4.1. Theoretical Contributions
In the turn to the twentieth-first century a lot of questions regarding ‘gravity control’ had not been answered, thus at least three projects were running simultaneously.
The first is Project Greenglow (The Quest for Gravity Control), which started in the mid-1990s, and has run officially under management of the mathematician Ron Evans at BAE (British Aerospace) [
69,
70,
71,
72,
73].
For the second project, called GRASP (Gravity Research for Advanced Space Propulsion), it has been written that took place at Boeing Co. although the company said that did not spent money on that [
73,
74,
75]. The latter is related with Eugene Podkletov for sure but not much info is available in the public domain [
76].
Regarding the third project, in 1996, a team of (USA) government, university and industry researchers proposed a program to seek the ultimate breakthroughs in space transportation: propulsion that requires no propellant mass, propulsion that can approach and, if possible, circumvent light speed, and breakthrough methods of energy production to power such devices [
77]. In the period 1996-2004, NASA funded the Breakthrough Physics Program, managed by Marc Millis (open access reports are [
77,
78,
79]), and the very final report was documented in a book of 740 pages [
80].
Within the context of the abovementioned NASA’s project, it was concluded that the ‘mechanical antigravity’ is impossible thus a particular report was released in December 2006 to prevent future inventors from disturbing them [
16].
Despite the suggestions by NASA, the USA Patent Office continued to grant patents related to the term “antigravity” or other closely related ones. An internet search in Google Patents in January 2024 shows about 10,000 entries on the word “antigravity”, 7,777 entries for the word “gravity control”, 17 entries for the word “antigravity propulsion” and another 18 entries for the word “reactionless propulsion”. Considering that more than 20 alternative principles of physics can be used to achieve propulsion (as has been documented in [
80,
81]), of which one such methodology is the inertial propulsion, it becomes evident that this review paper cannot cover the whole topic. Instead of this, the present paper restricts to the use of masses and gyroscopes and occasionally refers to other methods only when it makes sense.
As already mentioned, the conventional Dean-drive consists of two contra-rotating out-of-balance masses (eccentrics) thus each mass traces out a complete closed circular path, when the cart is still immobile. As a result, as the mass rotates the impulse of the inertial (centripetal) force given in the upper half of this circle is cancelled by the impulse of the lower half of the same circle. To restore this shortcoming and break the symmetry (upper equals lower), some inventors have tried to modify the lower part (see, e.g., Hoshino [
82] and
Figure 7) or to introduce a secondary motion on the articulation of the rotating masses on the vehicle or to modify the ideally symmetric shape of the curve [
83,
84] thus causing a sort of time delay (as they claim).
Another way to modify the symmetrical shape of the circle traced by the two contra-rotating masses is as follows. A patent filled in October 2008 [
85], smoothly modifies the lower part of the circular path of the rotating masses, using a special differential mechanism was explained in [
86,
87]. In more detail, the rotating masses trace a figure-eight-shaped path (the symbol of infinity, ∞) which is also inscribed into a half-spherical surface. This configuration (i) preserves the motion of the rotating masses in the upper part (either above or below the center of the sphere) and (ii) allows for the orientation of the axis of symmetry (of the said half-spherical surface) toward the desired direction of motion. But although this setup introduces a sort of asymmetry, it is obvious that after every 360 degrees of rotation (at angular velocity
) the masses always take the same initial position thus the time integral of the inertial force over a period vanishes. To overcome this shortcoming, the axis of symmetry of the said half-spherical surface is subject to a second rotation (at angular velocity
) thus fully breaking the symmetry except of the case that its angular velocity is a multiple of the angular velocity of the contra-rotating masses (i.e.,
, where
k is an integer number) (for details, see [
87]). Nevertheless, despite the title of that paper, and even though it is unlikely that two upper points of the traced curve will be at the same altitude (height) on the vertical
-axis, during the almost chaotic motion of each rotating mass there will always two successive points along the curve (say the first point is the local lowest the next will be the local highest) at which the horizontal velocity component will both vanish (see,
Figure 8). In conclusion, despite the title of [
87], within the context of Newtonian mechanics, it is not possible to continuously obtain net thrust.
It has been widely written that the most simplistic way to break the symmetry is to force the rods of the rotating masses to trace a circular arc of only 180 degrees, like car wiper blades. However, a mechanical study reveals vanishing impulse per period thus null net thrust (see,
Appendix A).
Another idea has come from the axial impact of an elastic beam (
Figure 9). It is well-known from the theory of elasticity, that when a Heaviside axial force
is exerted on an elastic bar it takes some time until the elastic wave reaches the fixation and then the reaction force becomes
double (i.e., equal to
) and remains constant during the
half period [
88]. This fact is not against Newton’s third law (action equals to reaction) because the total impulse is preserved (see,
Figure 9b). Based on this observation, it was though that the rectification of the axial force would be possible if the beam was rotating with a period equal to the period of the elastic wave. The initial thought was that the dead time-interval in which the reaction force vanishes could correspond to positions of the beam below the horizontal line passing through the articulation point
O, thus when the reaction force becomes non-zero then the beam would be found in the upper half space and would be pushed upwards. Unfortunately, this supposed “brilliant” idea did not work for the following two reasons: (i) because the reaction force is internal to the system and (ii) because a Coriolis force cancels it action. And since the Coriolis force does not exist in electromagnetics, a mechanical model of the Dean drive was compared with a Tesla-like electromagnetically based model [
89] (a paper approved by
Boeing Co.). In other words, both models, i.e. the mechanical Dean drive (based on circular track) and the electromagnetic model showed null thrust. A report describing the atmosphere of a relevant Conference meeting, with the eyes of a specialized journalist, is [
90].
The reader may easily find many Internet (e.g., YouTube) movies in which a black box swims or jumps up by itself or a cart moves aided by a sort of inertial drive. In other words, the inertial propulsion is an experimental fact, but its limits had not been explored, or at least had not been published until the end of the first decade of 2000. The only “scientific” work is that of Davis [
30] in which a ‘fourth law of motion’ is proposed (in addition to the three ones by Isaac Newton). On the other hand, one may still find a lot of (pseudo-scientific) explanations that have been posted to the Internet.
To the best of our understanding, till the end of the twentieth century the literature was reduced to rather the description of existing patents and/or qualitative characteristics of them [
91,
92,
93,
94]. In the beginning a the 21rst century (2006), a PhD thesis focusing on some existing inertial propulsion patents, without encouraging results on the possibility of net thrust, was defended [
95]. Another relevant paper is [
96].
To show the capabilities of the conventional Dean-drive (let us call it so) operating into the air, Provatidis [
97,
98,
99,
100] published his first four papers considering that the rotating masses track circular curves (the simplest case). To make robust conclusions, several formulations were applied such as (i) decomposition of the mechanical system into free body-diagrams and further application of Newton’s laws, (ii) study of the center of mass, (iii) application of Lagrange equations, and (iv) conservation of linear momentum. Obviously, all these four approaches have led to the same result, which for ‘motion in the air’ is as follows:
An inertial drive attached to a vehicle or cart, which initially lies on the ground, causes alternating (sinusoidal) support forces on it. For an immobilized vehicle, the total linear momentum of the contra-rotating masses varies in time and its temporal derivative equals to the vertical support force (ground reaction exerted on the vehicle or cart). For the continuous motion of the contra-rotating masses at a constant angular velocity , external energy is generally required to withstand the friction loses [
97].
When the magnitude of the constant angular velocity, , is adequately high, the vehicle (cart) can perform a vertical jump. This happens because in the upward motion of the rotating masses (i) the reaction force is higher than the weight, and (ii) the center of mass of the system (cart + rotating masses) has an adequately large initial velocity which allows for a vertical shoot.
An alternative explanation for the motion of the vehicle due to the attached inertial drive is as follows. In the beginning the rotating masses of the inertial drive possess a certain linear momentum toward the vertical -axis. When the orientation of the connecting rods (radii of out-of-balance masses) becomes vertical, the velocity vectors of these masses become horizontal thus the linear momentum of the rotating masses vanishes. If -for example- the angular velocity is high, the change of linear momentum per revolution () is a small percentage of the total initial value, thus practically the linear momentum of the system is preserved. Due to the said conservation of linear momentum in the vertical -axis, the lost momentum is undertaken by the vehicle. But since after 90 degrees the connecting rods will become horizontal with peak velocities, the instantaneous velocity of the vehicle vanishes, and so on.
Obviously, if no extra energy is transmitted to the inertial drive, the initial angular velocity of the rotating masses cannot be preserved at a constant value
but again the vehicle can jump [
98].
The maximum height the mechanical system “vehicle + drive” can reach depends on the initial velocity of the center of mass of this system.
The initial velocity of the center of mass occurs when the two connecting rods to which the masses are attached are found on a horizontal position and at the same time the ground suddenly opens like the cover of a well. Then, the conservation of the linear momentum toward the vertical axis is ensured [
98]. Again, it should become clear that while the vehicle stands on the ground the linear momentum is not preserved.
Depending on the level of the initial velocity, the vehicle may elevate following an oscillating mode with the rods having performed usually a lot of revolutions, until the center of mass takes a zero value. Then the vehicle starts falling, again elevate following an oscillating mode until it takes its initial velocity in the opposite direction.
During an extremely short time interval, it is possible to keep the vehicle immobile into the air. This phase ends when the rods which carry the rotating masses become vertical, thus the denominator of a closed-form expression vanishes, and the fraction becomes infinite [
97].
In some sense, the sinusoidal support forces are very similar to those exerted on the ground by a spring-mass system [
100]. To better understand this issue, note that when the topic of oscillations is presented in high-schools or colleges, teachers say that the oscillation is the projection of a moving material point on a circle determined by the extreme positions of that oscillation.
Therefore, the center of mass of the system performs a vertical shoot, but also an oblique shoot is possible [
98].
From the above conclusions, one may understand that an inertial drive practically offers the initial velocity to the center of mass (as far as the vehicle presses the ground), which velocity could be alternatively given through a catapult. In conclusion, within the context of inertial propulsion the inertial drive practically replaces the catapult.
Now, the above observation can justify the ‘peculiar’ behaviour of an untransparent box within which an inertial drive exists, i.e. that the said ‘magic’ box may perform an unexpected upward jump, vertical or oblique, or even to climb an inclined plane. The only condition is that when the box is left to move or fall, the connecting bars (carrying the rotating masses) must be close to the horizontal direction to ensure linear momentum in the upward vertical -direction.
Similar conclusions may be derived for a floating object on water. Since the influence of the gravity is indirect (it affects only the weight), the difference with the previous case is that now a water resistance appears, depending on Reynolds number. The boat travels a certain distance on the water and when it reaches a maximum distance from the starting point it practically stops (it performs a slight oscillation). A publication dedicated on this issue is [
101] accompanied with some movies.
Of major practical importance is the motion on the ground. Typical cases are the possible motion of a washing machine or the motion from the bumper of a mobile cellphone. In general, the inertial drive induces centripetal forces which may overcome the static friction thus causing motion. This is a rather old story which has been applied to vibrational transportation [
7,
8,
9,
10,
11], and to micro- and nano-robots [
102,
103,
104,
105,
106,
107]. Later, the concept of Dean drive (although not clearly mentioned therein) was applied to the modeling and control of micro-robotic systems by Vartholomeos and Papadopoulos [
108,
109]. Quite independently, the theoretical capability of Dean drive to perform motion of a vehicle has been further elucidated by Provatidis [
110,
111] and a particular study for an alternative figure-eight shaped drive was reported in [
112]. Also, a state-of-the-art report until 2011, including more propulsive methods than those by inertial drives, is [
113,
114].
In August 2015, Mike Gamble, a senior electrical engineer at Boeing Co., presented a released paper regarding the history of Boeing (CMG) Control Moment Gyros [
115]. His presentation was mainly a non-technical pictorial history of Boeing’s CMG work, which started back in the 1960s and continued into the 1990s. He got involved with it in 1995 when he took over operations of the (GN&C) Guidance, Navigation and Controls lab at the Boeing Kent (WA) Space Center. This lab and the building that housed it were badly damaged in the 2001 Seattle earthquake and later demolished. The pictures in his presentation show many of the different types of test articles built and used along with some of the lab facilities.
In the same Conference Mike Gamble presented a second paper [
116], from which the extracted chart of
Figure 10 shows the sawtooth input torquing rate waveshape (scissoring) for generating the pulsed output force. He has claimed that torquing fast in one direction and slow in the other, generates a pulsed (average) output force which is similar in waveshape to that of the examples based on rotating masses.
Remaining within the field of gyroscopes, a remarkable work is that by Alexander Dmitriev, a former professor at the University of Saint Peterburg in the laboratory of Optics, who has shown that a horizontally spinning rotor loses more weight the faster it spins [
59,
60,
61,
62,
63]. Based on these experiments as well as of those by V.N. Samokhvalov [
117,
118], the American mathematician Dennis Allen, Jr., has been highly influenced thus he has developed a new theory according to which Newton’s Second Law has to be revised by modifying the mass to its updated form:
where
are experimentally determined constants,
is an angle in degrees,
is the magnitude of the acceleration vector and
is the magnitude of the surge vector (i.e.
). Details may be found in a Chapter of his revised
book jointly with the senior lecturer Jeremy Dunning-Davies [
119].
Until now the review has reduced to rotating masses
and gyroscopes. Since the friction causes a fast decay in both, there are some
patents introducing the concept of the pendulum in which the decay is much
smaller [
120]. A particular patent of the
talented inventor Mr. Veljko Milkovic in which an inclined pendulum could offer
inertial propulsion to a cart was studied by Allen and Provatidis [
121]. The latter study was based on mechanics and
computer methods that had been previously developed and documented in [
122,
123].
4.3. Other Patents and Broadcasts
Since the period of “Dean drive”, a lot of somehow relevant patents have appeared (see, Hoshino [
82], Cook [
128], Thornson [
129], Foster [
130], Robertson [
131], Chung [
132], Tanner [
133], Farral [
134], among others) and have attracted the interest of technicians and scientists, mostly in the United States of America (USA), perhaps because this attempt is also encouraged by established scholars such as [
135,
136].
Furthermore, it has also even occupied educational television programs such as the German Chanel 2 (Zweiter Deutsche Fernsehen: ZDF), in which the Austrian mechanical engineer Johann Klimpfinger has participated on February 26, 2012, in a 30 minutes-documentary on oscillating devices and applications such as toys [
137].
In Austria, there are relevant publications (e.g., Hilscher [
138]) and YouTube videos such as (Gyro Precession Drive) [
139].
In the United Kingdom the influence of Eric Laithwaite is still alive. The author has been recently informed that William Stoney, owner of the “
Thermo Inertial Research” company, with background aircraft prototyping and business jet flying, has met Eric Laithwaite and Bill Dawson in the late 1990s and have kept up an ongoing discussion with Bill Dawson who kindly gave him most of their experimental gyroscopes including the 14 kg gyro that featured in the BBC documentary. Since then, he has done many thousands of hours testing different configurations and measuring them. Currently he has filled a relevant patent [
140] in which an existing prototype is described.
But the most difficult-to-review information is coming from Russia, where (because of the language) it is not at all easy to distinguish science from pseudo-science. On this issue the author tries to remain neutral. For the sake of completeness, we will not avoid the temptation to expand the review a little beyond the narrow margins of inertial propulsion, but in no case can we fully cover the subject. As was mentioned in the beginning, the Russian academicians used inertial propulsion for practical reasons such as motion of heavy objects by vibrational techniques [
7,
8,
9,
10]. Nevertheless, they denied the capability of Dean’s drive to be useful for space applications [
33]. They did the same with their compatriot Vladimir Tolchin who introduced the term “inertiod” [
39].
As previously said, due to the Russian language, it is not an easy task to overview the topic in a complete way, but it seems that tens of unexplainable microscopic and macroscopic effects in natural sciences and especially in physics and biology have been revealed and investigated. It should be emphasized that a large part of these phenomena was demonstrated by objects having spin or angular momentum. Probably the first researcher who experimentally detected the unusual effects associated with torsion was professor Myshkin of the Russian physical-chemical society [
141], who at the end of the nineteenth century conducted a series of experiments using scales. These experiments were successfully repeated in the 1960s by professor N.A.Kozyrev [
142,
143] and V.V.Nasonov and later by V.S.Belyaev, S.P.Mikhailov, A.G.Parkhomov and others. For more details, the interested reader can consult an Internet website [
144], while an opposing reference for the more recent activities of this “School” is [
145]. Recent out-of-stream works are (Linevich and Ezshov [
146], Kouznetsov [
147], as well as Ventura & Shipov [
148] among others).
According to the Russian NTV correspondent Vladislav Sorokin [
149], in April 2, 2006, members of the ‘Suburban Institute of Space Systems’ invented the engine called ‘gravitsapu («гравицапу»)’, which, according to the creators, can accelerate to infinity; their final wish is to send it into space. However, the Russian Academy of Sciences did not recognize the theory underlying the creation of this invention. Later, in 2009, details were reported by the newpaper “Pravda” [
150]. For more relevant information the interested reader can consult Internet resources such as [
151,
152].
In China, the use of shaking masses has been explored for sure in terrestrial applications (Zhao et al. [
153,
154,
155]), while publications refer to novel electromagnetic drives as well [
156].