History of the HL-20

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In Aeronautics industries, the simulation does the key role, because of the complexity. When we do the testing by prototype it takes considerable time, quite expensive, and difficult to check the results while change the parameters.

The spacecraft landing has an account with considerable practical and analytical problem because of the unpredictable external-environmental parameters i.e. weather, wind.

The HL 20 is the NASA designed model for a manned spaceplane, known as Crew Emergency Return Vehicle (CERV) or Personal Launching System (PLS). The concept of the PLS has been developed to carry six to eight men to space stations. 


Total length - 8.9 m (29 feet)

Maximum Diameter-wingtips - 7.2m (23.5 feet)

Total habitable volume - 16.3 m3

Total mass - 10 884 kg

Total payload - 545 kg


A lifting body is basically a wingless vehicle that flies due to the lift generated by the shape of its fuselage. researchers  including Alfred Eggers at the NASA Ames Research Center conducted early wind tunnel experiments find that half of a rounded nose -cone shape that was flat on top and rounded on bottom could generate Lift Drag ratio of about 1.5 to 1.

Literature Survey

History of the HL-20

After the 2nd world war the powerful countries were competing to dominate the revolution of the space world, yield in 1969 the America attained their first step at moon. After revolution in space world, the requirements to use the spacecrafts are rapidly increased in last decades.

In 1983, Vehicle Analysis Branch began the investigation of BOR small space plane being orbited several times by the Soviets starting in 1982 and recovered at in the Indian Ocean and Black sea. During the recovery operations of the space plane in the Indian ocean, an Australian P-3 Orion aircraft obtained photographs of the vehicle both floating in the water and being hauled aboard the recovery ship.  [2]. this provided the valuable insights into the shape, weight, and center of gravity of the vehicle. Based on this information, small wind tunnel models were manufactured and tested by NASA.the results demonstrated that, the vehicle had got good Aerodynamic characteristics throughout speed range from orbital entry interface to low supersonic speeds. Wind tunnel tests configuration directional stability at all speed from Mach 20, trimmed to maximum L/D with 10 degree elevon deflections in subsonic range.

Lifting Body Heritage

Lifting body concepts were proposed for transporting people to and from space in late 1950s. In those days NASA Langley Research Center developed a lifting body known as HL 10 it could carry 12 people and be launched on a Saturn IB booster with about 15000lb of payload to service an orbiting space station. But the HL-20 design approach was received Dec 10 1992  rivision received Feb 15 1983accepted for the publication Feb 17 1993. [1]. The NASA Ames Research center developed the M2-F2 lifting body concept, for this mission whereas the US AirForce developed the X-24 lifting body concept for military purposes.each of these configurations was propelled the extensive research and wind tunnel testing.

Very beginning of the research periods, the primary goals included the definition of concepts that would be reusable and have minimal operational refurbishment requirements, low entry accelerations, fixed geometries, runway landing capability, and a minimum of a once-per-day return capability to the USA. The specific vehicles goals were the achievement of a Lift Drag ratio grater than 1 at hypersonic speeds, high trim-lift coefficient, Lift Drag ratio grater than 4 at subsonic speeds, high volumetric efficiency, static stability and controllability of all speeds and of course compatibility with projected launch vehicle. [1].

HL 10 Lifting Body

The vehicle length was 21.17 feet. The launch weight with propellants was 10 009 lb and the landing weight was 6473 lb, the center of gravity range from 53.14 percent of the body length for the launch weight configuration to 51.82 percent for the landing condition.

From: NASA Reference publication 1332 1994 HL-20 chronologies

  • 1983 January 1-NASA Langley begins studies leading to HL-20
  • The vehicle analysis branch began investigation of the Soviet BOR-4 small models were tested in NASA wind tunnels and demonstrated that the vehicle had good aerodynamic characteristics throughout the speed range orbital entry interface to low supersonic speeds. The Soviet design had a 2400 km cross-range capability and outstandingly benign thermal profile at peak heating conditions. Therefore Langley adopted it as a baseline for a Crew Emergency Rescue Vehicle to backup or replace the shuttle after 1986 Challenger accident.

  • 1989 October 1 -Rockwell begins year long contracted study of HL-20
  • Rockwell International (Space System Division) began a year-long contracted effort managed by the Langley Research Centre to perform in an in-depth study of personal Logistics system design and operations with HL-20 concept as a baseline. The space plane would supplement the shuttle in support of the space station freedom.

  • 1991 October 1 - Lockheed Feasibility studies of HL-20
  • Lockheed Advance Development Company began a study to determine the feasibility of developing a prototype and operational system. The study objectives were to access technical attributes, to determine flight qualification requirements, and develop cost and schedule estimates.

  • 1991 December 1 - HL-20 Mock-up tests completed
  • NASA, North Carolina State University and North Carolina A & T University built a full-scale model of the HL-20 for human factors research on the concept. In the end, Space station Freedom became the International Space Station. As the initial crew emergency rescue vehicle, the Russian Soyuz spacecraft was selected. However NASA, looking for a higher-capacity alternative and concern about reliable availability of the Soyuz in the future, did begin development of the X-38CERV in 1997. The X-38 was however based on the Johnson concept of parachute-assisted landing, and used the pure -USA X-24 lifting body shape.

History of simulation

In last decades of the twentieth century, AIAA Modeling and Simulation Technical Committee were involving to develop the aircraft/spacecraft models. When they developed the simulations they had identified and include the basic simulation parameters for airframe model such as function tables, block diagram, mathematical equation (nonlinear partial equations) and verification test data to check the data before shear the data with another modal. The data should be able to interpret to the standard format or code by the internal architect of the simulation.

In late 1990s they developed candidate format to the aerodynamic section of the simulation model i.e. if want to clear or exchange the data, mathematical equations, definitions and the function tables are required.

When we consider the HL-20 NASA model the aerodynamic model contains 51 variables such as 168 one - and two dimensional table, four breakpoint sets, and total of 6240 data point. It defines the outputs for six aero dynamic coefficients i.e. Cx, Cy, Cz, Cl, Cm, Cn as a function in angles of attack (AOA) angle of slide ship, Mach number, Airspeed and angular body rate. This includes the non linear function as interpolated tables, switches and absolute value elements in the variable definitions.

From Evaluation of a Candidate Flight Dynamics Model

Simulation Standard Exchange Format

E. Bruce Jackson* NASA Langley Research Center, Hampton, VA 23681

Bruce L. Hildreth† SAIC, Lexington Park, MD 20653

Brent W. York‡ Naval Air Systems Command, Patuxent River MD 20670 and

William B. Cleveland§ Northrop Grumman Information Technology, Moffett Field, CA 94035

Chapter 2

Theoretical analysis

Modelling assumptions and limitations

The simulation of the spacecraft system is complicated system, so for easy work and analysis we assume the model or geometry of HL 20 as follows.

  • HL 20 airframe is laterally symmetrical
  • The airframe consists of three type of movement during flying i.e. the pitch movement, Yaw movement, and Roll movement. If we don't model laterally symmetrical we are not able to manage the steady state on rolling movement. It makes more complicate. Therefore we assume the airframe is laterally symmetrical.

  • It s incompressible i.e. the compressibility effects can be negligible

The Mach number is an important parameter in flight mechanics; it can be calculated by the following equation

According above equation the Mach number is depended on fluid velocity. If the fluid can be compressible, the surrounded fluid of the airspeed indicator velocity can't be the same as outside fluid velocity. And also the compressibility can vary with respect to the speed of the airframe. Therefore we assume the fluid is incompressible.

  • The airframe is rigid and consists of steady mass
  • The airframe contains three main components such as fuselage, wings, tail. If the airframe is not rigid the components can deform from the original shape. So the moments of the airframe can be varied due to the deformation.

    If they don't consist steady mass the force can vary during the motion of the airframe. It makes the fluctuation in the inertia. Therefore we assume that, the model is rigid and consists of steady mass.

  • The control effectiveness is varied nonlinearly with the angle of attack AOA, and linearly with the Angle of Deflection
  • he environmental model is non-linear 6 degree of freedom. When we consider the spacecraft, there are four forces acting on it. By adjusting them we can control the airframe and glide it.
  • Lifting force

    The lifting force is produce by the dynamic effect of the air acting on the airfoil i.e. due to the pressure difference - Bernoulli's principal. It acts perpendicular to the spacecraft's path through the centre of lift, which depends on the shape of the spacecraft and airspeed. If the airframe flies in the vacuumed space the lifting force is zero. Because the density tends to zero.

    Thrust force

    The forward force produces by the power plant or propeller/rotor. It opposes or overcomes the force of drag.  As general rule, it acts parallel to the longitudinal axis. For spacecraft moving the thrust force must be exerted and be greater than drag force. The spacecraft has to move until equalize the drag force by thrust force to maintain the constant speed. 

    The thrust force can be calculated by the following equation. If airframe flies in the vacuumed region there is no thrust force. Because mass of the air equals to zero.

    Drag force

    The drag force is a friction force, which is generated by the interaction and contact of the solid body with fluid. We can reduce the drag force by model the appropriate shape of the airframe. Drag force can be calculated by the following equation.

    Centre of gravity

    Generally the spacecraft is design like a Kite, so we can anaysis the position of the gravity by analysis the centre of the gravity of the kite.

    The product of centre of gravity and weight equals the sum of the products of the component weights and distances. i.e  W*cg= ?W*d

    So W*cg =WL*dL+ WF*dF+WW*dW+WU*dU

    The above diagram illustrates the mass distribution of the spacecraft. So, in order to change the angle of descending we can use the weight favourably i.e. which associates the trust force during the landing but during the lifting it acts on opposite direction of the spacecraft path. 

    The coordinate system

    When we consider the space craft motions, the motion is calculated and guided according three sets of coordinate system.

    • The wind axis
    • o X axis - positive in the direction of the ongoing air

      o Y axis - positive to right of  X axis perpendicular

      o Z axis - positive  downwards, perpendicular  to X-Y plane

    • Inertial axis
    • o X axis - positive forward through nose of the aircraft/spacecraft

      o Y axis - positive to right of X axis

      o Z axis - positive  downwards, perpendicular  to X-Y plane

    • Earth axis
    • o X axis - positive in direction of north

      o Y axis - positive in the direction of the East

      o Z axis - positive towards the centre of the earth

    The communication system

    Basic mechanism

    The spacecraft landing is a wide range of analysis with several parameters. Some of them related with spacecraft elements and radar system, some parameters related with atmosphere/ environmental factors and some are related with the control system from the ground/ space station. The following table illustrates the parameters relationship.

    The spacecraft consists of several systems to detect the changes in parameters.

    • Static Pressure system
    • The ALT meter is functioned by the static pressure system and also the airspeed indicator is functioned by the static pressure system and pilot pressure system. The static pressure system is placed - opened to the exterior of the airframe to sense the atmosphere pressure. The narrow opening is described as static port. This system has to fixed very accurately with the airframe, i.e. we have to consider the all possible angle of attack and make sure the static pressure is very close to the atmosphere pressure.

    • ALT meter
    • The ALT meter is used to measure the altitude of the airframe. The ALT meter is calibrated to show the pressure directly as an altitude above mean sea level according the International Standard Atmosphere ISA.

    • The attitude Indicator
    • It is also known as artificial horizon, which gives the instruction about the relative attitude to the horizon. According this data we can say that, whether the wings are level and if the aircraft nose is pointing above or below to horizon.

    • The airspeed indicator
    • The airspeed indicator gives the airspeed. It functions by the static pressure and the pilot pressure system. The airspeed indicator also calibrate for the sea level atmosphere. When the temperature/pressure combination yields the density altitude higher than sea level, the airspeed indicates the lower airspeed.  In other hand, if the density altitude is less than sea level the airspeed indicator detects the faster airspeed.

    • True airspeed

    True airspeed can be defined by, the relative speed of the airframe with respect to the air mass. The navigation system is worked by gathering the true speed and some other data. The true airspeed can be calculated by the following equation.

    • Indicated airspeed
    • The airspeed indicator ASI works according the pitot tube principle. It reads directly by the airspeed indicator and also it is directly related with calibrated airspeed.

    • Calibrated airspeed

    The airspeed indicator has got two types of errors such as in instrument error, and position error. After correct those errors, the airspeed indicator will show the new value, which is called calibrated airspeed. The CAS can be calculated by the following equation.

    • True altitude
    • The true altitude is measured by the airframe from mean sea level, but unfortunately the airframe can't measure the true altitude. It can measure only indicated altitude. During the landing i.e. approach very close to ground the indicated latitude is very close to the true altitude.

    • Indicated altitude
    • The indicated altitude is directly measured by the ALT meter of the airframe. It is useful to maintain the terrain/obstacle clearance and maintain the vertical separation to next airframe that passes over the airframe.

    • Pressure altitude
    • When the airframe flies above 18000 feet with high speed (subsonic speed or hypersonic speed), getting up-to-date ALT meter setting is not practically possible. And also we can't assume the Indicated altitude is same as true altitude due to the high difference between them by the high above reporting stations. So the pressure altitude   does not contain terrain, it consists only vertical separation.

    • Absolute altitude
    • The absolute altitude means the height of the airframe from the ground. If the airframe flies over the coastal area the absolute altitude is same as true altitude. But if the airframe flies over the hill the absolute altitude is varying with the fluctuation of the height of mountain. This is very important to prevent from the airframe crashes with ground.

    • Density altitude

    The lifting force is depends on the density atmosphere, during the flying the the density is varying with the altitude changes. And also the engine wants oxygen for the combustion, if the density of air decreases the concentration of the oxygen in air also decrease. So it makes the Chemically Oxygen Demand (COD) effect, Yield the engine generates less power so the thrust force can be decreased. So the density altitude is used to observe and maintain the air density of surrounded air.

    From http://www.meretrix.com/~harry/flying/notes/altitudes.html

    • The drift angle
    • The drift angle means the angle between longitude and the path of the airframe. The drift indicator measures the drift angle.

    • The heading Indicator
    • The heading indicator is also known as directional gyro or gyro compass. It displays the aircraft heading belongs the geographical north. The horizontal Situation Indicator can be replacing to the heading indicator.

    • Turn indicator
    • The turn indicator measures the direction of turnings and the rate of turnings. The quality of turn is also can be observable by using the turn indicator.

    • Slats
    • Slate is a instrument fixed at the aerodynamic surface of the leading edge in wings. It used to change the wings shape artificially to make higher angle of attack. Slates contribute the safety and slow taking off or landing.

    • Static Air Temperature
    • The static air temperature is measured by specially modified temperature mounded on the airframe surface. The probe is designed to bring the air to rest relative to the airframe i.e. the speed of the air is same as airframe's speed. But practically the air is seemed as compressed (Adiabatic process). So the total temperature is bigger than the static temperature. The relationship between the static temperature and total temperature is given by the following equation.

    • Pitch
    • Roll
    • Yaw

    Basic lows and Principles

    During the flying time the airframe is hold in the air due to the lifting force. There are several explanation for the generation of the lifting force. Generally the proponents  of the arguments belong into two places.

    1. Bernoulli's principle
    2. Newton's postion

    Bernoulli's principle

    The Bernoulli's principles states that increase in the speed of the fluid occurs simultaneously with a decrease in pressure or decrease in a fluid's potential energy. The Bernoulli's principle can be applicable for incompressible laminar flow. It can be describes in mathematical form as follows.

    Newton's first low

    The body at rest will remain at rest and a body in a motion will continue in straight line motion unless subjected to an external applied force.

    Newton's third low

    For every action, there is an equal and opposite reaction.