APOLLO 15 Mission Report Excerpt 983 Lunar Surface Mobility Systems Performance

Lunar roving vehicle: The major hardware innovation for the lunar exploration phase of the Apollo 15 mission was the lunar roving vehicle. Because of geological requirements during surface traverses, time was limited for evaluating the characteristics of the vehicle. However, during the traverses, a number of qualitative evaluations were made. The following text discusses the performance, and the stability and control of "Rover 1", as well as other operational considerations pertaining to the vehicle.

The manual deployment technique worked very well. Simulations had demonstrated the effectiveness of this technique and, with several minor exceptions, it worked exactly as in preflight demonstrations. The first unexpected condition was noticed immediately after removing the thermal blanket when both walking hinges were found open. They were reset and the vehicle was deployed in a nominal manner. The support saddle, however, was difficult to remove after the vehicle was on the surface. No apparent cause was evident. Additionally, both left front hinge pins were out of their normal detent positions; both were reset with the appropriate tool. After removal of the support saddle, the rover was manually positioned such that "forward" would be the initial driving mode.

Front steering was inoperative during the first extravehicular activity. All switches and circuit breakers were cycled a number of times during the early portion of the first extravehicular activity with no effect on the steering. Subsequently, at the beginning of the second extravehicular activity, cycling of the front steering switch apparently enabled the front steering capability which was then utilized throughout the remaining traverses.

Mounting and dismounting the rover was comparable to preflight experience in 1/6-gravity simulations in the KC-135 aircraft. Little difficulty was encountered. The normal mounting technique included grasping the staff near the console and, with a small hop, positioning the body in the seat. Final adjustment was made by sliding, while using the footrest and the back of the seat for leverage. It was determined early in the traverses that some method of restraining the crew members to their seats was absolutely essential. In the case of Rover 1, the seatbelts worked adequately; however, excessive time and effort were required to attach the belts. The pressure suit interface with the rover was adequate in all respects. None of the preflight problems of visibility and suit pressure points were encountered.

The performance of the vehicle was excellent. The lunar terrain conditions in general were very hummocky, having a smooth texture and only small areas of fragmental debris. A wide variety of craters was encountered. Approximately 90 per cent had smooth, subdued rims which were, in general, level with the surrounding surface. Slopes up to approximately 15 per cent were encountered. The vehicle could be maneuvered through any region very effectively. The surface material varied from a thin powdered dust, which the boots would penetrate to a depth of 5 to 8 centimeters (2 to 3 inches) on the slope of the Apennine Front, to a firm rille soil which was penetrated about 1 centimeter (one-quarter to one-half inch) by the boot. In all cases, the rover's performance was changed very little.

The velocity of the rover on the level surface reached a maximum of 13 kilometers (7 miles) per hour. Driving directly upslope on the soft surface material at the Apennine Front, maximum velocities of 10 kilometers (5.4 miles) per hour were maintained. Comparable velocities could be maintained obliquely on the slopes unless crater avoidance became necessary. Under these conditions, the downhill wheel tended to dig in and the speed was reduced for safety.

Acceleration was normally smooth with very little wheel slippage, although some soil could be observed impacting on the rear part of the fenders as the vehicle was accelerated with maximum throttle. During a "Lunar Grand Prix", a roostertail was noted above, behind, and over the front of the rover during the acceleration phase. This was approximately 3 meters (10 feet) high and went some 3 meters forward of the rover. No debris was noted forward or above the vehicle during constant velocity motion. Traction of the wire wheels was excellent uphill, downhill, and during acceleration. A speed of 10 kilometers per hour could be attained in approximately three vehicle lengths with very little wheel slip. Braking was positive except at the high speeds. At any speed under 5 kilometers (2.7 miles) per hour, braking appeared to occur in approximately the same distance as when using the 1-G trainer. From straight-line travel at velocities of approximately 10 kilometers per hour on a level surface, the vehicle could be stopped in a distance of approximately twice that experienced in the 1-G trainer. Braking was less effective if the vehicle was in a turn, especially at higher velocities.

Dust accumulation on the vehicle was considered minimal and only very small particulate matter accumulated over a long period of time. Larger particles appeared to be controlled very well by the fenders. The majority of the dust accumulation occurred on the lower horizontal surfaces such as floorboards, seatpans, and the rear wheel area. Soil accumulation within the wheels was not observed. Those particles which did pass through the wire seemed to come out cleanly. Dust posed no problem to visibility.

Obstacle avoidance was commensurate with speed. Lateral skidding occurred during any hardover or maximum-rate turn above 5 kilometers per hour. Associated with the lateral skidding was a loss of braking effectiveness. The suspension bottomed out approximately three times during the entire surface activity with no apparent ill effect. An angular 30-centimeter (1-foot) high fragment was traversed by the left front wheel with no loss of controllability or steering, although the suspension did bottom out. A relatively straight-line traverse was easily maintained by selection of a point on the horizon for directional control, in spite of the necessity to maneuver around the smaller subdued craters. Fragmental debris was clearly visible and easy to avoid on the surface. The small, hummocky craters were the major problem in negotiating the traverse, and the avoidance of these craters seemed necessary to prevent controllability loss and bottoming of the suspension system.

Vehicle tracks were prominent on the surface and very little variation of depth occurred when the bearing on all four wheels was equal. On steep slopes, where increased loads were carried by the downhill wheels, deeper tracks were encountered - perhaps up to 3 or 4 centimeters (an inch or two) in depth. There was no noticeable effect of driving on previously deposited tracks, although these effects were not specifically investigated. The chevron tread pattern left distinct and sharp imprints. In the soft, loose soil at the Apollo lunar surface experiment package site, one occurrence of wheel spin was corrected by manually moving the rover to a new surface.

The general stability and control of the lunar roving vehicle was excellent. The vehicle was statically stable on any slopes encountered and the only problem associated with steep slopes was the tendency of the vehicle to slide downslope when both crewmen were off the vehicle. The rover is dynamically stable in roll and pitch. There was no tendency for the vehicle to roll even when traveling upslope or downslope, across contour lines or parallel to contour lines. However, qualitative evaluation indicates that roll instability would be approached on the 15-degree slopes if the vehicle were traveling a contour line with one crewmember on the downhill side. Both long- and short-period pitch motions were experienced in response to vehicle motion over the cratered, hummocky terrain, and the motion introduced by individual wheel obstacles. The long-period motion was very similar to that encountered in the 1-G trainer, although more lightly damped. The "floating" of the crewmembers in the 1/6-G field was quite noticeable in comparison to 1-G simulations. Contributions of short period motion of each wheel were unnoticed and it was difficult to tell how many wheels were off the ground at any one time. At one point during the "Lunar Grand Prix", all four wheels were off the ground, although this was undetectable from the driver's seat.

Maneuvering was quite responsive at speeds below approximately 5 kilometers per hour. At speeds on the order of 10 kilometers per hour, response to turning was very poor until speed was reduced. The optimum technique for obstacle avoidance was to slow below 5 kilometers per hour and then apply turning correction. Hardover turns using any steering mode at 10 kilometers per hour would result in a breakout of the rear wheels and lateral skidding of the front wheels. This effect was magnified when only the rear wheels were used for steering. There was no tendency toward overturn instability due to steering or turning alone. There was one instance of breakout and lateral skidding of the rear wheels into a crater approximately 0.5 meter (1.5 feet) deep and 1.25 meters (4 feet) wide. This resulted in a rear wheel contacting the far wall of the crater and subsequent lateral bounce. There was no subsequent roll instability or tendency to turn over, even though visual motion cues indicated a roll instability might develop.

The response and the handling qualities using the control stick are considered adequate. The hand controller was effective throughout the speed range, and directional control was considered excellent. Minor difficulty was experienced with feedback through the suited crewmember to the hand controller during driving. However, this feedback could be improved by a more positive method of restraint in the seat. Maximum velocity on a level surface can be maintained by leaving the control stick in any throttle position and steering with small inputs left or right. A firm grip on the handle at all times is unnecessary. Directional control response is excellent although, because of the many dynamic links between the steering mechanism and the hand on the throttle, considerable feedback through the pressure suit to the control stick exists. A light touch on the hand grip reduces the effect of this feedback. An increase in the lateral and breakout forces in the directional hand controller should minimize feedback into the steering.

Two steering modes were investigated. On the first extravehicular activity, where rear-wheel-only steering was available, the vehicle had a tendency to dig in with the front wheels and break out with the rear wheels with large, but less than hardover, directional corrections. On the second extravehicular activity, front-wheel-only steering was attempted, but was abandoned because of the lack of rear wheel centering. Four-wheel steering was utilized for the remainder of the mission. It is felt that for the higher speeds, optimum steering would be obtained utilizing front steering provided the rear wheels are center-locked. For lower speeds and maximum obstacle avoidance, four-wheel steering would be optimal. Any hardover failure of the steering mechanism would be recognized immediately and could be controlled safely by maximum braking.

Forward visibility was excellent throughout the range of conditions encountered with the exception of driving toward the zero-phase direction. Washout, under these conditions, made obstacle avoidance difficult. Up-sun was comparable to cross-sun if the opaque visor on the lunar extravehicular visor assembly was lowered to a point which blocks the direct rays of the sun. In this condition, crater shadows and debris were easily seen. General lunar terrain features were detectable within 10 degrees of the zero phase region. Detection of features under high-sun conditions was somewhat more difficult because of the lack of shadows, but with constant attention, 10 to 11 kilometers (5.5 to 6 miles) per hour could be maintained. The problem encountered was recognizing the subtle, subdued craters directly in the vehicle path. In general, 1-meter (3.25-foot) craters were not detectable until the front wheels had approached to within 2 to 3 meters (6.5 to 10 feet).

The reverse feature of the vehicle was utilized several times, and preflight-developed techniques worked well. Only short distances were covered, and then only with a dismounted crewmember confirming the general condition of the surface to be covered.

The 1-G trainer provides adequate training for lunar roving vehicle operation on the lunar surface. Adaptation to lunar characteristics is rapid. Handling characteristics are quite natural after several minutes of driving. The major difference encountered with respect to preflight training was the necessity to pay constant attention to the lunar terrain in order to have adequate warning for obstacle avoidance if maximum average speeds were to be maintained. Handling characteristics of the actual lunar roving vehicle were similar to those of the 1-G trainer with two exceptions: braking requires approximately twice the distance, and steering is not responsive in the 8- to 10-kilometer (4- to 5.5-mile) per hour range with hardover control inputs. Suspension characteristics appeared to be approximately the same between the two vehicles and the 1/6-G suspension simulation is considered to be an accurate representation with the exception of the crewmembers' weight.

The navigation system is accurate and a high degree of confidence was attained in a very short time. Displays are also adequate for the lunar roving vehicle systems.

Lunar communications relay unit: The lunar communications relay unit and associated equipment operated well throughout the lunar surface activities. The deployment techniques and procedures are good, and the operational constraints and activation overhead are minimum. Alignment of the High-Gain Antenna was the only difficulty encountered, and this was due to the very dim image of the Earth presented through the optical sighting device. The use of signal strength as indicated on the automatic gain control meter was an acceptable backup alignment technique.

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