Chemical Forums

Please login or register.

Login with username, password and session length

Sponsored links

Pages: [1]   Go Down

Author Topic: Atomic nitrogen for scramjet propulsion?  (Read 639 times)

0 Members and 1 Guest are viewing this topic.

RobertGC

  • New Member
  • **
  • Mole Snacks: +1/-0
  • Offline Offline
  • Gender: Male
  • Posts: 8
Atomic nitrogen for scramjet propulsion?
« on: April 25, 2017, 07:20:58 AM »

A problem with scramjets, which attempt to achieve combustion at hypersonic speeds, is that the high speed means the air is producing so much drag that it is difficult to achieve net thrust when combusting with just the oxygen in the air.

So what if we also combusted with the nitrogen? Nitrogen makes up 80% of the air mass so perhaps this could provide sufficient thrust. The problem is nitrogen is commonly present in the atmosphere as N2, a molecule that is very stable, i.e., non-reactive.

But at hypersonic speeds so much heat is produced that the N2 is "cracked", generating various nitrogen species, such as N, NO, NO2, etc.

So how much energy could be produced by reacting hydrogen fuel with the various nitrogen species? How much for methane fuel?

The energy density and thus Isp would be less than for reacting with oxygen but conceivably the thrust could be greater because of the greater mass of the nitrogen.
Logged

Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Mole Snacks: +1550/-389
  • Offline Offline
  • Gender: Male
  • Posts: 23544
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Atomic nitrogen for scramjet propulsion?
« Reply #1 on: April 25, 2017, 10:36:05 AM »

And what would the "combustion with nitrogen" reaction be?

How much of the nitrogen is ionized/decomposed? Have you tried to check numbers, or is it just a wild speculation?
Logged
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +162/-48
  • Offline Offline
  • Posts: 2198
Re: Atomic nitrogen for scramjet propulsion?
« Reply #2 on: April 26, 2017, 12:26:19 AM »

Same opinion.

The formation of N2 from two N atoms releases very much heat, so
  • Combustion of hydrogen and methane occurs with the oxygen and leaves N2 untouched. Forming NH3 would release 45.9kJ/mol only, and don't expect it to happen at a few bar and 2000°C.
  • Aluminium nitride would be less bad, releasing 318kJ/mol, but is quite impractical.
  • Even when a spacecraft re-enters our atmosphere from low-Earth orbit, hence around 8km/s rather than targeted 2km/s hypersonic speed, N2 doesn't atomize significantly. Only O2 does, in small amount.
  • But if using an amine as a fuel, the recombination of N atoms separated in the amine into N2 is an essential contribution to the combustion heat.

"Achieving a net thrust" is difficult... for slightly different reasons. At 2km/s, the compression to subsonic speed prior to combustion heats the air to almost 2000°C, but the engine needs to expand hotter air so it's ejected faster than it came in and achieves a thrust. If you accept a dirty exhaust and you cool the engine properly, for instance aluminium burnt with the oxygen would bring the temperature increase. If you want a hydrocarbon, it's less easy, but its flame is hotter than in cold air.

The alternative known as scramjet seeks to burn the fuel in lukewarm air so the temperature increase is big. This implies a small compression, hence the air speed remains supersonic at the burner. The big difficulty is to stabilize the flame at supersonic speed (hence "supersonic combustion" ramjet), for which hydrogen fuel hopefully brings the necessary detonation speed.

Somewhere, I've proposed to create in a scramjet combustor local shock waves to permanently ignite a fraction of the fuel there before the flame propagates sidewards: the deflagration and detonation speeds wouldn't be a limit any more.

Still an other possibility has emerged with the Sabre engine
https://en.wikipedia.org/wiki/SABRE_(rocket_engine)
which evaporates the liquid hydrogen fuel to liquefy oxygen from the intake air for subsequent combustion. While I consider it pointless for space launch, it's a seducing idea for atmospheric flight.
Logged

Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +162/-48
  • Offline Offline
  • Posts: 2198
Re: Atomic nitrogen for scramjet propulsion?
« Reply #3 on: April 27, 2017, 10:43:23 AM »

Out of curiosity, I've used Propep to estimate the compression from 2km/s and 55mbar, -60°C (20km altitude). At stagnation, the air would attain 220bar (wow!) and 1920K, so heating it further is difficult.

Flame      Expansion      Fuel
  K           m/s        (liquid except noted)
======================================================
3138          2846        Methane
3204          2864        RP-1 (~Diesel)
3180          2875        Diethylene Triamine
3183          2905        Diaminoguanidine (s) (much)
3269          2910        Cyclopropane
3318          2910        Bicyclo[1.1.0]butane
3285          2910        Diazetidycyclopropane
3340          2915        Cyclooctatetraene
3307          2923        Ethylene
3326          2936        Spiropentane
3329          2936        Di-spiropentyl (storable)
3394          2948        Cubane (s)
3224          3008        Nitromethane (much)
3215          3046        Hydrogen
3763          3213        LiAlH4 (s)
4733          3258        Aluminium (s) (much)
3982          3276        AlH3 (s)
4818          3357        Aluminium (molten) (much)
=====================================================

The ones meant to consume less oxygen (nitromethane, amines...) consume much fuel.

With 2km/s intake, expansion isn't much faster, so this parameter matters a lot. Methane is bad.

Cyclooctatetraene
(COT) is the highest performing with limited drawbacks. "Only" carcinogen.
Di-spiropentyl and cubane must first be synthesized and mass-produced.
Among the easily flammable ones, ethylene is cheap and easier than hydrogen.
Hydrides consume less fuel than metals.
Logged

Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +162/-48
  • Offline Offline
  • Posts: 2198
Re: Atomic nitrogen for scramjet propulsion?
« Reply #4 on: May 14, 2017, 04:12:40 AM »

Fuels that heat by recomposing may ease the supersonic combustion in a scramjet. After recomposition in a pre-chamber, they would be hotter than their ignition temperature when meeting the air, and hopefully light permanently, needing no flame propagation faster than the supersonic air.

Such fuels must avoid soot while recomposing, which imposes more hydrogen per carbon than most amines offers. I've given examples based mainly on the guanidine family and ethylene diamine, there
http://www.scienceforums.net/topic/83156-exotic-pumping-cycles-for-rocket-engines/#entry805383
and beginning there
http://www.scienceforums.net/topic/82965-gas-generator-cycle-for-rocket-engines-variants/
in the tables with light grey background.

The no-soot proportions depend on the pre-chamber pressure. This pressure can be attained by a gas in the strong tank or by a pump rotated by a turbine built between the recomposition chamber and the combustion chamber. The proposed mixes also limit to 700°C or 1100°C the temperature after recomposition to fit nickel alloys and prospectively molybdenum alloys; active cooling seems difficult here too.

Marc Schaefer, aka Enthalpy
Logged

Enthalpy

  • Chemist
  • Sr. Member
  • *
  • Mole Snacks: +162/-48
  • Offline Offline
  • Posts: 2198
Re: Atomic nitrogen for scramjet propulsion?
« Reply #5 on: June 15, 2017, 08:48:53 AM »

The unsaturated rocket fuels suggested there
http://www.chemicalforums.com/index.php?topic=91735.msg327577#msg327577
would be efficient as ramjet fuels too, here after air compression from 2000m/s:

Flame      Expansion      Fuel
  K           m/s
=====================================================
3289          2911        Divinylcyclobutane, cis-1,2
3340          2915        Cyclooctatetraene
3377          2952        [3]-syn-Ladderdiene
3367          2960        Spirohex-1-ene
3430          2989        Dispirooctadiene
=====================================================


Cis-1,2-divinylcyclobutane is as efficient as cyclooctatetraene (COT) and possibly cheaper to produce by dimerization of butadiene, see for instance patent US 3,594,434:
www.google.com/patents/US3594434 (click on Pdf for images)

[3]-syn-Ladderdiene and spirohex-1-ene outperform cyclooctatetraene and cubane, but their cost and ease of use are unclear, while dispirooctadiene may not even exist.

Marc Schaefer, aka Enthalpy
Logged
Pages: [1]   Go Up
 

Mitch Andre Garcia's Chemical Forums 2003-Present.

Page created in 0.078 seconds with 23 queries.