# why some engine thrust so different atmos vs. orbit ? (rhino, poodle etc)

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hello, question as titled - if engines are, generally speaking aerodynamically similar (i.e. a cone with thrust coming out of the end...), and the mass of said engines is somewhat "negligable" when stuck on the end of a MASSive rocket (liquid being liquid).

then how can there be such a difference between some engines atmos vs orbital thrust, vs. say the mosotodon, (being pretty similar ?).

thanks.

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That is a rather complicated topic.

In short, rocket nozzles are often optimized for best performance in intended environment. Lower in the atmosphere the air pressure confines the exhaust gasses kind of like a nozzle, so you don´t need to put a big and heavy metal one on the bottom of your rocket. The lower the surrounding atmospheric pressure, the less  it pushes against the exhaust gasses. You can observe that when you watch videos of rocket launches. At launch the exhaust gasses look like a column of fire, but higher up they spread much wider at the bottom of the nozzles. This lowers the efficiency of the motor, so you need bigger nozzles to force the gasses to go out to the back and not so much to the sides.

In KSP this is just simulated with different engine specs. There are just some engines that are really good at launch, some are good higher up in the atmosphere, and some are best used in vacuum.

The aerospike rocket engine that we have in KSP but not quite in real life is an attempt to deal with this problem, but has draw backs on its own.

This is a crude simplification and there are other factors like the actual design of the motor and so on.

I suggest you read further into this topic. It´s quite fascinating and understandable without a physics degree.

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thanks; yeah i suck too bad at maths for physics - they can keep their squiggles...

so it's basically the effect of the output matter on the surrounding air ? / vs void there of higher up... accounting for the numbers ? hence me not understanding (focusing on the engine) "ohhhhh"

- can you direct me to any specific reading about the above, that would be great (dunno what the field is called even)

thankyou!

Edited by k00b
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Here's a decent crash course to get you up to speed

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The book I read was in german, and I can´t find it at the moment, so that´s useless.

Just start with the wikipedia article "rocket engine nozzle". That´s pretty good and in the external links there is a NASA pdf "NASA Space Vehicle Design Criteria, Liquid Rocket Engine Nozzles" that goes in much... much, much more detail.

Maybe some of the other fine folks here on this forum would be able to provide more literature?

Edited by KerrMü
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thankyou very much!, i will attempt to get learning...

Edited by k00b
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Thread moved to Science subforum due to actual spaceflight content.

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can anybody expand re: 4:31 of the video "this can lead to... undesirable phenomena" what the "phenomena" is ? - does he mean "heat vs mettallurgy" ?

seems like "over expansion" (or what would be "linear output" ?) is a "good thing" (if they weren't cones... / re: aerospike), or that they should custom build conical engines for individual rockets every single time (if building a new rocket) ?.

...think maybe mother nature wants to keep her children grounded.

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Undesireable phenomena could include, for instance, combustion-driven acoustic feedback (aka "screech") that literally shakes the rocket nozzle apart.

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5 hours ago, k00b said:

seems like "over expansion" (or what would be "linear output" ?) is a "good thing" (if they weren't cones... / re: aerospike), or that they should custom build conical engines for individual rockets every single time (if building a new rocket) ?.

Overexpansion creates risk of the exhaust stream starting to peel off the nozzle before reaching nozzle exit, which can easily became asymmetrical and do something drastic, like tear apart the gimbal actuators or the nozzle itself.

So all rocket engines tend to be designed underexpanded. And yeah, this means you need different nozzles for different altitudes, and that an SSTO would take massive performance penalties, especially high up where most of the dV is produced.

Edited by DDE
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So what would your rocket nozzle look like if you were trying to take off from the surface of Venus?

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1 hour ago, KG3 said:

So what would your rocket nozzle look like if you were trying to take off from the surface of Venus?

You'd have practically no choice other than an aerospike, in order to cover the enormous pressure range from the Venusian surface to vacuum (~90atm to 0)

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6 hours ago, DDE said:

Overexpansion creates risk of the exhaust stream starting to peel off the nozzle before reaching nozzle exit, which can easily became asymmetrical and do something drastic, like tear apart the gimbal actuators or the nozzle itself.

this is suggestive that humans need to get better at mettalurgy and stop making things that they know are going to tear to pieces though doesn't it ?

like last time i saw they were messing about on the space station growing lettuce and talking to a monochrome computer face "robot" that doesn't even work ?.

like i am saying; my maths isn't great but... "expenditure of organic matter (fossil fuels and expensive metals (and cost of harvesting therein) x chemistry and mathematics (human resource) dividied by "it might not work anyway" (baring in mind the human knowlege it is not "efficient" (as above)) = ..."growing lettuce"

doesn't seem to quite "add up" ?.

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2 minutes ago, k00b said:

this is suggestive that humans need to get better at mettalurgy and stop making things that they know are going to tear to pieces though doesn't it ?

No. It's not worth the bother to engineer perfect pieces of equipment, for the best is enemy of good enough.

The efficiency of the ISS science program is another matter entirely.

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42 minutes ago, DDE said:

No. It's not worth the bother to engineer perfect pieces of equipment, for the best is enemy of good enough.

The efficiency of the ISS science program is another matter entirely.

whats the point of sending rockets into space in the real world environment though - at all ? like really ?. "good enough" to do what ??? (BEYOND physicisits quanitifying their squiggles in order to "create the illusion of [their own, alleged] intelligence".... (and growing lettuce)

logically we need to wait for an electric propulsion system (i.e. if elon musk is reading; please hurry up...) or put the toys away completely ?. (i would rather use a stainless steel then an aluminium frying pan also... but maybe it is about time we maybe stop plagurising a trains engine ?).

Edited by k00b
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4 minutes ago, k00b said:

whats the point of sending rockets into space in the real world environment though - at all ? like really ?. "good enough" to do what ???

A long list of political, military and economic objectives.

4 minutes ago, k00b said:

logically we need to wait for an electric propulsion system

We don't. ISP is hardly everything.

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39 minutes ago, k00b said:

whats the point of sending rockets into space in the real world environment though - at all ? like really ?. "good enough" to do what ??? (BEYOND physicisits quanitifying their squiggles in order to "create the illusion of [their own, alleged] intelligence".... (and growing lettuce)

logically we need to wait for an electric propulsion system (i.e. if elon musk is reading; please hurry up...) or put the toys away completely ?. (i would rather use a stainless steel then an aluminium frying pan also... but maybe it is about time we maybe stop plagurising a trains engine ?).

Electrical rocket engines als ion engines are much used today. You might know them from KSP, the real world version has far less trust. So little that the probe who went to Ceres was not able to circulate on ions alone.
However in real world you can have an probe do an week long burn.

Generally you can select either high trust or high ISP. With very high energy fuel you can do both, project orion who would use atomic bombs as fuel is an obvious one. Antimatter would work much better.

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Spoiler

The main point of sending rockets into space is to troll flat-earthlings and sell space-related souvenirs.

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i was thinking we need to "teleport" electricity from the sun to gain thrust because "rockets don't work very good" you know how they have mobile phones that get "charged through the air" (i dunno if that works where there is no air).

51 minutes ago, DDE said:

A long list of political, military and economic objectives.

oh right; for no reason then just as i thought.... (they already have enough satelites / and politicians can "suck my balls" / PRETTY SURE the military looks after "the economy" (thus you have stated the same one thing 2x and also included idiots....)

- the operation paperclip rocket guy from germany SHOULD have been able to have a bash at getting to mars (how much time and resources later ....and now ....we want to goto mars ...when he could have done it already in year whatever, with "poo poo technology" - BORING).

(enjoy drawing squiggles people, because that is all  you are doing (- beyond wasting resources)).

- thanks for the help though... "seems the engines with the differential are the best ones" (pertaining to my long jeb and bill rescue flight mission to laythe.)

P.S. i bet space station lettuce tastes worse then earth lettuce.

Just now, kerbiloid said:
Hide contents

The main point of sending rockets into space is to troll flat-earthlings and sell space-related souvenirs.

I loveING KNEW IT !

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2 hours ago, k00b said:

i was thinking we need to "teleport" electricity from the sun to gain thrust because "rockets don't work very good" you know how they have mobile phones that get "charged through the air" (i dunno if that works where there is no air).

oh right; for no reason then just as i thought.... (they already have enough satelites / and politicians can "suck my balls" / PRETTY SURE the military looks after "the economy" (thus you have stated the same one thing 2x and also included idiots....)

- the operation paperclip rocket guy from germany SHOULD have been able to have a bash at getting to mars (how much time and resources later ....and now ....we want to goto mars ...when he could have done it already in year whatever, with "poo poo technology" - BORING).

(enjoy drawing squiggles people, because that is all  you are doing (- beyond wasting resources)).

- thanks for the help though... "seems the engines with the differential are the best ones" (pertaining to my long jeb and bill rescue flight mission to laythe.)

P.S. i bet space station lettuce tastes worse then earth lettuce.

You can use lasers to generate trust, this should work well but require stuff like gigawatt laser arrays in orbit for smaller stuff.

And yes we could gone to mars planted an flag and gone home and not gone again.
Until the time of reusable rockets any space operations was idiotic expensive worth it for com sats and weather satellites, rest was security, military or prestige.

On the other hand it should not be very hard to scale up starship 10x, you would need larger engines and even then might need an wider base on ultraheavy.
250 meter high and an starship diameter of 23 meters. Guess it could put 2000 ton into leo.

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7 hours ago, magnemoe said:

On the other hand it should not be very hard to scale up starship 10x, you would need larger engines and even then might need an wider base on ultraheavy.
250 meter high and an starship diameter of 23 meters. Guess it could put 2000 ton into leo.

Even a small Starship is still just a claim and a toy hopper. And looking at the rapid evolution of the Starship design, one could say it was more pictured than engineered. In fact, it's just getting closer to the earlier Energy-2 project (the winged 2nd stage with retractable shroud on a hinge to open) which was looking similar but was never implemented irl.

And the bigger is the engine - the harder and more expensive is its engineering.
Big chamber and nozzle make conditions not uniform and cause local instabilities, like those they were dealing with in F1.
Huge thrust makes testing equipment huge. Together with huge fuel consumption it makes required funds huge and limits number of explosions tests.

So, irl we have that still nothing heavier than 150 t was delivered to LEO by single-use and overexpensive rockets.

***

About the significance of space flights in the world economics.

Spoiler

(It's strange, though, that they just add dollars of 1977 and 2017 together without the currency rate recalculation)

Probably only dinosaurs are more economically significant that the space. Both are hype drivers for adventure franchises.

Edited by kerbiloid
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19 hours ago, magnemoe said:

Electrical rocket engines als ion engines are much used today. You might know them from KSP, the real world version has far less trust. So little that the probe who went to Ceres was not able to circulate on ions alone.
However in real world you can have an probe do an week long burn.

Generally you can select either high trust or high ISP. With very high energy fuel you can do both, project orion who would use atomic bombs as fuel is an obvious one. Antimatter would work much better.

On the other hand, the Dawn probe had 10km/s delta-v in its ion engine for use *after* reaching escape velocity.  If you have the patience, ion thrusters are absolutely amazing (but I'd expect that you still want to use fancy gravity tricks: ions will simply get you in position for them faster).  If your patience depends on the cargo, you can practically break the rocket equation by using ions to move propellant to a depot in the right orbit, and then simply move your chemical-powered crewed rocket from ion-fed depot to ion-fed depot: LEO to escape velocity is ~3000m/s, while going to Mars is another ~1000m/s (and presumably send any return propellant not ISU-generated on the surface via ions as well).

Electric propulsion with Isp>1000s is a thing (generally speaking Isp isn't the limiting factor with ion propulsion, if you want more you can probably get it at a price).  Remember Robert Heinlein's statement that "LEO is halfway to anywhere": with ions (and especially ion-fed depots), LEO is nearly all the way there (no known way to use electric propulsion to orbit, not to say Escape Dynamics didn't try).

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18 hours ago, wumpus said:

no known way to use electric propulsion to orbit, not to say Escape Dynamics didn't try

ARCA and their flying electric teapot demand a mention.

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On 8/18/2019 at 2:03 AM, k00b said:

i was thinking we need to "teleport" electricity from the sun to gain thrust because "rockets don't work very good" you know how they have mobile phones that get "charged through the air" (i dunno if that works where there is no air).

Charging mobiles phones through the air is due to EM induction creating currents to charge the battery. You need a base to produce the magnetic field needed.

About "Teleporting" energy, it can be done through using solar panels to capture Sunlight. The problem is how to convert that energy to thrust.

All rocket engine do this by throwing mass out of your rocket. Either by electric propulsion as mentioned by @wumpus, or by traditional "burn the fuel and shoot exhaust gas out very fast", aka chemical engines.

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Spoiler

Image from JPL about the operation of ion engines. These engines are very fuel-efficient, as electrical energy, instead of chemical energy (burning), is used to accelerate the exhaust (Xe). Hence there wouldn't be so much volume needed to store the propellant, though you would require a lot of electricity: this explains the gigantic solar panels of Dawn compared to the size of the probe.

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