ANALYSIS OF HIGH-SPEED AEROCAPTURE AT MARS USING HYPERPASS, A NEW AEROASSIST TOOL

AIAA-03-2172AIAA 03-2172ANALYSIS OF HIGH-SPEEDAEROCAPTURE AT MARS USINGHYPERPASS, A NEW AEROASSIST TOOLA. D. McRonald, K. L. Gates, and K. T. Nock Global Aerospace Corporation 711 West Woodbury Road, Suite HAltadena, CA 91001AIAA Aerodynamic Decelerator Systems TechnologyConferenceMay 19-22, 2003 / Monterey, CAFor permission to copy or republish, contact the American Institute of Aeronautics and Astronautics17th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar 19-22 May 2003, Monterey, CaliforniaAIAA 2003-2172ANALYSIS OF HIGH-SPEED AEROCAPTURE AT MARS USING HYPERPASS, A NEW AEROASSIST TOOLAngus McRonald* Kristin Gates† and Kerry T. Nock+Global Aerospace Corporation711 West Woodbury Road, Suite HAltadena, CA 91001-5327ABSTRACTA revolutionary interplanetary rapid transit concept1for transporting scientists and explorers between Earth and Mars has been developed by Global Aerospace Corporation under funding from the NASA Institute for Advanced Concepts (NIAC). This architecture uses highly autonomous spaceships, dubbed Astrotels; small Taxis for trips between Astrotels and planetary Spaceports; Shuttles that transport crews to and from orbital space stations and planetary surfaces; and low-thrust cargo freighters. Earth-Mars cycler orbits have been proposed for the interplanetary transfer because these both provide fast flight times and propellant savings. Some cycler orbits developed thus far can have some undesirable characteristics, however, such as high approach and flyby velocities at Mars. In this paper we address the high approach speed at Mars and its implications to aerocapture. In addition, we describe the new simulation tool, called HyperPASS, used in part for this investigation.INTRODUCTIONEarth-Mars cycler orbits have been proposed2-6 as the logical choice for the interplanetary transfer because these both provide flight times of 5-6 months, and propellant savings by not stopping at one or both planets. Some of these cycler orbits, Aldrin Cyclers (see Figure 1) in particular, have some undesirable characteristics, such as high approach and flyby velocities at Mars. As a result, high-speed Mars aerocapture has been investigated for entry speeds up to 12.5 km/s using a crewed vehicle with a similar geometry to the Aeroassist Flight Experiment8 (AFE) vehicle proposed in the eighties, with an L/D(lift/drag) value up to 0.63.Figure 1 Aldrin Cycler OrbitsThe exit speed is about 4,200 m/s to go to an apoapsis at the distance of Phobos, which is used as a staging point. This vehicle is a “Taxi” which brings astronauts from the continuously cycling Earth-Mars-Earth vehicle (“Astrotel”) to Phobos, using aerocapture at Mars. At the highest speeds expected, a Taxi using only aero lift and drag will require peak g-loads up to 7 to stay in the atmosphere at the initial level-off point. By using onboard propulsion, the peak g-load can be reduced to lower, acceptable values. Trajectories have been evaluated using an onboard engine to limit g-loads to various values, and a surprisingly small amount of propellant is used.________________The analysis of this aerocapture problem was facilitated by a new aero-assist software tool developed by Global Aerospace Corporation called Hypersonic Planetary Aero-assist Simulation System or HyperPASS, which is also described in the paper. HyperPASS enables the user to perform simulations at any planet using any of the pre-programmed vehicles or a user-entered vehicle. It allows the user to perform trade study simulations by way of graphical user interfaces (GUIs). Current functions include manual and guided aerocapture and ballute simulations, aerobraking simulation, entry descent and landing, calculation of various parameters of interest, and a variety of output visualization options. After allowing the user to enter a selected set of entry and exit conditions, the guided simulation creates an optimal trajectory based on the chosen function. AEROCAPTURE VEHICLE DESIGN The proposed Taxi vehicle uses aerodynamic orbit capture (aerocapture) at both Earth and Mars. The entry speed at Earth is modest and the velocity to be lost is consistent with a relatively short-duration aerocapture flight. At Mars, the entry speed is much higher than the exit speed desired, so that the aerocapture vehicle has to cruise around the planet at nearly constant altitude for a relatively long period of time in order to bleed off enough speed. As discussed earlier, a vehicle with relatively high lift-to-drag ratio is required at the start of the cruise in order to supply the required centripetal acceleration and to stay under a total g-load of about 5. The current baseline Taxi vehicle aeroshell is known as an elliptical raked cone9 that has a maximum lift-to-drag ratio of 0.63. The design in Ref. 7 was known as the Aerobraking Orbit Transfer Vehicle (AOTV). The AOTV was sized for a smaller payload than is being considered forthe current Taxi. The AOTV had a Cd ofabout 1.6 and a Clof 0.48, i.e., lift-to-drag ratio (L/D) of 0.3 at angle of attack (AOA) ofzero, and it had a maximum L/D of about 0.63at an AOA of –20° (Cd= 0.95 and Cl= 0.6).For the AOTV with a diameter 12 m and mass13,200 kg (dry mass 6,800 kg plus fuel) them/CdA was 72.9 kg/m2 at an AOA of zero and123 kg/m2 at an AOA -20 deg. At an AOA of+20°, the Cdis about 1.75 (m/CdA = 60kg/m2), and L/D is zero and there is no lift.The Taxi being considered here has about a19,730-kg entry mass (including propellant tocircularize at Phobos orbit) with a 12-mdiameter aeroshell.Taxis utilize LOX/LH propulsion to escapeplanets and place them and their crew ontohyperbolic rendezvous trajectories with theinterplanetary orbiting Astrotels. Figure 2depicts a Taxi departing the Earth Spaceportwith the Moon in the background and a Taxiduring aerocapture at Mars arrival. This figureillustrates the crew module, propellant tanks,rocket engines (in their deployed position),and the aeroshell. Propellant capacity of thebasic Taxi vehicle is 20.6 tonnes.Figure 2 Taxi Departing the Earth-Moon34duration of crew time in the expected cramped quarters. Crew volume is comparable to what the Apollo astronauts had on their flights to the Moon and back.Figure 3 is a scale drawing of the Taxi as it undergoes aerocapture at Mars. The view is as seen from 50 km above Valles Marineris during aero-cruise. Note the rocket engines are in their stowed position. During this time the tanks are almost empty, containing only the propellant necessary to rendezvous with the Mars Spaceport after aerocapture. The crew module is shown in see-through mode so one can observe the crew g-seats, which rotate in order to accommodate the varying g-load direction and the quite different thrustdirection during propulsive maneuvers.CHARACTERISTICSTable 1 was generated to examine what approximate g lift is required for a full cycle of opportunities for the Aldrin Cycler orbits. We also display three maximum g-load limits as a function of required L/D as a function of Mars opportunity. The dates of the opportunities (2012-2025) are representativeof a full cycle of 15 years of transfers. For this table, an altitude of 50 km is assumed, and the V horizontal /V e is an empirical fraction of V e , the entry speed. The required g lift value is shown for each opportunity. Below these values are the required vehicle L/D as a function of peak g-load. It is assumed that the vehicle guidance arranges the entry angle and initial lift to level off at a speed and altitude consistent with keeping L/D and g drag at the values shown.It can be seen that L/D for a 5-g limit is above 0.63 of the proposed Taxi vehicle design for the dates 2012, 2014 and 2016. Even for the 6-g total limit, 2014 and 2016 require higher L/D (about 0.8) than the proposed vehicle.For a 7-g limit the proposed Taxi vehicle design would cover all cases. One question that can be asked is what is the acceptable peak g-load for astronauts arriving at Mars.Some U.S. roller coasters have peak g-loads up to 6.5 for a few seconds. It may be that since these aerocapture g-loads last only a few seconds at peak, they can be tolerated. If, on the other hand, they can not be tolerated another question might be asked, namely, can the total g-load be reduced in some way, e.g.by applying a propulsive delta-V before entry,changing the entry path, using a ballute, etc.We have determined that level flight for Mars aerocapture will use bank control of the Taxi at a fixed angle of attack (AOA). The Mars requirements dictate that the AOA will be set for maximum L/D. The conventional guidance process calls for an initial period of quasi-equilibrium glide to evaluate the error in entry angle, if any, and apparent variation of the atmospheric density and scale height from the values expected. Angle of Bank (AOB) is defined as the roll angle about the wind or ram direction. Generally, a constantly changing AOB will be used to maintain level flight, with a roll reversal if needed to reduce the cross-track deviation to an acceptable level.Table 1 Aeroassist Parameters as a Function of g-Loads and Needed L/DAutomated and guided simulations were run on our integrating aeroassist software, HyperPASS. The main control variable is the AOB that rotates the lift vector to be able to modulate vertical component of lift.The following figures display several parameters from a Taxi aerocapture trajectory simulation that results in a velocity that would take the Taxi to the radius of Phobos orbit. The Taxi was assumed to have an entry mass of 19,730 kg (which includes propellant to circularize the orbit at Phobos) and a diameter of 12 m. For this profile the Angle of Attack (AOA) is fixed at -20 degrees for a lift coefficient of 0.60 and a drag coefficient of 0.95. The AOA is the difference between where the axis of symmetry of the aeroshell is pointed relative to the direction of the air flowing past the aeroshell. At zero AOA there is no lift. For particular values of AOA,the lift varies with drag. Convention is that when AOB is 180° the lift is pointed positive toward the planet. Initially the AOB is set at 0°, or full lift up away from Mars, in an attempt to attempt to level off at a slightly higher altitude than the predicted aerocruise altitude. Then full lift down is applied to descend to the cruise altitude. After reaching the approximate altitude for aero-cruise, the AOB is automatically adjusted to apply the right amount of lift toward Mars to offset centrifugal forces. Because of the short time scale for this part of the entry it is expected that the approach to aerocruise flight will be automated, using accelerometers to measure total g-load and to compute velocity, and measure stagnation pressure to determine altitude (atmospheric density). Bank angle and thrust will be used to maintain flight at the desired g-load. The AOB is adjusted as needed throughout aero-cruise until the56required budget of velocity is lost (at ~250 s),where upon the AOB rolls to zero to align the lift vector away from Mars so that the Taxi swiftly leaves the atmosphere on its way toPhobos.Altitude vs Time50100150200250300350400450Time, sA l t i t u d e , mFigure 4 Taxi Aerocapture Altitude ProfileVelocity vs TimeTime,sV e l o c i t y , m /sFigure 5 Taxi Aerocapture Velocity ProfileFlight Path Angle vs Time-15-10-505101550100150200250300350400450Time, sF l i g h t P a t h A n g l e , d e gFigure 6 Flight Path Angle ProfileRange vs Time01,0001,5002,0002,5003,000Time, sR a n g e , k mFigure 7 Taxi Range as a Function of Time-40-20020406080100120140160180050100150200250300350400450Time, sA n g l e s , d e gFigure 8 Taxi AOA and AOB ProfilesHeating vs Time102030405060708050100150200250300350400450Time, sH e a t i n g , W /c m ^2Figure 9 Heat Load vs. TimeOverall g-load vs. Time123456050100150200250300350400450Time, sg -l o a d , g e eFigure 10 Various components of g-load vs.TimeREDUCING HIGH G-LOADA number of aeroassist strategies exist that can reduce the crew g-loads during the aerocapture maneuver. These strategies include 1) the baseline of descending into the atmosphere at a shallow angle and using drag to reduce velocity before reaching aero-cruise altitude, 2) a steeper entry angle to bleed off more speed prior to aero-cruise, 3) the use of a ballute device at entry, 4) the reduction of velocity by propulsive means before entry, and 5) propulsive thrusting in the velocity direction (which is very counter-intuitive) during the initial aero-cruise phase.Steeper EntryExamination of some entry trajectories intoMars at Ve = 12.502 km/s, with ballisticcoefficient, B = m/Cd A of 30 and 60 kg/m2indicates that entering at a slightly steeper angle (than the nominal 10 deg) can remove about 700 m/s prior to arriving at about the 5-g deceleration level.Ballute Drag Augmentation at EntryA ballute alone could be used to remove almost all necessary delta-V. However such a ballute would be excessively massive and result in excessive g-loads for human crews; greater than 17.Alternatively, using a smaller ballute on the inward path could generate over 1.0 km/s more delta-V prior to the point of level flight. The characteristics of such a ballute design are initial 50-m radius, a peak temperature about 500°C, a ballute envelope mass of about 100 kg and an envelope structure (a strong net) mass of about 350 kg. A pressure relief valve responding to the g level could partially deflate the ballute, reducing its cross-section area, to maintain the g-load below 5 or 6. Ballutes of this kind have not been flight-tested but NASA is presently considering a development program leading to a flight test.Propulsive Pre-entry Delta-VHere we examine how a pre-entry delta-V of1.0 km/s could improve the 2016 opportunityshown in the Table 1 for 5, 6 and 7 g. We compute the gliftneeded, the allowed peakgdrag, and the corresponding L/D, for the new lower initial horizontal aero-cruise speed of10.9 km/s (11.9 –1.0). For this situation, thepeak initial cruise total g-load of 6 enables all the Mars entry cases with the current Taxi vehicle design.Table 2 Peak Allowable g-load Due to DragTotal g-load gliftPeak gdragRequired Allowed Allowed L/D5.0 3.13 3.900.806.0 3.13 5.120.617.0 3.13 6.260.50The implication of a 1.0 km/s delta-V equates to a propellant requirement of 4,897 kg assuming the delta-V is accomplished at 200 km altitude prior to entry. Delta-Vs much higher than 1 km/s may be required to reduce the g-load fully to 5 or less. For example, the required delta-V for a 12.5 km/s entry speed to reduce the g-load to 5 is 1674 m/s if done just prior to entry, or 1858 m/s at “infinity.”THRUSTING AEROCAPTURE One promising new and innovative scheme to reduce crew g-load is to fly a little higher,reducing gdrag, and to use propulsive thrust to augment the limited lift of the vehicle during the critical few seconds at the start of aero-cruise. With a Mars entry velocity of 12.5 km/s, the start of aero-cruise is at a velocity ofabout 11.9 km/s. The allowed gliftto keepg < 5 g is 2.67. Since the required gliftis 3.81,a propulsive thrust level of 1.14 g willaugment the lift of the vehicle and allow it to continue aero-cruise and still keep the resultant total crew g-load below 5 gees.From observing the placement of the engines78on the vehicle of Ref. 1, it seems that the nozzle axes would be about 45 deg to the velocity vector, and to generate vertical force would also generate a horizontal force opposing drag. This has the unexpected effect of reducing the total g-load. For example, in the above case a propulsive vertical force of 1.14 g would give the required 3.81 of g lift and would reduce g drag by 1.14 g, resulting in 3.81vertical and 3.10, horizontal, giving a total g-load of 4.91 g. There will evidently be an optimum value and direction of the engine thrust to meet the cruise conditions and give minimum total g-load.The total burn delta-V to augment cruise lift was evaluated for a 45° case of entry at 12.5km/s, starting cruise at 11.8 km/s. The delta-V is 358 m/s (1,630 kg of propellant),compared with a burn of 1674 m/s (8,867 kg of propellant) just prior to entry to reduce the speed enough to perform cruise with only lift.The vehicle of Ref. 7 is derived from a set of “canoe” type entry vehicles evaluated for a range of cone angles and rake angles. Data published 9 for a vehicle close to that of Ref. 7indicate that higher L/D, up to 0.9, is generated at greater angle of attack, and one can infer that the Ref. 7 vehicle has also this capability.Thus the rocket engines can be used to keep the vehicle within a safe corridor of g-load in the early part of aerocruise. The following figures display the key parameters for the case where we are thrusting at an angle of 45° to the velocity vector and in the same general direction, i.e. opposite to the aerodynamicdrag.Figure 11 Propulsive Thrust vs. TimeFigure 12 Total g-load vs. Time forPropulsive Thrusting CaseFigure 13 Integrated Delta-V vs. Time forPropulsive Thrusting Case AEROCAPTURE SIMULATION TOOL Global Aerospace Corporation has been developing a state-of-the-art aero-assist tool called the Hypersonic Planetary Aero-assist Simulation System or HyperPASS. This toolwas used in generating much of the data analyzed in this paper. HyperPASS is an aero-assist simulation system, coded using the MATLAB language, which enables the user to perform simulations at any planet using any of the pre-programmed vehicles or a user-entered vehicle. It allows the user to perform trade study simulations without any prior knowledge of MATLAB, by way of graphical user interfaces (GUIs). The program incorporates equations of motion10for a spacecraft in three-degrees-of-freedom (3-DOF) to describe the dynamics.The current capabilities of HyperPASS are: Simulation at Mercury, Venus, Earth,Mars, Jupiter, Saturn, Uranus, Neptune, and TitanManual / Guided / Thrusting Aerocapture SimulationManual / Guided Ballute SimulationGuided Aerobrake SimulationEntry Descent and Landing (EDL)Several Entry Vehicle TypesInfinite Mid-flight Trajectory Transitions G-Load CalculationStagnation Point Heating Rate Calculation Dynamic Pressure CalculationTrajectory VisualizationView Previous SimulationThe View Previous Simulation option allows the user to view output/plots of previously saved simulations. The Aerocapture, Ballute, and EDL Simulation options allows the user to run a regular simulation, after which one has the option to view results and add an infinite number of trajectory transitions. The implementation of transitions, such as AOB changes, added thrust, change in cross-section area, etc., makes it possible to manually guide the vehicle to the desired final conditions. The more advanced, Guided Aerocapture and Ballute Simulations, allow the user to specify a desired exit velocity or apoapsis altitude.The simulator then picks the optimal entry FPA and guides the vehicle during level off (and ballute release for the guided ballute case) by means of bank angle modulation, in order to exit at the specified velocity. The Aerobrake Simulation calculates the time and number of orbits as a function of the initial and final apogee altitudes and ballistic coefficient entered by the user.During mission setup, the planet, atmosphere, gravity model, and vehicle are chosen.Atmosphere models available are of several types including exponential, table interpolation, Mars simplified homosphere, or the users may enter their own atmosphere.Gravity models include inverse-square rotating, J2 rotating, and inverse-square non-rotating. As for vehicle selection, the current options are Apollo, Viking, Elliptical Raked-Cone (AFE), or the user may enter vehicle specifications of their own choosing. Once all parameters are entered, simulation commences. Simulations can be automated in order to carry out trade studies on the variation of key aerocapture parameters.Afterwards, output can be viewed and plotted for further analysis.Future versions of HyperPASS will be upgraded to include a thrusting option during guided aerocapture simulation, launch/ascent to orbit, and powered landing. Potential uses of HyperPASS include mission studies of aerocapture systems at most any planet that has an atmosphere. Trades studies can investigate performance with alternate aeroshell types, varying flight path angle and entry velocity, different g-load limits, angle of attack and angle of bank variations.SUMMARY AND CONCLUSIONS The studies reported here indicate that high-speed aerocapture at Mars (up to 12.5 km/s entry speed), associated with Earth-Mars9cycler orbits, may be achieved with a high L/D vehicle (L/D on the order of 1.0), or with a vehicle with less L/D and capable of thrust during the early part of aerocapture. An example is the AFE vehicle with L/D of about 0.63. By using engine thrust to add to downward lift and oppose drag (which is counter-intuitive) the peak total acceleration can be limited to an acceptable level with a relatively low expenditure of propellant during a short period as the vehicle approaches level flight. In this phase bank angle and thrust are used to achieve flight at the desired g-level, with inputs from accelerometer and stagnation pressure measurements.A user friendly trajectory simulation program, HyperPASS, based on MATLAB, was used to generate much of the data analyzed in this paper. HyperPASS has been developed to model several types of atmospheric flight such as aerocapture, aerobraking and ballute aerocapture. It offers a flexible range of options, using rational file creation and plot capabilities.ACKNOWLEDGEMENTSThe authors wish to acknowledge the support of the NASA Institute for Advanced Concepts through their Universities Space Research Association Grant and contract Nos. 07600-25 and 07600-58.REFERENCES1.Nock, K. T., et. al., "Cyclical Visits toMars via Astronaut Hotels", GlobalAerospace Corp., Altadena, CA, Report510-04911-007, USRA Research GrantNo. 07600-049, 30 November 2000.2.Holister, W. M., "Castles in Space",Astronautica Acta, 1967.3.Aldrin, E. E., "Cyclic TrajectoryConcepts", SAIC presentation to theInterplanetary Rapid Transit StudyMeeting, Jet Propulsion Laboratory,1985.4.Friedlander, A. L., Niehoff, J. C.,Byrnes, D. V., Longuski, J. M.,"Circulating Transportation OrbitsBetween Earth and Mars", Paper AIAA86-2009, presented at the AIAA/AASAstrodynamics Specialist Conference,Williamsburg, VA, 18-20 August, 1986.5.Nock, K. T., Friedlander, A. 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