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#26 2016-06-21 17:26:17

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Air breathing engines on Mars

elderflower I did a quick topic search on solar sterling and here are the topics that we have on Newmars.

Sterling engine+solar= neat powersource - EETimes article

Solar thermal power - Fathers Day gift

Solar thermal power

Some topic get good discussion while other get duplicated after a period of time as the originals are not sought out to continue in.....

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#27 2016-07-11 06:26:45

Antius
Member
From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Air breathing engines on Mars

A pulse-jet engine may be a good choice for an air-breathing silane engine on Mars.  On Earth, these engines are relatively inefficient, partly because they are effectively sea-level rocket engines with a poor expansion ratio.  On Mars, atmospheric pressure is low, so a pulse-jet could achieve a much better expansion ratio.  Also, as CO2 is the oxidiser and is 95% abundant in the Martian atmosphere, flame temperatures will be high, providing an additional boost to ISP.  As the external environment is close to zero pressure, the outlet from the combustion chamber could be a diverging nozzle, thereby reducing the potential for solid build-up.  The pulse jet could reach take-off speed by blasting compressed CO2 into the engine until dynamic pressure on the inlet is great enough to open the injection valves.

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#28 2016-07-11 08:24:38

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,712
Website

Re: Air breathing engines on Mars

I dunno about pulsejet on Mars.  But I do know pulsejet,  and there's nothing about them that functions like a rocket,  other than thrust = jet momentum.  There's a whole lot more flow and combustion chemistry and physics going on in such a device than just thrust = jet momentum. 

Here on Earth,  the peak of the oscillating pressure-versus-time trace for the combustion chamber is only about 2 to 2.5 times the ambient atmospheric pressure,  on an absolute-pressure basis of calculation,  in the best designs,  less for the poor ones.  The min chamber pressure is around 70% ambient atmospheric in the best designs.  The thrust you get is directly proportional to average chamber pressure,  and to engine cross section area. 

That's more-or-less true whether you build a valved or valveless pulsejet tube.  Neither form's chamber pressure responds to inlet dynamic pressure;  that affects mixing of fuel and air,  but not the basic cycle pressure.  Most designs develop static thrust that is actually higher than thrust when moving. 

In the valved form,  the valves must be very lightweight,  and must mechanically vibrate open-and-closed at the same frequency as the engine oscillates.  Such valves have very short lifetimes,  usually measured in a few dozens of minutes.  They are subject to combustion backflow impact,  and would be fatally affected by any slagging at all,  which changes the mechanical frequency as well as destroys the sealing. 

The shape of the combustion chamber and exhaust tube strongly affects whether the engine will resonate at all.  It has to reflect the waveform effectively at the exit,  and it has to flow efficiently for exhaust pipe inflow as well as collimate the exhausting jet flow.  Slag would really upset all of this.  If any of this is not done right,  the engine will not run at all. 

The exact shape and size of the inlet is not as important,  as the inlet is nonresonant.  But the size of the inlet relative to the chamber needs to be in the correct range to achieve best performance,  as well as good combustion physics and flameholding.  If any of that last is wrong,  it will not run. 

The other critical factor is combustion chemistry rate,  and how that interacts with device size and frequency (in turn intimately linked).  Here on Earth,  really big ones the size of the V-1 Buzz Bomb engine can be made to run on gasoline.  Smaller ones like the hobbyists play with can only be made to run on properly-vaporized propane.  Those range from 3 to 10 inches combustion chamber diameter,  and from 100 to about 500 Hz frequency.  Really little ones (about 1 inch diameter,  1000 Hz stuff) can only be made to run on acetylene,  sometimes with raw oxygen injection added.  Getting this wrong means the pulsejet will not run at all.   

If that +2.5/0.7 x atmospheric pressure trace description holds on Mars with the silane fuels discussed here,  it means your pressure numbers and resulting thrust numbers are going to be very low for the size and mass of the equipment you have to build.  Precisely because that is such a thin atmosphere.  Pressures achievable would oscillate between around 15 and 4 mbar in a 6 mbar atmosphere,  all else equal. 

GW

Last edited by GW Johnson (2016-07-11 08:47:02)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#29 2016-07-11 10:10:23

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,712
Website

Re: Air breathing engines on Mars

Just following up about pulsejets ---

My best design recommendations for the valveless form of the pulsejet derive from work done cooperatively by Hiller Aircraft here in the US and SNECMA in France circa 1960,  based on earlier French work in the 1950-1960 time frame. 

I collated this immense body of work as best I could,  and turned it into usable design recommendations for the subcases not limited by fuel-air chemistry reaction rates.  Consequently,  to employ these design recommendations,  you must pay strict attention to the chamber diameter limits. 

That collation work is summarized and posted over at http://exrocketman.blogspot.com as “Recommended Broad Design Guidelines for Valveless Pulsejet Combustors”,  dated 5-20-2012,  under keyword “pulsejet”.  It’s a ways down;  use the date tool for year 2012,  and scroll down from there. 

The valved pulsejet is not as geometrically-sensitive as the valveless,  but all the valved designs have lower performance and more noise than any of the successful valveless designs.  If you can find the ancient thing,  there is a good report on the German version:  USN PQ-TM-4 (1948) ''The Aero-Resonator Power Plant of the V-1 Flying Bomb'' by G. Diedrich, translated by A. Kahane.  This is long-declassified intelligence information collected just after WW2.  It was part of something called “Project Squid” by the Navy.  That valved engine ran on gasoline,  but had very high fuel consumption. 

Over here in the 1950’s and 1960’s,  there were a couple of small valved pulsejets available to hobbyists.  These were known variously as Tigerjet,  Dynajet,  and similar brand names.  All featured a single reed valve instead of the bank of German flapper valves,  which was mechanically feasible only because of the small size.  I think these were propane-fueled devices,  of rather low performance. 

In the 1970’s,  there were a couple of modest-sized valveless pulsejets available as an alternative for launching sailplanes.  One of these was offered by Thermo-Jet.  I believe these were also propane-fueled.  Performance was better than the valved engines,  but lower than the best valveless designs of Hiller/SNECMA. 

The very best of the Hiller/SNECMA designs added an augmentor shroud to the exit and the inlet,  and bent the engine into a U-shaped tube,  since about 40% of the total jet momentum gets “spit” out of the inlet tube during the blowdown phase of the cycle.  The augmentor increased thrust by a factor of roughly 1.5,  and decreased thrust-specific fuel consumption (TSFC) by roughly a factor of 2. 

They were testing items of 9 or 10 inch diameter chamber,  several feet long,  operating about 100 Hertz,  for potential military aircraft applications.  Shrouded like that,  they achieved about 500 lb of thrust from a device massing about 30-40 lb,  and (at the most favorable power setting) a TSFC of just under 1 lbm/hour fuel per lb of thrust.  Typically,  they measured about 125 dB noise up close. The form of the successful augmentor shroud was very specific:  a rounded-lip entrance into a short divergent exit cone,  located just off the inlet or exit tube exit planes.  It was very definitely an unsteady-flow device. 

In contrast,  the German engine was about a foot diameter and about 11 feet long,  operating about 47 Hertz on gasoline.  It achieved a max static thrust of 1100 lb,  and got at best about TSFC = 8 at around 800 lb of thrust.  Its noise level was about 145 dB.  Hiller maintained that about the top 20 dB of that was valve clatter noise.  Valve lifetime averaged 45 minutes.  The flight time from sites in France to London was 40-43 minutes. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#30 2016-07-11 17:27:04

SpaceNut
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From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

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#31 2016-07-12 10:18:44

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 2,712
Website

Re: Air breathing engines on Mars

Amazing.  The U-tube Lockwood designs I have characterized for you over at "exrocketman" appear in two of those sites whose links you gave.  The reversed-inlet designs I have characterized over there,  as well.  That pulsejet article I put on "exrocketman" is one of the two or three most widely read items I ever posted.  Lots of people play with this stuff. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#32 2016-07-25 14:56:35

Antius
Member
From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Air breathing engines on Mars

Apologies for the late reply.

The traditional valved pulse-jet would appear to be useless for anything other than a short range missile.  Too slow to be useful on Earth anyhow.  The non-valved version, sounds like some sort of pulsed ram-jet engine.  I struggle to understand why engine dimensions would be quite so critically important.

With so little pressure on Mars, maybe a detonation rocket engine would work better?  Inject both fuel and oxidiser as micron sized droplet and ignite before significant phase transition has time to occur.  A powerful enough electric arc (1-10KJ) will trigger detonation in most combustible mixes.  At lower energy it takes time for the flame front to gradually accelerate as turbulence increases.  Turbulence tends to accelerate flame fronts.  To a certain extent the droplets themselves will promote turbulence.  Hydrogen ions have high molecular speed compared to any other excited molecule.  Ignition using a plasma torch with hydrogen as a carrier gas may be a more efficient detonation trigger than an electric arc in heavier gas.  These ignition systems are energy intensive but reliably give rise to dentonation.  Some kind of free piston internal combustion engine coupled with a linear electric generator could generate the required electric pulse whilst keeping weight down.  But the two systems must be perfectly aligned, with injection preceding the pulse by microseconds.

On Mars, liquid CO2 could be used as the oxidiser, with a mixture of silane and hydrogen as fuel.  The ignition source would most likely be a plasma-jet, discharging hydrogen rich plasma into the combustion chamber.  With so little air on Mars, the noise would hardly matter.  How could we pulse the injection?  Globe valve seats would fail very quickly due to excessive wear.  Maybe some kind of rotating ball valve, releasing propellants from a reservoir, which is in turn pressurised from something like a diesel injection pump?  We can do that if we are injecting liquids instead of gases, as volume flow rates are low.

Last edited by Antius (2016-07-25 15:13:47)

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#33 2016-07-25 20:22:36

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 10,650

Re: Air breathing engines on Mars

I found that we have a topic where the interest was using silane and hydrogen with co2 for good measure.....
Silane Hoppers - Use the CO2 man...
Got some cleanup to do of that topic and wow its from 2003

Burning silane

https://www.linkedin.com/pulse/what-sil … ert-morlan
AAEAAQAAAAAAAAW9AAAAJDhiYzdmNjk1LTZiMDYtNGU1Yy1hZGNhLTFiMGE1ODg0N2Y1NA.jpg

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