Chapter 3 - Prerequisites For Space Drive Science

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Notes by David A Roffman on Chapter 3 of


Chapter by Marc G. Millis, NASA Glenn Research Center, Cleveland, Ohio

   A space drive is a device that uses indigenous mass as a means to propel itself.  With that in mind, the main issue with a space drive is, what can is the reaction mass?  The answer to this question can vary, but there is no apparent winning solution at the moment.  Galactic Hydrogen, Dark Matter, Dark Energy, Cosmic Background Radiation, Quantum Vacuum Fluctuations, and other means are possibilities.  However, the mass densities for many of these are too minute.  Due to our lack of knowledge though, the small values (energy densities) provided in the book many be off by as much as 120 orders in magnitude (J/m3).  Ever hear of a google?  Well, imagine being off by that (a one with 100 zeros after it).  See Table 1 from Chapter 3 below:


Frontiers in Propulsion Science, Chapter 3, Table 1


Known forms of mass and energy


In terms of mass density, (kg/m3)

In terms of energy density, (J/m3)

Total matter in the universe (critical density)


Ω = 1.00

9.5 X 10-27

8.6 X 10-10

Dark Energy

L = 0.73

6.9 X 10-27

6.2 X 10-10

Dark Matter

DM = 0.22

2.1 X 10-27

1.9 X 10-10

Baryonic matter (normal matter)

B = 0.04

3.8 X 10-28

3.4 X 10-11

Photons and relativistic matter

rel = 8.3 X 10-4

7.9 X 10-31

7.1 X 10-14

Cosmic background radiation

CMB = 10-5



Quantum vacuum fluctuations



Inferred as dark energy



Up to nucleon Compton frequency (1023 Hz)



U to Planck limit (2043 Hz)



Galactic hydrogen

3.3 X 10-21

3.0 X 10-4

Spacetime itself


In terms of mass density

9.5 X 10-27

8.6 X 10-10

General Relativity analogy to Young’s modulus

5.3 X 1025

4.8 X 1042



     Ignorance is not the only problem.  A major issue with space drives is conservation of momentum.  With space nearly empty according to our incomplete measurements, we probably cannot easily use it as a medium.  Proposals for how to circumvent this involve using space-time as a means of propagation.  Not much is known here.  Some more problems are the lack of being able to incorporate general relatively into a frame-dependent version of Mach’s principle, and not knowing if Geometric or the Euclidean view is correct for relativity.  Mach’s principle states that an inertial frame is created by and connected to the surrounding matter in the universe.  If there is a universal reference frame, then it might be possible to use it as a propulsion source.

     When considering using space-time itself as a means of propulsion, one must consider how inflexible space-time is.  According to a calculation in the book, the stiffness (a measure of how much an object in strained as a result of distortion) is about 4.8 X 1042 N/m2 (Pa).  This very high figure indicates that space time is not so easily bent, and requires an enormous amount of energy or mass to do so.  Difficulties implied by our ignorance have left us in the “questioning” stage of the scientific method.

      There are some common concepts that must be discarded when considering the space drive.  One such notion is infinite specific impulse (ISP).  Just because the fuel supply is unlimited, doesn’t mean the same for ISP.  ISP =F/[g X (dm/dt)].  The derivative shown here means propellant mass flow rate.  What propellant is flowing from the original wet mass out?  The answer is none.  A space drive uses exterior resources as “fuel,” not interior (organic) ones that deplete (as shown in the standard rocket equation).  Because the denominator of the ISP equation is now worthless, so is the formula.

      Another standard equation that is now worthless, is E=.5 X Mp X (ISP X g)2.  It would state that a space drive would require infinite energy, or that zero energy would be required if there was no propellant.  This makes no sense, so the equation must go.  Cancelling out the effects of inertia on a craft is currently not possible, so inertial dampeners as seen in Star Trek and Star Gate are not possible.  The positive uses of altering inertia are currently not fully known.

     The Gravity Shield is asserted to be an infeasible option.  It has failed under numerous tests, and it would defy the Equivalence Principle.  This states that if gravitational mass is modified, then the inertial mass must be too.  Gravity would not necessarily be reduced by such a device.  The mass of the rocket above any such g shield could be altered, or gravitational mass could be altered.

     Maneuvers with a space drive are thought of in terms of kinetic energy (a space drive converts potential energy to kinetic), while a rocket’s maneuvers are thought of in terms of velocity increases.  The space drive hence has a different equation for energy input.  It is also true for non-relativistic flight (speed less than 10% that of light) that a space drive uses two to three times less energy than a rocket.  Specific impulse for the last situation has the space drive win by 150 orders in magnitude for a rendezvous mission, and 72 orders of a magnitude for a one way trip.  This data is for hypothetical discoveries pertaining to deep space flight. Space drive propellant type is not considered.

     Instead of pondering deep space flight like the last paragraph, Earth to orbit energies will be discussed.  The book is very brief here with mostly equations, but the bottom line is that the space drive is 3.65 times more efficient with energy use than rockets.  Thus the space drive has another win here. 

     In terms of comparing the space drive and the rocket engine for levitation, the space drive has an advantage because it expends no propellant, and doesn’t fall when it runs out of fuel.  How to view this levitation concept depends on how “force” is used in equation, amongst other factors.  An approach here could be to remove an object from a gravitational field (i.e., as if moving the object to infinity).  The book says to levitate a 1 kg object near the Earth’s surface would take 62 Mega joules (about twice the requirement for low Earth orbit).  But the book doesn’t specify how to do it, as the method is unknown.

     There are several hypothetical space drives.  FRONTIERS discusses ten versions in this chapter.  The Bussard Interstellar Ramjet collects galactic hydrogen for fusion use.  Two problems are introduced (aside from the fact that cold fusion is currently infeasible): the large amount of space needed to be covered to collect hydrogen and the need for a laser to start acceleration of the craft.  The later is probable as the jet cannot collect adequate hydrogen until it’s at the appropriate speed.  For the first, about 1024 m3 of space must be covered in order to collect 1 metric ton of hydrogen (Table 7 on page 152 of Chapter 3).

    The next section deals with multiple types of “sail” drives.  These include: Differential sail, Induction sail, and the Diode sail.  A concept familiar to all variations is the maximum speed of .99997c due to drag forces in space.  The differential sail uses the concept of an ideal radiometer.  This is a device that has photons push the sails in a vacuum.  In a pure vacuum (hence ideal) the sails push from white to blacks, with the white sails reflecting two units of momentum, and the black sails absorbing one unit of momentum.  However, if the environment has some air left over, and then the situation is that of a real radiometer.  The sails (in this case, paddles) are pushed the opposite way due to interactions with air particles.  The induction sails uses the last principle, by altering the energy-density of the medium.  Unlike the differential sail that becomes useless as equilibrium approaches, the induction sail doesn’t due to continuous energy flow.  The last sail is the diode sail.  It doesn’t have a problem with removing absorbed energy, because it is a one-way mirror.  However, the sail for this would be massive, about 100 million square kilometers.  As a final note, quantum energy might be used, but there is not much known here when pertaining to sails or to quantum energy in general.

   The book discusses inertia modification, but the main point is that the equivalence principle is the problem.  The main inertial modification device proposal is the oscillatory inertia thruster.  In the Woodward approach, inertia changes, not just position or velocity.  In this system a device “cyclically changes the distance between two masses, while the inertia of each mass is oscillating about its nominal mean so that the system as whole shifts its position to the right.”  There is an issue about conservation of momentum.  Millis asks whether inertia is an intrinsic property of matter only, or does it measure a relationship between matter and space-time?  He points out the need to revisit Mach’s principle.



     The field drive idea uses a field (gravitational, electromagnetic, etc.) to propel the craft.  There are two issues here.  Since the field completely surrounds the craft, all the forces would seem to be internal, and thus useless.  Also, conservation of momentum is a problem yet again.  There are four variants: (1) Diametric, (2) Disjunction, (3) Gradient Potential, and (4) Bias Drives. 

1.  Diametric Drive uses negative mass for propulsion.  This hypothetical concept is defined as an object that travels in the opposite direction of the force exerted on it.  It can have negative inertia. To continue, some terms must be defined: 

Inertial mass is a characteristic that defines the force and acceleration relationships. 

Active gravitational mass creates a gravitational field only. 

Passive gravitational mass reacts to a gravitational field. 

With regard to the diametric drive, electric charges are incompatible with it.  This is because all charges have positive inertia. Furthermore, the magnitude of a charge’s inertia is not directly linked to the magnitude of the electrical charge. The drive can work by creating a gradient by positive and negative point sources (negative mass and normal mass).  

2.  Disjunction Drive relies on separating passive and active mass.  To conserve momentum though, the active and passive masses will be accelerated toward each other.  However, if the two are separated by a sturdy device (so they can’t collide), then the whole system (both masses) will accelerate due to the gravitational force of the active mass on the passive mass.

3.  Gradient Potential Drive, unlike the last two, relies on a whole field being altered, and not just points.  The first problem is the net external force requirement.  Usually a field acts on a device and the field in question.  In this case though, it seems as if all forces are internal, and nothing would happen.  However, if a gradient can be created then, the craft would travel across it, and hence move.

4.  The Bias Drive alters the properties of space-time itself.  The textbook provides the “soap-boat” example.  By simply adding soap to a solution of water, the boat will move forward.  In this model, space itself is the reaction mass, much like how the soap is in the last example.

    It is important to note that not much is known in any of the last areas, and that the conservation of momentum and net-external force requirements are impediments.  Energy must also be conserved too.  Flight must be stable, and craft must be controllable.  Space-time itself must also be researched, as well as inertial frames.  Levitation must also be further studied.  To summarize, there is still a very long way to go in this area.


Additional Notes: Review paper on Negative Matter, Repulsion Force and Dark Matter by Yi-Fang Chang, Department of Physics, Yunnan University, Kunmin, 650091, China 


Note: There is no copyright on this chapter because it is a work of the U.S. Government.