Introduction

Most of the material in this section was abstracted from presentations prepared by Professor Robert Braun for his students at Georgia Tech.

It is amply demonstrated in this book that the most important technology needed to enable human missions to Mars is efficient aero-assist technology for Mars orbit insertion and for descent to the surface from Mars orbit. Aero-assist technology would reduce IMLEO by typically more than 1,000 mT as compared with use of chemical propulsion for entry, descent, and landing (EDL).

We define the following aero-assist operations:

Direct entry is a direct flight into the planet's atmosphere from hyperbolic approach without first injecting into Mars orbit. The entry vehicle can be passive (ballistic) or actively controlled. The passive vehicle is guided prior to atmospheric entry and proceeds into the planet's atmosphere as dictated by the vehicle shape and the atmosphere. An actively controlled direct entry vehicle may maneuver autonomously while in the atmosphere to improve landed location, or modify the flight environment. Direct entry was successfully performed on Viking, Apollo, Shuttle, PioneerVenus, Galileo, Mars Pathfinder and MER missions.

In direct entry, the spacecraft is pointed toward Mars at some flight path angle, and it gradually speeds up as it enters the gravitational attraction of Mars. As it impinges on the upper atmosphere, drag forces will cause intense heating on the frontal surfaces of the spacecraft causing it to slow down. To protect the spacecraft, a heavy aeroshell loaded with thermal protection material is placed in front of the spacecraft. In addition, a backshell is likely to be needed to prevent hot gases from flowing into the spacecraft behind the aeroshell. Although very intense heating takes place, the heat transferred to the vehicle is limited due to the short duration of the deceleration period. The disadvantages of this method are that high deceleration loads are encountered, and the weight penalty of the thick aeroshell. The accelerations encountered in direct entry are likely to be excessive for human landings but might be acceptable for cargo.

Table 4.14. Sequence for direct entry.

Step

Altitude (km)

Time from previous step

Comment

Separate entry system from

cruise stage

12,000

0

Begin entry process

Reach outer edge of Mars

atmosphere

131

~30min

Begin deceleration

Attain peak heating rate

37

~70 sec

Endure peak gr-load

30

~10 sec

About 15-20 g's

Deploy parachute

8

~80 sec

Heat shield jettison

Land

0

~2min

Terminal descent includes

several steps several steps

Propulsion/Aerobraking is a process for inserting a spacecraft into a low Mars orbit (typically a circular orbit). If aerobraking is not employed, and an incoming spacecraft is inserted into Mars orbit using only chemical propulsion, orbit insertion would take place in two steps:

(1) Insert into an elongated elliptical orbit (Au — 1.7 km/s).

(2) With burns at apoapsis and periapsis, reduce the orbit to a circular orbit of perhaps 400 km altitude (Au —0.8km/s).

Aerobraking is a technique for eliminating the second step, thereby reducing Au for orbit insertion from —2.5 km/s to about 1.7 km/s. Aerobraking is a relatively low risk maneuver that consists of repeated dips into an atmosphere to generate drag and lower velocity. Large performance margins are maintained to accommodate significant atmospheric variability. Generally, the total heat flux and peak temperatures are low enough to fly without a thermal protection system. The primary drag surface is typically the solar array panels, and the maximum heating rate (typically — 0.6 W/ cm2) is dictated by the temperature limit of the solar array (typically — 175°C). Aerobraking has been used on a number of missions at Earth, Venus, and Mars. Despite advances in aerobraking automation, aerobraking remains a humanintensive process that requires 24/7 maintenance for several months with close cooperation between navigation, spacecraft, sequencing, atmosphere modeling, and management teams. The time-sequence of aerobraking is shown in Figure 4.16.

Aerocapture is a maneuver that takes advantage of a planet's atmosphere to slow a spacecraft to orbital capture velocities and results in orbit insertion in a single pass. Descent into the atmosphere causes sufficient deceleration and heating to require a massive heat shield (aeroshell). The trajectory of the inbound spacecraft is bent around Mars as shown in Figure 4.17. As the spacecraft exits the atmosphere, the heat shield is jettisoned and a propulsive maneuver is performed to raise the periapsis. The entire operation is short-lived and requires the spacecraft to operate

Initial capture orbit (48 hr)

Figure 4.16. MGS aerobraking process. [Based on NASA GSFC website.]

Initial capture orbit (48 hr)

Figure 4.16. MGS aerobraking process. [Based on NASA GSFC website.]

Figure 4.17. Schematic sequence of events in aerocapture.

autonomously while in the planet atmosphere. Later, a final propulsive maneuver is used to adjust the periapsis.

In one design78 an elongated elliptical orbit with a major axis of ^ 10,000 km and an eccentricity of ^0.34 was generated by initial aerocapture. Subsequent maneuvers to raise the periapsis to a safe 400 km altitude (while reducing the apoapsis to ^3,800 km) require a Av of about 250 m/s.

Aerocapture has been proposed and planned several times, and was even developed for the MSP 2001 Orbiter, but it has never actually been implemented. A change in the MSP 2001 Orbiter was made to propulsive capture followed by aerobraking. Nevertheless, it is instructive to review the design for the MSP 2001 Orbiter. In this design, the minimum altitude (^50 km) was reached about two minutes after entry into the atmosphere. The maximum deceleration was reached at that point (about 4.5 Earth g). Exit from the atmosphere occurred at about 9 minutes after minimum altitude, or 11 minutes after entry. Various masses associated with this system are listed in Table 4.15.

A comparison of aerocapture and aerobraking options for the MSP 2001 Orbiter is given in Table 4.16. It should be noted that the mass of the entry system for aerocapture was about 60% of the mass inserted into Mars orbit. There is little advantage mass-wise for aerocapture over aerobraking. However, aerobraking has the disadvantage of taking several months whereas aerocapture is accomplished in a few days.

78 Sizing of an Entry, Descent, and Landing System for Human Mars Exploration, John A. Christian, Grant Wells, Jarret Lafleur, Kavya Manyapu, Amanda Verges, Charity Lewis, and Robert D. Braun, Georgia Institute of Technology, preprint, 2006.

Table 4.15. Some characteristics of the MSP 2001 Orbiter aerocapture system (as of the end of Phase B of the mission).

Maximum deceleration ~4.4g's at 48 km altitude

Minimum altitude ~48 km

Time duration at altitude <125 km ~400sec

Mass prior to Mars entry 544 kg

Heat shield mass 122 kg

Backshell mass 75 kg

Total entry system mass (includes all structures and mechanisms) 197 kg

Entry system mass/Mass prior to Mars entry 0.36

Entry system mass/Mass placed into orbit 0.6

Approach mass/Mass placed into orbit 1.66

Post-aerocapture periapsis-raise maneuver propellants

(400 km circular orbit) 20 kg

Payload mass in Mars orbit 327 kg

Table 4.16. Comparison of aerocapture and aerobraking options for the MSP 2001 Orbiter.

Component Aerocapture system mass Propulsive capture/

aerobraking system mass (kg) (kg)

Launch mass (wet) 647 730

Cruise stage 71 71

Cruise propellant 32 32

Entry system 197 0

Propellant at Mars 20 321

Payload mass 327 377 Payload mass ratio 0.51 0.52

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