational drag.

snip

Last but not least, why is the Saturn V value of the Drag Loss so drammatically low??? What made it so efficient - assuming this is not just a typo?

A lot of good information about the Saturn V here.

http://www.braeunig.us/apollo/saturnV.htm

Braeunig says that the low aerodynamic drag was because the Saturn V thrust to weight ratio was low resulting in lower velocity within the lower, thicker atmosphere. He also says that the drag coefficient at max q was about 0.51. I note that Cd= 0.51 is quite low for a rocket moving just above Mach 1, which is when max q happens.
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Offline aero

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Re: Basic Rocket Science Q & A
« Reply #896 on: 04/16/2014 06:38 AM »
I got a few questions about gravitational drag.

Frankly speaking, I haven't been able to fully understand how this drag depends on the vertical and horizontal components of the speed. Why GD (gravitational drag) isn't described by an analytic function of the trajectory(x,y), weight(t) and speed(t)?

snip


Oh there is. It's equal to -g + ac where ac is centrifugal acceleration. You just have to integrate it along the trajectory you want to fly. Of course g is a function of altitude and ac is a function of horizontal velocity and altitude so the integration is a little complicated...
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Offline fatjohn1408

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Re: Basic Rocket Science Q & A
« Reply #897 on: 04/16/2014 10:14 AM »
I got a few questions about gravitational drag.

snip

Last but not least, why is the Saturn V value of the Drag Loss so drammatically low??? What made it so efficient - assuming this is not just a typo?

A lot of good information about the Saturn V here.

http://www.braeunig.us/apollo/saturnV.htm

Braeunig says that the low aerodynamic drag was because the Saturn V thrust to weight ratio was low resulting in lower velocity within the lower, thicker atmosphere. He also says that the drag coefficient at max q was about 0.51. I note that Cd= 0.51 is quite low for a rocket moving just above Mach 1, which is when max q happens.

In addition, drag loss is negatively correlated with vehicle length.
If you have a vehicle with a certain thrust to weight and you double its length, you need to double its thrust to keep the ratio the same and to be able to fly exactly the same launch trajectory.
However, since your diameter hasn't expanded the drag force will not have doubled and the size of your drag relative to your thrust will have been cut in half (or about that). Therefore your drag loss, which is just the total deceleration your rocket undergoes during flight, will also have been cut roughly in half.
Note also that drag loss is totally uncorrelated to vehicle diameter following the same logic.
Drag loss (during gravity turn) is all about length and thrust-to-weight (which dictates your velocity-height profile).
« Last Edit: 04/16/2014 10:16 AM by fatjohn1408 »

Offline baldusi

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Re: Basic Rocket Science Q & A
« Reply #898 on: 04/16/2014 08:05 PM »
Drag loss id directly related to diameter, but inversely to thrust. If you scale a rocket linearly on all directions, surface growths quadratically while mass (and thus, thrust) has to grow cubically. Thus, as rockets get bigger they could decrease the drag loss. The fact is that drag is also directly related to aerodynamic stress. And also fatter vehicles are more mass efficient (because of surface to volume reasons) than thinner ones. Thus, you'll notice that bigger vehicles tend to be squatter. Also the reason why the Shuttle had not higher drag losses whiles having such a huge frontal area.

Offline pagheca

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Re: Basic Rocket Science Q & A
« Reply #899 on: 04/16/2014 08:19 PM »
Oh there is. It's equal to -g + ac where ac is centrifugal acceleration. You just have to integrate it along the trajectory you want to fly. Of course g is a function of altitude and ac is a function of horizontal velocity and altitude so the integration is a little complicated...

Thanks very much to you and all the others replying on this issue.

Any reference to this equation and how it can be derived?