N ASA’s Demonstration of Autono-mous Rendezvous Technologies(DART) space experiment will test

the sensors, propulsion systems, and softwarethat future U.S. spacecraft will need for con-ducting complicated maneuvers in close prox-imity to other spacecraft, without any help fromhuman controllers or astronauts.

The mission was delayed last Novemberwhen Orbital Sciences, the prime contractor forboth DART and its Pegasus launch vehicle, in-formed NASA that it had new data suggesting

that DART’s ride to orbit might be rougherthan expected. Jim Snoddy, the DART pro-gram manager at NASA Marshall, resolved the

issue by ordering new hardware tests thatshowed DART could survive the rougherride. The mission is now scheduled forlaunch this month.

In orbit, DART will attempt tolocate a 47-kg retired military satel-lite and use on-board computersand propulsion systems to performa series of maneuvers around it. Themission will last 16-24 hr, depend-ing on how long it takes DART to

catch up to the target spacecraft. Thechase time could be as long as 9.5 hr,

depending on the date of launch and howaccurately Orbital’s L-1011 carrier jet releases

the air-launched Pegasus rocket.

DART aims at

DART will be the first space-craft to perform a rendezvouswithout human assistance. Itsautonomous rendezvous andproximity operations softwarewill test additional algorithmsby calculating and executingcollision avoidance maneuversand circumnavigation. To con-clude the mission, DART willfly away from the MUBLCOMsatellite (left).

26 AEROSPACE AMERICA/MARCH 2005

AEROSPACE AMERICA/MARCH 2005 27Copyright© 2005 by the American Institute of Aeronautics and Astronautics.

The DART technologies willbe valuable for both humanmissions to deep space andmilitary satellite operationsin Earth orbit, say DARTofficials. On the civilianside, the technologiesare now funded underthe space explorationvision announced byPresident Bush.

The technologycould enable the firstfully automated dock-ings between U.S. space-craft. Autonomy will beessential in deep space be-cause of the time it would takecommunications signals to crossmillions of kilometers of space. Thattime lag makes it impossible for con-trollers on the ground to orchestrate dockingsor a rendezvous at great distances. In addition,there is always the possibility that astronautsmight be injured or sick and unable to guide acargo craft to a docking, Snoddy says.

In Earth orbit, the technologies could en-able DOD to spy on rival satellites up close, theofficial says. For DART personnel, any deeperdiscussion of the military applications are pro-hibited, notes the official.

A tricky fitDART is a cylindrical space-

craft about 1.8 m long and1 m wide. It weighs 360

kg, including its hy-drazine and nitrogenfuels. To build DART,engineers had to at-tach the cylindricalfourth stage of thePegasus rocket to a slightly narrowercylindrical structureprocured from Ver-

mont Composites inBennington, Vt. This

carbon-fiber structureholds DART’s experimental

guidance sensor and propul-sion system.

The fourth stage will stay attachedto the DART structure through the entire mis-sion, and the two should be considered onespacecraft, Snoddy says.

The fourth stage contains a flight computersupplied to Orbital by SBS Technologies of Al-buquerque, N.M., and Augsburg, Germany. Itwill run the experiment using 110,000 lines ofsoftware code written by Orbital. The stage alsocontains a hydrazine thruster that will begin thetest by boosting DART out of its 500-km park-

NASA’s on-orbit experiment may soon lead to

the first fully automated dockings of U.S. spacecraft

by Ben IannottaContributing writer

At Vandenberg AFB, workers maneuver the DART spacecraft,suspended by a crane, over theupper stage in preparation forlaunch.

space rendezvous

28 AEROSPACE AMERICA/MARCH 2005

ing orbit and toward the retired satellite calledMUBLCOM, for multipath beyond-line-of-sightcommunications. At the end of the experiment,the fourth stage will send DART back towardEarth so that it does not become dangerousspace debris. By requirement, it must reenterthe atmosphere and burn up within 25 years.

The Pegasus fourth stage is wider in diam-eter than the DART structure. Vermont Compos-ites supplied two inverted-cone-shaped rings tolink the two dissimilar diameter cylinders. Onering forms the tapered neck that joins the two;the other ring is located inside the vehicle andconnects the fuel tanks of the fourth stage to theDART structure.

Program engineers knew the juncturewould be the most vulnerable part of the space-craft, partly because of the mass of the fuel andthe fact that joining two dissimilar diameterstends to concentrate stress, Snoddy says. Ver-mont Composite technicians tested the invertedcones to 1.4 times the loads engineers calcu-lated DART would experience as the Pegasusrocket stages fired.

One day equals six monthsDays before the scheduled November 9 launch,Snoddy learned that the jolt from the Pegasussecond stage might be much harder than previ-ously thought. Orbital’s Pegasus team had re-ceived data about another launch vehicle thathad used an Orion solid rocket motor similar tothe one in DART’s second stage. The result ofthat launch showed that the load might bemuch higher than predicted.

DART had come close to being launchedwith the potentially fatal misjudgment. The firstlaunch date, October 26, was scrubbed because

of a GPS problem on MUBLCOM that was suc-cessfully resolved. MUBLCOM must report itsposition in space to DART engineers prior to thespacecraft’s launch so it can be loaded into theDART computer.

The next launch date, October 28, wasscrubbed when members of the launch close-out team discovered flakes of aluminum foilfrom the rocket’s fairing inside the enclosure.The mission was rescheduled for no earlier thanNovember 4.

When the new load issue arose, Snoddy as-sembled his NASA team to decide whether ornot the launch could proceed. “Our biggest con-cern was the composite [inverted cone] struc-ture, with a big mass, 70-80 lb of tank sittingthere like a big hammer,” Snoddy says.

A delay would require lots of rescheduling.Snoddy had arranged time at four ground sta-tions to receive S-band data transmissions fromDART, and he had won a coveted launch slotwithin the Vandenberg AFB, Calif., rocket range,which was about to be closed temporarily formaintenance. With a $100-million mission atstake, Snoddy decided he had no choice but toconduct new analyses and hardware tests basedon the new load anticipation.

“It took me a day to make the decision, butit cost me six months,” he says.

There was one fact that prevented an ex-pensive and possibly much longer delay. “Wehappened to have a piece of hardware made atthe same time,” Snoddy says. Engineers testedthe spare inverted cone on a hydraulic press atVermont Composites to imitate forces far greaterthan the new estimate for the stage-two jolt. “Wetook it up to a factor of 3.0. We even thoughtabout breaking it,” Snoddy says. The hardware

At Vandenberg AFB, the PegasusXL launch vehicle and DARTspacecraft, which are mated,were attached to the underbellyof the Orbital Sciences L-1011(for a launch that was sub-squently scrubbed).

AEROSPACE AMERICA/MARCH 2005 29

passed the test without delaminating or break-ing. DART is now back on track for launch.

Snoddy will not say exactly how he learnedof the faulty estimate. “As a project manager,you have many sources. I can’t recall who in-formed me first,” he says. He also will not sayhow far off the load estimate was. “I’d hate toput a number on it. It was a significant increasein our load [that] we had to recertify to,” he says.He expresses no rancor toward Orbital, how-ever. “I appreciate the notification,” he says.

Does the eye have it?While the L-1011 is on the runway, technicianswill load a final set of GPS coordinates forMUBLCOM into the DART flight computer.“The last command is given prior to drop. Fromthat point on, there’s no human in the loop,”says Tim Rumford, Orbital’s DART programmanager. As DART climbs toward MUBLCOM’sorbit of 760 km, for much of the trip it will relyon the last MUBLCOM GPS readings. Once it isabout 40 km behind MUBLCOM and 7.5 kmbelow it, DART will initiate a space-to-spaceUHF link with MUBLCOM to update the GPSposition information, Rumford explains.

The goal is to chase down MUBLCOM with-out passing it. “One of the critical things is, youcan never get in front of somebody, becauseyou’d have to do a whole other orbit, and youdon’t have enough propellant,” Snoddy warns.

At a range of 1,000 m, DART will turn onan experimental eye called the Advanced VideoGuidance Sensor. The AVGS was developed byOrbital based on a prototype called the VideoGuidance Sensor. The prototype was designedat Marshall in the late 1980s and tested on thespace shuttle’s robotic arm. Marshall softwareexperts provided 13,000 lines of computercode for the new AVGS.

The AVGS will bounce low-powered lasersoff MUBLCOM to calculate the relative distanceand attitudes of the spacecraft. DART’s flightcomputer will compare the GPS and AVGS read-ings as the distance closes, to ask, “Who’s giv-ing you a better answer?” Snoddy says. At a dis-tance of between 500 and 300 m, depending onAVGS’s performance, DART will hand overguidance to AVGS. “Once we get within 300 m,the AVGS will trump all other sensors in termsof importance,” Rumford says.

Engineers do not yet know exactly whenAVGS will be able to see MUBLCOM, and thatwill be one of the interesting findings of the mis-sion, according to Rumford and Snoddy.

Though engineers consider AVGS to beDART’s eye, Snoddy says the sensor does notproduce traditional images or videos. It sees

points of light. Software within AVGS uses thevideo to calculate distance and attitude. Theflight computer then turns that informationinto propulsion commands.

MUBLCOM has two sets of three laser retro-reflectors, one long-range set for when DART isrelatively far from MUBLCOM and a short-rangeset for when DART is so close that all three long-range reflectors would not be in the field ofview. The transition point varies according tothe maneuver but is usually when DART is me-ters away from MUBLCOM. “By definition, youneed three points to calculate distance. You cal-culate the time of the return signal. It’s almostakin to a GPS,” Snoddy explains.

A separate camera on DART will returnvideo images to Earth in near real time so man-agers can monitor the mission in a control roomat Vandenberg.

DART’s flight computer will use the AVGSinformation to command DART’s ProximityOperations Reaction Control System. This con-sists of 16 cold-gas nitrogen thrusters arrangedin groups of four on the aft, forward, starboard,and port sides of DART. “Each points in a dif-ferent direction to give six degrees of motion,”Rumford says.

AVGS can be testedtoday only be-cause in the1 9 9 0 s

N A S Ao f f i c i a l surged DARPAand Orbital to includelaser reflectors on MUBLCOM. Thelasers would have no effect on MUBLCOM’s pri-mary mission, which was to test the ability ofArmy soldiers and Marines to achieve over-the-horizon battlefield communications with aquickly assembled satellite constellation. Thesatellite was launched in 1999. It is retired butstill functional.

At Vandenberg AFB, DART waitsfor fairing installation. The fair-ing will encapsulate the space-craft and protect it while it ison the launch pad and duringits ascent.

30 AEROSPACE AMERICA/MARCH 2005

Dancing in space“Just finding [MUBLCOM] is an exercise in it-self,” Snoddy says. The most challenging part ofthe mission will be the series of “proximity op-erations” DART performs near MUBLCOM. “It’sdoing tens of thousands of miles an hour, andwe’re trying to dance around it. It’s not a trivialfeat,” Snoddy says.

The dance will start cautiously. DART willmaneuver behind MUBLCOM along its orbitalpath to a distance of 15 m. It will hover theremotionless relative to MUBLCOM for 1.5 hr todemonstrate the ability of AVGS to track a targetunder the varying lighting conditions of a fullorbit. Stationkeeping there will enable man-agers to receive video of the mission under afull array of light conditions.

DART will then move to another point andhead toward MUBLCOM as though it were on adocking axis. However, it will stop at 5 m. “Itactually has logic that says if you come within 5m, ‘safe’ yourself by backing up,” Snoddy says.After that, DART will back off to a distance of100 m and then approach MUBLCOM and per-form a simulated collision avoidance maneuver,as if a docking were being aborted.

Next, it will move slowly away from DARTto determine the maximum range of sight for theAVGS eye. Once AVGS loses tracking, DART willmove back within range for a new series of ma-neuvers from beneath DART. These maneuverswill occur along tracks perpendicular to Earth.

The dance culminates with DART drop-ping back to a distance of 1 km to circumnavi-gate MUBLCOM over the course of 75 min, orthree-quarters of an orbit.

DART will repeat the maneuvers from thebeginning until its internal clock tells it time hasrun out. The mission will end when the fourthstage fires its hydrazine motor to send DART to-ward the atmosphere.

Assuming the AVGS performs well, DARPAplans to fly a version of it on its Orbital Expressautomated docking experiment in 2006, saysSnoddy. “Most Marshall DART people are goingto work on Orbital Express,” he says.

Snoddy says that he is confident that allmajor technical issues are behind DART, al-though he expects to be busy right up to thelaunch. “It’s fairly routine for me to get calls inthe middle of the night: ‘What do you want todo, Mr. Snoddy?’”