Long-range UUVs

Abstract

In one aspect, the invention is a vehicle working in ambient-pressure liquid or gas medium; in a second, it is a fuel tank for such craft—usually with a housing and related propulsion; the tank is craft-mounted, at least partly outside all other parts, contacting the medium; and best at least partly pressure compliant; propulsion consumes fuel; drag depends on structure shape, and fuel volume. Shape decreases longitudinally and/or laterally, reducing drag with fuel use; housing is cylindrical, structure annular (outside the housing); drag depends partly on structure radial thickness, in turn on fuel quantity. Pressure may compress fuel against housing. The structure best is made of or has embedded skin-drag reducing material; materials (soaking-replenished) are best low-friction and/or high-compliance; the surface-noise-absorbing (for underwater craft) structure has shape and size reduction some 20 to 80% of like fixed-geometry craft.

Description

BACKGROUND

This invention relates to a long-range (or long-endurance) unmanned underwater vehicle (“UUV”) that is powered by chemical propellant(s)—e. g., Otto Fuel, hydrogen peroxide or other materials, including separate fuel and oxidizer combinations. In such a vehicle, a large percentage of the initial weight (and volume) is likely to be the propellant(s).

Underwater vehicles are generally designed for economical maneuverability in three dimensions. This calls for neutral buoyancy relative to the surrounding medium. That medium is most typically seawater, although some such submersibles are operated in freshwater, brackish water, etc.

If the vehicle has conventional, fixed geometry, then—as propellant is consumed—maintenance of neutral buoyancy requires replacement of propellant weight with seawater or the like. An alternative is to decrease the overall volume of the vehicle, modifying the geometry, as the vehicle consumes the propellant(s). Preferred embodiments of my invention (e. g. FIGS. 1 and 2) incorporate this volume change into the design of the vehicle itself.

As illustrated, in this first embodiment propellant (“fuel”) is stored in a collapsible bladder that surrounds the structural part of the UUV. Utilizing this invention can gain these significant benefits:

• • 1) increase the maximum range (more than 50% is achievable);
  • 2) eliminate the need for fuel pumps, by using the external pressure of the seawater or other medium;
  • 3) decrease the structural weight of the vehicle (more than 50% is achievable);
  • 4) decrease radiated noise, due to the shielding effect of the externally carried fuel;
  • 5) deter transition to turbulence and thus significantly decrease the skin-friction drag; and
  • 6) for military or other applications (e. g. mine-clearing) that call for demolition, ignite the residual fuel to provide additional destructive energy.

These are discussed in the following paragraphs and in Appendix A—which develops a simple analytic model for potential gain in range (or endurance) provided by the invention.

The form of my invention shown in FIGS. 1 and 2 is by no means the only way to achieve these benefits. One other configuration, for example, has a variable-volume fuel reservoir inside the vehicle structure, optionally even wholly out of contact with the surrounding medium in which the vehicle operates—but the vehicle structure is able to expand and contract, accordion-style or in the manner of a hydraulic jack, or analogously to the operation of a mechanical claw, or in any of numerous other ways that are well known in the mechanical arts. The pressure of the surrounding medium, however, is ideally transmitted by the expansible/contractible structure to the internal reservoir, so that the overall effect emulates the external reservoir of FIGS. 1 and 2—namely, the overall outside shape and size of the vehicle is automatically trimmed as the vehicle consumes the fuel.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a conceptual diagram of a preferred embodiment of the invention having an external collapsible bladder, for a time when significant fuel remains;

FIG. 2 is a like diagram, for a time near the end of a mission—i. e., when relatively little fuel remains.

FIG. 3 is a conceptual diagram of another preferred embodiment of the invention having instead an internal collapsible bladder but an external collapsible structure (e. g., accordion-like, or telescoping as shown) that preferably transmits ambient-fluid pressure to the bladder—analogous to that of FIG. 1 in representing a time when significant fuel remains;

FIG. 4 is a like diagram, but for a time when relatively little fuel remains;

FIG. 5 is a conceptual diagram like FIGS. 1 and 3, but for a variant configuration of the FIG. 1 embodiment; and

FIG. 6 is a diagram like FIGS. 2 and 4, but for the FIG. 5 variant.

CALCULATION OF THE GAIN IN RANGE, FOR PREFERRED EMBODIMENTS

Appendix A derives the results below—assuming a simplified model for the decrease in required power, as a function of decrease in drag from the decrease in vehicle volume. All these derivations assume a fixed ratio of the fuel volume (Vol_f) to the total vehicle volume (Vol_tot), and use the fixed vehicle-geometry case as a reference.

The variation in the vehicle radius with time r(t), compared to the initial radius r₀, will depend on the fuel-use rate, designated as ni_t. Ideally this parameter is selected according to the type of mission contemplated. Here consider three possibilities:

1. constant average velocity;

2. fixed mission time; or

3. constant power.

Each is discussed below.

1. Constant Average Velocity

It will turn out (see Appendix A) that the optimal strategy actually is to maintain constant velocity V, and the resultant r(t) profile becomes:

$\frac{r\left(t\right)}{r_0}={}^{\mathrm{ct}}$ $\mathrm{where}$ $c=\frac{-C_D}{4}\frac{{\rho }_f}{{\rho }_w}\frac{V^3}{\chi L}$

Further, χ represents the propulsion energy per unit mass of fuel—and includes the heat-energy output multiplied by the efficiency of the propulsion system. C_D is the drag coefficient based on frontal area, and L is the length of the vehicle. For this case, the gain in range of this invention compared to the case of constant geometry becomes:

$\mathrm{Gain}\equiv \frac{\mathrm{Range}\mathrm{for}\mathrm{Variable}\mathrm{Geometry}}{\mathrm{Range}\mathrm{for}\mathrm{Fixed}\mathrm{Geometry}}=-\frac{\ln \left(1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}\right)}{\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}}$

2. Constant Mission Time

Again it turns out that the optimum is to run at constant velocity; however, in this case the velocity should be higher for the new variable-geometry invention compared to the constant-geometry baseline. The radius profile is still given by the exponential above (with the higher Velocity). The gain in range in this case becomes:

$\mathrm{Gain}\equiv \frac{\mathrm{Range}\mathrm{for}\mathrm{Variable}\mathrm{Geometry}}{\mathrm{Range}\mathrm{for}\mathrm{Fixed}\mathrm{Geometry}}=\frac{{\left[-\ln \left(1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}\right)\right]}^{1/3}}{{\left[\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}\right]}^{1/3}}$

The resultant gain in range in this case is considerably less, due to the need to run at higher velocity to maintain the mission-time constraint. Finally consider the constant-power case:

3. Constant Power (or Equivalently Fuel-Flow Rate)

The r(t) profile in this case becomes:

$\frac{r\left(t\right)}{r_0}={\left(1-\mathrm{At}\right)}^{1/2}$

where A is the volumetric fuel-flow rate V{dot over (o)}l_f divided by the total vehicle volume Vol_tot. The gain in this case becomes:

$\mathrm{Gain}\equiv \frac{\mathrm{Range}\mathrm{for}\mathrm{Variable}\mathrm{Geometry}}{\mathrm{Range}\mathrm{for}\mathrm{Fixed}\mathrm{Geometry}}=\frac{3}{2}\frac{\left(1-{\left[1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}\right]}^{2/3}\right)}{\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}}$

In this case the maximum gain is limited to 1.5. Also, velocity increases as fuel burns—thus lowering gain relative to the case of constant average velocity (case 1 above).

Table 1 summarizes the above results:

TABLE 1
Gain as a function of fuel-volume ratio for the different constraints
	Volf/	Volf/	Volf/	Volf/	Volf/
	Voltot =	Voltot =	Voltot =	Voltot =	Voltot =
Constraints	0.4	0.5	0.6	0.7	0.8
fixed average velocity	1.28	1.38	1.52	1.71	2.11
fixed mission time	1.08	1.11	1.15	1.19	1.26
fixed power	1.08	1.11	1.15	1.18	1.23

Structural Considerations:

For a conventional underwater structure of cylinder-like geometry, the required structural thickness—to maintain a sea-level internal pressure in the interior—is proportional to the radius. With fuel stored in the collapsible bladder (FIGS. 1 and 2), the radius and therefore the required structural thickness is smaller by the factor below.

$\mathrm{Structural}\mathrm{Radius}\mathrm{Reduction}\mathrm{Factor}={\left[1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}\right]}^{1/2}$

Since the structural weight is proportional to the radius times the thickness, the weight factor becomes:

$\mathrm{Weight}\mathrm{Factor}\equiv \frac{\mathrm{Weight}\mathrm{for}\mathrm{Variable}\mathrm{Geometry}}{\mathrm{Weight}\mathrm{for}\mathrm{Fixed}\mathrm{Geometry}}=\left[1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}\right]$

Table 2 illustrates this result for different values of

$\frac{{\mathrm{vol}}_f}{{\mathrm{vol}}_{\mathrm{tot}}}\text{:}$

TABLE 2
structural reduction factors with the variable-geometry invention
Factor	$\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}=0.4$	$\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}=0.5$	$\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}=0.6$	$\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}=0.7$	$\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{tot}}}=0.8$
thickness	0.77	0.707	0.632	0.547	0.447
weight	0.6	0.5	0.4	0.3	0.2

These are clearly very large factors as the

$\frac{{\mathrm{vol}}_f}{{\mathrm{vol}}_{\mathrm{tot}}}$

increases for long-range missions.

Fuel-Pump Elimination:

For this invention, the external water pressure should be entirely sufficient to completely eliminate fuel pumps. Fuel flow to the conversion system can be controlled simply by operating valves (controllable openings). This reduces the weight of the volume of the internal components, increases the reliability, decreases the cost and—perhaps more important—in various underwater applications reduces a large source of radiated noise for the vehicle. Pumps should still be kept available, to maintain vehicle buoyancy and stability.

Transition to Turbulence:

The outer collapsible bladder, in preferred embodiments of my invention, will be very smooth due to the stretched condition of the bladder—strongly preferred for use with the invention. In addition, it is planned to form this bladder from materials that emphasize this smoothness and thus delay the onset of transition and turbulent flow over the body. Further, preferred design will work to reduce the effect of positive pressure gradients that increase the onset of transition to turbulent flow.

The surfaces of vehicles in the ocean can become contaminated with biological growth, such as barnacles. In operation this significantly increases the drag of the Navy's vessels. With the present invention, the outer bladder will be made of materials that can prevent this from becoming a problem. This is especially important in the case of long duration and very slow missions in the littoral zones. Finally, it is known that injecting certain polymers into the boundary layer can significantly reduce the skin-friction drag. In practice of the present invention an attempt can be made to “seed” the bladder with these polymers to reduce the drag coefficient even further: in operation such polymers—embedded into the external skin of the bladder material, are automatically drawn out by the forces between them and the surrounding medium. Later, further quantities of the same materials can again be soaked into the external skin to repeat (multiple times) the process with its drag-reducing benefits. Such use of boundary-layer polymers can equally well be employed in the internal-reservoir forms of the invention discussed earlier, i. e. injecting or soaking the polymers into the external claw, or accordion, etc. mechanism. I am not a materials scientist, but can suggest starting points for selection of the polymers here under discussion: Teflon of course is well known for very low friction, but may not be available in sufficiently compliant form for the variable-shape/size requirements of some forms of my invention; latex, conversely, is quite compliant but may not be available in sufficiently low-friction form.

Discussion of Other Aspects:

The general principle of reducing vehicle size, as the fuel (with oxidizer) is consumed, is applicable to a variety of geometries besides the simple one of FIGS. 1 and 2. If skin-friction drag dominates, then it will be desirable to seek an apparatus design that reduces the surface area more effectively than only as a function of the radius.

In addition to the application of the concept to unmanned underwater vehicles, the invention is also applicable to manned submarines, and to torpedoes and other underwater weapons, long-endurance ocean buoys or sensor systems that might use chemical energy for supplying required energy. The invention can also be applied to long-endurance and long-range airborne vehicles and surface ships. Although the advantage for airborne and sea-surface vehicles is smaller, reducing the drag by reducing surface and frontal areas—and thus drag—is always beneficial. The preferred embodiment of FIGS. 1 and 2 has the dual benefit of reducing both frontal-area drag and skin drag along, e. g., the entire surface of a vessel, or along a water surface. (Skin-friction drag rises with surface area, even if the frontal area is held constant.)

FIGS. 3 and 4, however, demonstrate that the invention does not depend upon having a fuel-containing bladder (or other structure) that is outside the vehicle structure. Here instead the bladder is inside, but the vehicle structure includes a collapsible/expansible outer shell preferably arranged to transmit ambient-pressure of the medium to the bladder.

In all or most embodiments of my invention, a significant point is to use the incompressibility (or at least relatively low compressibility) of the liquid fuel to bear the external pressure load, eliminating the need for additional structure that surrounds the fuel—and thereby eliminating the significant weight of such additional structure, for an underwater or like vehicle. A fundamental concept, for all or most embodiments, is to reduce the vehicle volume as fuel is consumed, thus maintaining preferably neutral or near-neutral buoyancy. The preferred embodiment of FIGS. 3 and 4 is particularly beneficial when skin-friction drag dominates over form drag.

The collapsing structure can maintain vehicle buoyancy as fuel is consumed, as in the previously discussed embodiment of FIGS. 1 and 2. Analysis of the potential relative benefit(s) can be performed analogously to that (illustrated in the Appendix) of the earlier discussion. The previously introduced benefits are also applicable to the embodiment of FIGS. 3 and 4.

Moreover, the concepts of the two pairs of drawings can be combined.

Specific examples and calculations presented in this document are based on reducing frontal-area drag; however, closely analogous derivations are applicable to reducing surface area. That is desirable for cases in which skin friction is a major (and especially a dominant) drag component. In addition to a continuously collapsible bladder storing fuel, other modes of volume variation—e. g., more-rigid sectional subcontainers that may be actuated stepwise, or staged in particularly advantageous sequences—are within the scope of this invention.

A refinement applicable to many preferred embodiments of my invention is incorporation of noise-absorbing materials. This is especially important for underwater vehicles, whether manned or unmanned, as so many applications of such vehicles call for stealth as a protective behavior.

Appendix: Calculation of the Gain (e. g. in Range):

The fuel-flow rate in terms of the drag of the vehicle, for the simple case of a cylindrical geometry with varying radius due to fuel flow, is:

${\stackrel{.}{m}}_f=\frac{\frac{1}{2}C_D{\rho }_wV^3\pi r^2}{\chi }=2{\mathrm{\pi \rho }}_fr\stackrel{.}{r}l$

—in which the overdot ({dot over ( )}) represents a time derivative and χ is the propulsion-energy output per unit mass of fuel (and oxidizer if any) that is used. This is the product of the inherent heat energy of the fuel and the conversion efficiency of the heat to propulsion mechanical energy. The radius r for this invention varies with time, due to decrease in volume of the outside fuel bladder as desired to maintain correct buoyancy while the fuel is consumed.

From the above equation, velocity V can be expressed as:

$V^3=K\frac{\stackrel{.}{r}}{r}$

where K is given by:

$K\equiv \frac{-4L·\chi \frac{{\rho }_w}{{\rho }_f}}{C_D}$

As indicated previously, χ is the propulsion energy output per mass of fuel consumed.

Assuming a constant average velocity V_AVE for a contemplated mission, and given a fixed volume of fuel, the maximum range is obtained by maximizing the function:

Range=∫₀^tmaxV(t)dt

subject to the constraint that the average velocity defined by:

$V_{\mathrm{AVE}}\equiv \frac{1}{t_{\max }}{\int }_0^{t_{\max }}V\left(t\right)t$

is fixed. The velocity V(t) is a function of r(t), and t_max is a function of r(t) in the above expressions.

The resultant maximum in range is obtained when r(t) is an exponential in time given by:

$\frac{r\left(t\right)}{r_0}={}^{\mathrm{cz}}$ $\mathrm{where}$ $c=\frac{-C_D}{4}\frac{{\rho }_f}{{\rho }_w}\frac{V^3}{{\chi }^L}$

The gain in range, relative to a fixed-geometry vehicle that has the same ratio of fuel volume to total volume, becomes:

$\mathrm{Gain}\equiv \frac{\mathrm{Range}\mathrm{with}\mathrm{Varying}\mathrm{Radius}}{\mathrm{Range}\mathrm{with}\mathrm{fixed}\mathrm{Radius}}=\frac{-\ln \left[1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{Tot}}}\right]}{\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{Tot}}}}$

Instead of holding average velocity constant, it is feasible to fix the mission time; this leads to the same exponential form for the change in radius with time, but provides less gain than the average-velocity constraints—due to the need to run at increased velocity, to achieve increased range in the same time as for the fixed-geometry case. The resultant gain in this case becomes

$\mathrm{Gain}\equiv \frac{\mathrm{Range}\mathrm{for}\mathrm{Variable}\mathrm{Geometry}}{\mathrm{Range}\mathrm{for}\mathrm{Fixed}\mathrm{Geometry}}=\frac{{\left[-\ln \left(1-\frac{{\mathrm{vol}}_f}{{\mathrm{vol}}_{\mathrm{tot}}}\right)\right]}^{1/3}}{{\left[\frac{{\mathrm{vol}}_f}{{\mathrm{vol}}_{\mathrm{tot}}}\right]}^{1/2}}$

Finally, also consider the case of running both the present variable-geometry invention and a conventional fixed-geometry system at constant power. This may be an advantage for certain missions and systems—and would simplify vehicle-propulsion design, and increase reliability. In this case for the variable geometry the r(t) becomes

$\frac{r\left(t\right)}{r_0}={\left(1-\mathrm{At}\right)}^{1/2}$

where A is the volumetric fuel-flow rate, V{dot over (o)}l_f divided by the total vehicle Volume, Vol_tot.

The gain now becomes:

$\mathrm{Gain}\mathrm{in}\mathrm{Range}=\frac{3}{2}\frac{\left(1-{\left[1-\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{Tot}}}\right]}^{2/3}\right)}{\frac{{\mathrm{Vol}}_f}{{\mathrm{Vol}}_{\mathrm{Tot}}}}$

Here the maximum gain is limited to 1.5. Also, in these circumstances, the velocity rises as fuel burns; therefore, less gain can be achieved than in the case of the average-velocity constraint.

The disclosures in this document are merely exemplary, and are neither limiting nor exhaustive. The scope of the invention is to be determined from the accompanying claims.

Claims

1. A vehicle that operates in a liquid or gaseous medium that is at an ambient pressure; said vehicle comprising:

components including a housing, and propulsive apparatus associated with the housing;

a fuel container mounted to the vehicle, at least partially outside all other components of the vehicle and in contact with the medium; and wherein:

the container is of a structure that is at least partially compliant, and responsive to said ambient pressure of the medium;

the propulsive apparatus consumes fuel from the fuel container;

drag of the vehicle in moving through the medium depends at least partially on shape of the container structure that is in contact with the medium; and

the shape of the container structure that is in contact with the medium depends on quantity of fuel within the container.

2. The vehicle of claim 1, wherein:

said container-structure shape becomes smaller in either a longitudinal or lateral dimension, or both, and produces less drag, as the fuel is consumed by operation of the vehicle.

3. The vehicle of claim 2, wherein:

the vehicle housing is generally cylindrical;

the container structure is annular and just outside the cylindrical housing; and

part of said drag of the vehicle depends on radial thickness of the annular container structure.

4. The vehicle of claim 3, wherein:

the radial thickness of the annular container structure depends upon quantity of fuel within the annular structure.

5. The vehicle of claim 4, wherein:

the fuel within the annular structure is compressed against the vehicle housing by said ambient pressure.

6. The vehicle of claim 2, wherein:

range of the vehicle in moving through the medium is maximum when the vehicle is operated at constant velocity through the medium.

7. The vehicle of claim 1, wherein:

the fuel container structure that is in contact with the medium is made of, or has embedded, materials that further reduce skin-friction drag.

8. The vehicle of claim 7, wherein:

the materials are drawn out of the surface into a boundary layer with the medium.

9. The vehicle of claim 8, wherein:

the materials are replenishable plural times, by soaking the container surface in them.

10. The vehicle of claim 7, wherein the materials are selected from the group consisting of:

Teflon or other materials having low friction;

latex or other materials having high compliance; and

mixtures, or other combinations, of low-friction and high-compliance materials.

11. The vehicle of claim 1, wherein:

the container surface incorporates at least one noise-absorbing material.

12. The vehicle of claim 1, wherein:

the vehicle is an underwater vehicle.

13. The vehicle of claim 1, wherein:

the container-structure has variable shape and size that provide a reduction in vehicle structural weight, between twenty and eighty percent of structural weight in a corresponding fixed-geometry vehicle.

14. A vehicle that operates in a liquid or gaseous medium that is at an ambient pressure; said vehicle comprising:

components including a housing, and propulsive apparatus associated with the housing;

a fuel container mounted to the vehicle, at least partially inside other components of the vehicle and at least partially out of contact with the medium; and wherein:

said other components of the vehicle include at least some portions that are capable of mechanical expansion and contraction;

the fuel container is of a structure that is at least partially compliant, and responsive to mechanical force from said expansion- and contraction-capable portions of the vehicle;

the propulsive apparatus consumes fuel from the fuel container;

drag of the vehicle in moving through the medium depends at least partially on shape of the vehicle structure that is in contact with the medium; and

at least part of the shape of the vehicle structure that is in contact with the medium depends on quantity of fuel within the container, by virtue of the responsiveness of the compliant container structure to the vehicle structure that is in contact with the medium.

15. The vehicle of claim 14, wherein:

the container structure has variable shape and size that provide a reduction in vehicle structural weight;

said reduction being between twenty and eighty percent of structural weight in a corresponding fixed-geometry vehicle.

16. The vehicle of claim 14, wherein:

said expansion- and contraction-capable portions of the vehicle are compliant and responsive to said ambient pressure of the medium.

17. A fuel container for a vehicle that operates in a liquid or gaseous medium that is at an ambient pressure, neither the vehicle itself nor the medium being a part of the claimed invention; and wherein:

the container is mounted to the vehicle, at least partially outside all other parts of the vehicle and in contact with the medium;

the container is of a structure that is at least partially compliant, and responsive to said ambient pressure of the medium;

drag of the vehicle in moving through the medium depends at least partially on shape of the container structure that is in contact with the medium; and

the shape of the container structure that is in contact with the medium depends on quantity of fuel within the container.

18. The fuel container of claim 17, wherein:

said container-structure shape becomes smaller in either a lateral or longitudinal direction, or both, and produces less drag, as the fuel is consumed by operation of the vehicle.

19. The fuel container of claim 18, wherein the vehicle housing is generally cylindrical; and wherein:

the container structure is annular and just outside the cylindrical housing; and

part of said drag of the vehicle depends on radial thickness of the annular container structure.

20. The container structure of claim 19, wherein:

the radial thickness of the annular container structure depends upon quantity of fuel within the annular structure.

21. The container structure of claim 20, wherein:

the fuel within the annular structure is compressed against the vehicle housing by said ambient pressure.

22. The container structure of claim 17, wherein:

range of the vehicle in moving through the medium is maximum when the vehicle is operated at constant velocity through the medium.

23. The vehicle of claim 17, wherein:

the fuel container structure that is in contact with the medium is made of, or has embedded, materials that further reduce skin-friction drag.

24. The vehicle of claim 23, wherein:

the materials are drawn out of the surface into a boundary layer with the medium.

25. The vehicle of claim 24, wherein:

the materials are replenishable plural times, by soaking the container surface in them.

26. The vehicle of claim 17, wherein the embedded materials are selected from the group consisting of:

Teflon or other materials having low friction;

latex or other materials having high compliance; and

mixtures, or other combinations, of low-friction and high-compliance materials.