Metallic Hydrogen: The Most Powerful Rocket Fuel Yet To Exist

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Silvera, Isaac F. and John W. Cole. 2010. Metallic hydrogen: The most powerful rocket fuel yet to exist. In International Conference on High Pressure Science and Technology, Joint AIRAPT-22 & HPCJ-50 : [proceedings] : 26-31 July 2009, Tokyo, Japan. Journal of Physics Conference Series 215(1): 012194.
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Citation
Silvera, Isaac F. and John W. Cole. 2010. Metallic hydrogen: The
most powerful rocket fuel yet to exist. In International Conference
on High Pressure Science and Technology, Joint AIRAPT-22 &
HPCJ-50 : [proceedings] : 26-31 July 2009, Tokyo, Japan. Journal
of Physics Conference Series 215(1): 012194.
Published Version
doi:10.1088/1742-6596/215/1/012194
Accessed
January 29, 2017 9:39:25 PM EST
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http://nrs.harvard.edu/urn-3:HUL.InstRepos:9569212
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Metallic Hydrogen: The Most Powerful Rocket Fuel Yet to Exist 
 
Isaac F. Silvera1 and John W. Cole2  
1Lyman Laboratory of Physics, Harvard University, Cambridge MA 02138 
2NASA MSFC, Retired, Huntsville, AL 35801 
 
E-mail: silvera@physics.harvard.edu 
Abstract. Wigner and Huntington first predicted that pressures of order 25 GPa were 
required for the transition of solid molecular hydrogen to the atomic metallic phase.  
Later it was predicted that metallic hydrogen might be a metastable material so that it 
remains metallic when pressure is released.  Experimental pressures achieved on 
hydrogen have been more than an order of magnitude higher than the predicted 
transition pressure and yet it remains an insulator.  We discuss the applications of 
metastable metallic hydrogen to rocketry.  Metastable metallic hydrogen would be a 
very light-weight, low volume, powerful rocket propellant.  One of the characteristics 
of a propellant is its specific impulse, Isp .  Liquid (molecular) hydrogen-oxygen used 
in modern rockets has an Isp of ~460s; metallic hydrogen has a theoretical Isp of 1700 
s!  Detailed analysis shows that such a fuel would allow single-stage rockets to enter 
into orbit or carry economical payloads to the moon.  If pure metallic hydrogen is 
used as a propellant, the reaction chamber temperature is calculated to be greater than 
6000 K, too high for currently known rocket engine materials.  By diluting metallic 
hydrogen with liquid hydrogen or water, the reaction temperature can be reduced, yet 
there is still a significant performance improvement for the diluted mixture.  
 
 
Introduction 
Production of metallic hydrogen in the laboratory is one of the great challenges of high-pressure 
physics.  Not only is it predicted to have fascinating fundamental properties but also revolutionary 
applied properties. In this paper we discuss the progress towards producing metallic hydrogen and an 
application to rocketry as a powerful propellant. 
 
Over 70 years ago Wigner and Huntington [1] predicted that if solid molecular hydrogen was 
compressed to a pressure of 25 GPa it would have a dissociative transition from a molecular solid to 
an atomic solid with a half-filled conduction band so that it would be metallic. Later, Ashcroft [2] 
predicted that the putative metallic hydrogen might be a room temperature superconductor. It was also 
predicted by Ramaker, Kumar, and Harris [3] that high pressure molecular hydrogen would become 
metallic; a recent publication predicts high temperature superconductivity for the molecular phase [4]. 
Brovman, Kagan, and Kholas [5] showed that hydrogen would be a metastable metal with a potential 
barrier of ~1 eV.  That is, if the pressure on metallic hydrogen were relaxed, it would remain in the 
metallic phase, just as diamond is a metastable phase of carbon. However, Salpeter [6] showed that the 
metastability time of hydrogen might be short due to a tunnelling mechanism in which atoms on the 
atomic lattice tunnel into molecular states. Since metallic hydrogen is yet to be produced in the 
laboratory, none of these ideas have been tested. 
 
If metallic hydrogen is a metastable substance and can be economically produced in the 
laboratory then it would be the most powerful chemical rocket propellant in existence.  In the past 
such assertions were made because of the large energy of recombination and the very light mass of 
hydrogen; in this paper we demonstrate the advantages using detailed calculations.  In rocketry one of 
the important attributes of a propellant is its specific impulse Isp  (see ahead).  For example, 
 
 
 
 
 
 
(molecular) hydrogen-oxygen, currently the most powerful propellant, with an Isp ≈ 460 s, is the fuel 
for the main engine on the Space Shuttle.  A fuel with an increase of 50 s would have a significant 
impact on the payload of the Shuttle.  Metallic hydrogen has been cited as a fuel that could bring 
important improvements to rocketry.  Carrick [7] calculated the Isp of metallic hydrogen and found a 
value of ~1400 s.  In addition to the Isp advantage, metallic hydrogen is ~10 times denser than 
molecular hydrogen and probably would not need cooling as a cryogenic fuel, further reducing the size 
or increasing the payload. 
 
We decided to carry out a realistic analysis of the impact that metastable metallic hydrogen 
would have on the design of rockets propelled by this fuel using advanced codes for the analysis. With 
pure metallic hydrogen as the fuel, it was found that the temperatures in the combustion chamber 
would be too high for current construction materials used in rocket engines.  We then decided to dilute 
the metallic hydrogen to lower the combustion temperatures and analyzed mixtures of metallic 
hydrogen with molecular hydrogen and metallic hydrogen with water. Even with this dilution, 
revolutionary improvements would be made in rocketry, for example, a single stage to orbit.  In the 
following section of this paper we discuss the current status of efforts to produce metallic hydrogen 
using high-pressure techniques.  We then discuss rocket engines and conclude with designs of metallic 
hydrogen fuelled launch vehicles. 
 
The Status of Experiments to Produce Metallic Hydrogen 
The original predicted pressure of 25 GPa for the metallization of hydrogen was made early in the 
development of the quantum mechanics for many-particle systems.  Experimentally this pressure has 
been exceeded by more than an order of magnitude and hydrogen remains in the non-metallic 
molecular phase. Studies have been carried out on hydrogen and its isotope deuterium, as well as 
hydrogen deuteride. There are two methods of producing high pressures, static and dynamic. Almost 
all modern studies on solid hydrogen at static high pressures have been carried out in diamond anvil 
cells (DACs). Shockwave experiments achieve high pressures for very short periods of time, but the 
temperatures are thousands of degrees K and the hydrogen samples are in the liquid phase. In a 
dynamic shockwave experiment, Weir, Mitchell, and Nellis [8] observed liquid hydrogen to enter a 
conductive state, believed to be metallic at pressures of 140 GPa and temperatures of 2000 to 3000 K.  
The first very high-pressure DAC experiments were carried out at low temperatures and two 
new phases were discovered.  If we focus on para-hydrogen (or ortho-deuterium) then at lower 
pressures and down to T=0 K, the solids are in the hexagonal close packed (hcp) phase and the 
molecules are in spherically symmetric quantum mechanical states.  At high pressure (110 GPa for 
hydrogen; 28 GPa for deuterium) a phase transition to the so-called broken symmetry phase, or BSP, 
takes place in which the single-particle molecular states are no longer spherically symmetric and the 
molecules orientationally order their symmetry axes along crystalline directions.  At a still higher 
pressure of ~150 GPa another transition takes place to the so-called A-phase.  This phase is also a non-
conducting orientationally ordered phase.  A phase diagram from Cui et al [9] for ortho-deuterium is 
shown in figure. 1. The three phases discussed are sometimes referred to as I, II, and III. 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. 
 The high 
pressure phase diagram of 
ortho-deuterium 
showing 
the low pressure-LP, BSP 
and A phases.   
 
200
150
100
50
0
Te
m
pe
ra
tu
re
 (K
)
200
150
100
50
0
Pressure (GPa)
BSP (II)
D-A (III)
      LP (I)
 o-D2, IR, run 1
 o-D2, IR, run 2
 mixed crystal, Raman, run2
 mixed crystal, IR, run 2
 mixed crystal, Raman, Hemley et. al.
Phase Diagram
of Deuterium
 
 
 
 
 
 
The highest pressures on hydrogen, in the 300 to 400 GPa range, have been obtained by two 
groups.  Narayana et al [10] achieved pressures in the region of 340 GPa stating that hydrogen 
remained transparent. Loubeyre, Occelli, and LeToullec [11] found hydrogen to darken at 320 GPa as 
a band gap was closed through the visible. Both groups found that hydrogen was still in the molecular 
phase.  Extrapolations of data from Loubeyre, Occelli, and LeToullec indicate that the transition 
pressure is above 4.5 Mbar.  Some inconsistencies, as a result of the two groups using different 
pressure scales, have been explained by Silvera [12]. 
Since the work of Wigner and Huntington there have been numerous calculations of the critical 
pressure for the dissociative transition from molecular to solid atomic metallic hydrogen; many of 
these results have been reviewed (Silvera [13]). These calculations utilize various techniques such as 
density functional theory, quantum Monte Carlo, etc. Although quantum chemistry calculations have 
been developed to a high degree of sophistication, and in general, there is a close correlation between 
theory and experiment, this is not the case for hydrogen. Phase transition calculations that seek the 
structure with the lowest lattice energy have difficulty handling the zero-point energy contribution to 
the total energy and zero-point energy is very important in hydrogen. As a result, the predicted critical 
transition pressures have an enormous variation, from as low as 0.25 Mbar to over 20 Mbar, while 
recent predictions are in the 400 to 600 GPa range. 
In recent years a new pathway along the melting line has been under investigation.  Scandolo 
[14] predicted that at high pressure hydrogen would have a negative slope in its melting line.  This was 
followed by a detailed two-phase molecular dynamics study by Bonev et al [15] who showed that 
hydrogen should indeed have a peak in its melting line.  They extrapolated their curve with a negative 
slope to very high pressures and zero Kelvin, indicating that hydrogen may melt from the molecular 
phase into the metallic phase, so that in the 400 to 600 GPa range hydrogen would be an atomic liquid 
at T=0 K, shown in figure 2.  Attacalite and Sorello [16] have calculated a point at high pressure and 
lower temperatures that is in agreement with the extrapolation.  Babaev, Sudbo, and Ashcroft [17] 
considered the possibility of two component superconductivity with itinerant electrons and protons.   
On the experimental side, measurements of the melting line by Datchi et al [18] and Gregoryanz 
et al [19] found curvature in the line, indicating that a peak might exist.  Experimental challenges 
prevented them from extending the melt line to higher pressures and temperatures to investigate this 
possibility.  Recently Deemyad and Silvera [20] used pulsed laser heating and found a sharp peak in 
the melt line at 64.7  GPa and 1050 K, shown in figure 3.  Eremets and Trojans [21] have recently  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure  2.  Phase  diagram  of  hydrogen  showing 
extrapolation of  the melting  line  (below 700 K) 
to very high pressure. The melting line of Bonev 
et al. was carried out to 700 K. The recent work 
of Attaccalite and Sorella (filled circle) confirms 
the extrapolation to 400 K. 
1000
800
600
400
Te
m
pe
ra
tu
re
 (K
)
200
150
100
50
Pressure (GPa)
 Deemyad&Silvera
 Datchi et al.
 Gregoryanz et al.
 Bonev et al. Melt Line
 Bonev et al liq.-liq transition
 Eremets&Trojans
Figure 3.  Experimental measurements of 
the melting line, compared to theory.  
The solid line (upper right, green) is the 
liquid-liquid molecular to atomic line. 
 
 
 
 
 
 
extended the melt line to higher pressures and confirm  the existence of a peak.  Efforts to study 
hydrogen and its isotopes at still higher pressures, aimed at the transition to metallization, continue. 
 
Rocket Engines 
A liquid rocket engine is fundamentally a quite simple device.  Liquid fuel and oxidizer are injected 
into a reaction chamber where combustion releases the chemical energy producing a hot gas.  The hot 
combustion products exit the chamber at fairly high pressure through a throat and then expand out of a 
nozzle.  In thermodynamic terms as the gas expands the random motion of the gas particles in the 
combustion chamber are converted via collisions with each other and the nozzle walls into directed 
motion in the direction opposite of the acceleration of the rocket.  In the moving frame of the 
expanding gas the temperature (random component of the molecular motion) decreases.  The 
expansion can only proceed at the speed of sound, which is a function of temperature.  Constrained by 
the nozzle walls the moving gas expands at the speed of sound relative to the local speed in the gas.  
So the expanding gases push the gases in front of them that are also expanding, pushing the gases in 
front of them, etc., without locally exceeding the speed of sound constraint.  This result is an exhaust 
that can be very supersonic and directed optimizing the transfer of momentum to the rocket.  The 
kinetic energy from the exhaust represents the energy that can be obtained from the propellant by the 
engine for acceleration.  Lighter molecular weight exhaust products provide higher exhaust velocities 
for a given initial chamber temperature. 
The fuel and oxidizer must be injected into the chamber at a pressure higher than the pressure in 
the chamber.  In large rocket engines this pressure is obtained by pumps powered by turbines that use 
some of the vehicle fuel and oxidizer as their energy source.  Some rocket engines dump the turbo-
pump exhaust overboard so this propellant does not directly contribute to the engine thrust.  More 
advanced engines operate the turbo-pumps at very high pressure and the high-pressure exhaust is 
injected with the fuel into the combustion chamber.  This staged combustion approach allows most of 
the turbo-pump energy to be recovered improving the thrust and Isp.  Liquid rocket engine design 
techniques have progressed to the point that only minor improvements in performance are now 
achieved. These improvements come from lighter weight high temperature materials and from 
computer aided design and computational fluid dynamics to design more efficient pumps and 
injectors.  First-stage propellants are generally liquid oxygen and kerosene which has about half the 
energy content of the liquid oxygen and hydrogen upper stage propellants.  Hydrazine and nitrogen 
tetroxide provide slightly more specific energy than liquid oxygen and kerosene, as does liquid oxygen 
and methane.  A safe and affordable propellant with very high specific energy and with light-weight 
exhaust products would enable substantial improvements in rocket performance. 
 
Metallic Hydrogen Launch Vehicle Application 
Above some critical temperature the metastability of metallic hydrogen is overcome and the atoms 
recombine into hydrogen molecules releasing the energy of recombination, 216 MJ/kg.  This is more 
than twenty times the specific energy released by the combustion of hydrogen and oxygen in the 
Space Shuttle’s main engines, 10 MJ/kg.  Because of this very large potential specific energy, it is 
interesting to explore the possible use of metallic hydrogen as an energy source for rocket powered 
launch vehicles.  Since metallic hydrogen has not yet been produced and the material has only been 
characterized theoretically, some assumptions must be made to investigate its potential use in a rocket 
vehicle.  We assume that metallic hydrogen is a metastable solid or liquid at ambient conditions, that it 
is compatible with launch vehicle propulsion environments (vehicle vibrations, pumps, valves, etc.), 
and that it can be safely and affordably produced and handled in large quantities.  We also assume that 
it has a density of 0.7 gm/cm3 (liquid H2 density is about 0.07 gm/cm3) and that it’s metastability is 
overcome at around 1000 K for 40 bar pressure. 
We have considered pure metallic hydrogen and diluted mixtures, to reduce the combustion 
temperature.  Diluting the atomic hydrogen with water or with molecular hydrogen will reduce the 
 
 
 
 
 
 
chamber temperature to reasonable material limits while still delivering a high specific energy, 
.  
A rocket engine converts the energy available from the fuel into the kinetic energy of the exhaust,  
 
 
 
                                                                     
                   (1) 
where 
 is the exhaust velocity, E  is the available kinetic energy in the fuel, and 
 is the 
propellant mass.   Rocket scientists prefer to use specific impulse, Isp, rather than specific energy when 
discussing rocket engines.  Specific impulse represents the number of seconds that a thrust of one 
kilogram of force can be sustained by one kilogram mass of propellant, and is related to the square 
root of the specific energy,  
 
 
 
 .                                           (2) 
Here 
 is thrust, 
 is propellant flow rate, and 
 is the gravitational constant from the kilogram of 
force definition.   
We have used the NASA Glenn Chemical Equilibrium Calculation (CEC) [22] to evaluate the 
properties of the rocket engine. We find that a reaction chamber filled with atomic hydrogen at 100 
atm of pressure will reach recombination/dissociation equilibrium temperatures much greater than 
7000 K where ~50% of the hydrogen remains dissociated. This would provide a theoretical specific 
impulse of 1700 s, but clearly, the temperature is higher than any known chamber material can 
withstand.  Even at 40 atm pressure at equilibrium a temperature of ~6700 K will develop, far above 
the melting point of available rocket engine materials. 
Diluting the atomic hydrogen in the rocket chamber with water or with molecular hydrogen will 
reduce the chamber temperature, Tc, to reasonable material limits while still delivering a high specific 
impulse.  To achieve chamber temperatures between Tc = 3500 – 3800 K water diluents will provide 
 = 460- 540 seconds and hydrogen diluents will provide 
 = 1030 – 1120 seconds.  For 
comparison, as mentioned earlier, the Space Shuttle main engines provide an 
 ~ 460 seconds. 
The next step in conceptualizing metallic hydrogen propelled launch vehicles is to determine the 
size of each stage required to deliver a payload to a desired orbit or location.  For this we need the 
rocket equation. The momentum carried away by a small amount of rocket exhaust provides a reactive 
thrust on the vehicle that adds a little momentum to the vehicle.  From conservation of momentum, the 
changes in momentum must balance: 
 
 
 
 ,                                                          
                    (3) 
 
where dm is the differential vehicle mass (exhaust), m is the vehicle instantaneous mass, and 
 is 
the differential vehicle velocity.  Integration of equation 3 yields the Tsiolkovsky rocket equation 
which can be written 
 
 
 
                                                                      (4) 
 
where m0  is the initial mass, mi  is the injected mass (stage dry mass plus stage payload at stage burn 
out), and 
 is the change in ideal velocity (including velocity losses from drag, gravity, steering, 
etc.).  Equation 4 can be used to determine the mass ratios of each stage given a mission 
 
requirement and an 
 for each stage.  Propellant densities and mixture ratios permit the individual 
tank sizes to be determined with appropriate margins for engineering and safety. 
 
 
 
 
 
 
Because of the limited specific energy available from current propellants there are no existing 
launch vehicles that can reach orbit with a single stage.  A single stage to orbit would greatly reduce 
complications and cost of such missions.  Recently we calculated the size of several single-stage to 
orbit launch vehicles that deliver 25 MT (metric tons) of payload to low earth orbit with a 
 = 9.2 
km/s, figure 4 [23].  The vehicle is a little smaller than the Shuttle but significantly lighter in weight 
(see figure 4) and delivers the payload with only one stage.  The metallic hydrogen is carried in a 
small spherical tank inside the liquid hydrogen diluents tank. 
 
In another analysis we examined launch vehicles with two stages that deliver 35 MT of cargo to 
the lunar surface requiring a 
 = 16.4 km/s, figure. 5.  For this launch vehicle the 1st stage is cooled 
with water diluents and provides 45% of the mission
, the 2nd stage is cooled with hydrogen 
diluents, and both stages are 7.5 m in diameter.  With a vehicle that is slightly taller, but lighter than 
the Shuttle (shown only for size comparison), a much larger payload is delivered to a much more 
challenging destination.  Once again, because of propellant specific energy limits there are no current 
two-stage vehicles that can land payloads on the lunar surface. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4.  Sketch depicting the sizes of liquid hydrogen cooled metallic hydrogen single-stage 
to orbit (SSTO) launch vehicles that can deliver 25 metric tons to low earth orbit (LEO), 
compared to other vehicles. Important properties are tabulated, including the gross lift-off 
weight (GLOW). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Yet another analysis examined two-stage launch vehicles that deliver 30 MT into a 
geosynchronous transfer orbit (GTO) that will carry the payload to geosynchronous orbit altitude but 
not provide orbit circularization or plane change to achieve an equatorial inclination.  The vehicle and 
engine characteristics are the same as the lunar cargo mission except for the payload mass, the mission 
 = 11.7 km/s, the vehicle diameter of 5.5 m and the resulting vehicle height of about 53 m.  The 
initial mass for the vehicle concepts ranges from 242 to 323 MT.  For comparison Japan’s H-IIA 
vehicle is 53 m tall, 4 meters in diameter, Isp ~ 450 s, with an initial mass of 285 MT and delivers 
about 2 MT into GTO, figure 6.  This vehicle that utilizes liquid hydrogen-oxygen for both stages plus 
some 
solid 
launch 
assist 
motors 
can 
be 
seen 
in 
detail 
at 
its 
website 
(http://www.jaxa.jp/projects/rockets/h2a/design_e.html).  Thus, the metallic hydrogen propelled 
launch vehicles can deliver about 15 times the GTO payload for similar sized conventional launch 
vehicles. 
Metallic hydrogen propelled vehicles clearly can revolutionize rocketry.  The first step in this 
direction will be to prepare metallic hydrogen in small quantities in the laboratory and test to see if the 
assumptions used in this analysis are valid. 
 
 
 
 
Figure 5.  Two stage launch vehicle concepts that deliver a 35 metric ton payload plus an 
empty 2nd Stage to the Lunar Surface.  Chamber temperatures between these concepts vary 
from 3500 K to 3800 K.  The metallic hydrogen for the 1st stage is diluted with water, and for 
the 2nd stage with liquid hydrogen. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Acknowledgments 
One of us, IFS, acknowledges support for this research from the National Science Foundation, grant 
NSF DMR-0804378. 
References 
 
[1] Wigner E, Huntington HB. On the Possibility of a Metallic Modification of Hydrogen. J Chem 
Phys. 1935;3:764-70. 
[2] Ashcroft NW. Metallic Hydrogen: A High Temperature Superconductor? Phys Rev Lett. 
1968;21(26):1748-9. 
[3] Ramaker DE, Kumar L, Harris FE. Exact-Exchange Crystal Hartree-Fock Calculations of 
Molecular and Metallic Hydrogen and Their Transitions. Phys Rev Lett. 1975;34(13):812-4. 
[4] Cudazzo P, Profeta G, Sanna A, Floris A, Continenza A, Massidda S, et al. Ab Initio 
Description of High-Temperature Superconductivity in Dense Molecular Hydrogen. Phys Rev 
Lett. 2008;100:257001. 
[5] Brovman EG, Kagan Y, Kholas A. Properties of Metallic Hydrogen under Pressure. Sov Phys 
JETP. 1972:783-7. 
[6] 
Salpeter EE. Evaporation of Cold Metallic Hydrogen. Phys Rev Lett 1972;28(9):560-2. 
[7] Carrick PG. Specific Impulse Calculations of High Energy Density Solid Cryogenic Rocket 
Propellants 1: Atoms in Solid H2: Phillips Laboratory; 1993. Report No.: PL-TR-93-3014. 
Figure 6.  Metallic hydrogen propelled two stage launch vehicle concepts that deliver a 30 MT 
Payload into a Geosynchronous Transfer Orbit.   The vehicles have various chamber 
temperatures determined by the dilution ratio.  Stage 1 is diluted with water and stage 2 is 
diluted with liquid hydrogen. 
 
 
 
 
 
 
 
[8] Weir ST, Mitchell AC, Nellis WJ. Metallization of Fluid Molecular Hydrogen at 140 GPa (1.4 
Mbar). Phys Rev Lett. 1996;76:1860-3. 
[9] Cui L, Chen NH, Jeon SJ, Silvera IF. Megabar pressure triple point in solid deuterium. Phys Rev 
Lett. 1994;72:3048-51. 
[10] Narayana C, Luo H, Orloff J, Ruoff AL. Solid hydrogen at 342 GPa: no evidence for an alkali 
metal. Nature. 1998;393:46-8. 
[11] Loubeyre P, Occelli F, LeToullec R. Optical studies of solid hydrogen to 320 GPa: evidence for 
black hydrogen. Nature. 2002;416:613-7. 
[12] Silvera IF. Hydrogen under Pressure; Bizarre Behavior of a Simple Quantum Solid.   
 
AIRAPT; 
2005; 
Karlsruhe, 
Germany: 
Forschungszentrum 
Karlsruhe 
http://bibliothek.fzk.de/zb/verlagspublikationen/AIRAPT_EHPRG2005/. 
[13] Silvera IF. Metallic Hydrogen. In: Edwards PP, Rao CNR, eds. Metal-Insulator Transitions 
Revisited. London: Taylor and Francis 1995:21-42. 
[14] Scandolo S. Liquid-liquid phase transitions in compressed hydrogen from first-principles 
simulations. Proc Nat Acad of Sciences. 2003;100:3051-3. 
[15] Bonev SA, Schwegler E, Ogitsu T, Galli G. A quantum fluid of metallic hydrogen suggested by 
first-principles calculations. Nature. 2004;431:669-72. 
[16] Attaccalite C, Sorella S. Stable Liquid Hydrogen at High Pressure by a Novel Ab Initio 
Molecular-dynamics Calculation. Phys Rev Lett. 2008;100:114501. 
[17] Babaev E, Sudbo A, Ashcroft NW. A superconductor to superfluid transition in liquid metallic 
hydrogen. Nature. 2004;431:666-8. 
[18] Datchi F, Loubeyre P, LeToullec R. Extended and accurate determination of the melting curves 
of argon, helium, ice (H2O), and hydrogen (H2). Phys Rev B. 2000;61:6535-41. 
[19] Gregoryanz E, Goncharov AF, Matsuishi K, H.K. Mao, Hemley RJ. Raman Spectroscopy of 
Hot Dense Hydrogen. Phys Rev Lett. 2003;90:175701-4. 
[20] Deemyad S, Silvera IF. Melting Line of Hydrogen at High Pressures. Phys Rev Lett. 
2008;100:155701. 
[21] Eremets MI, Trojan IA. Evidence of Maximum in the Melting Curve of Hydrogen at Megabar 
Pressures. JETP Letters. 2009;89:174-9. 
[22] McBride B, Gordon S. Chemical Equilibrium Program CEA2: NASA Glenn Research Center, 
Cleveland, OH; 2004. Report No.: NASA RP–1311, Part I, 1994, and NASA RP–1311, Part II,. 
[23] Cole J, Silvera IF, Foote JP. Conceptual launch vehicles using metallic hydrogen propellant. In: 
978 ACP, editor. STAIF-2008; 2008; Albequerque, NM; 2008. p. 977-84. 
 
 
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