L11 - Returning from Space

Question 1

What are the competing requirements for entry design?

Answer 1

• Deceleration
• Heating
• Accuracy

Question 2

What are the requirements for re-entry deceleration?

Answer 2

• Structure + payload limit maximum deceleration
• Human payload, max 12g for short periods
• High enough deceleration to avoid bouncing of atmosphere

Question 3

What are the requirements for re-entry Heating?

Answer 3

Good design must prevent air-molecules and vehicle friction from damaging the spacecraft.

Question 4

What are the requirements for Accuracy during re-entry?

Answer 4

The trajectory needs to be aimed at a precise landing location on Earth.

Question 5

What is an entry corridor?

Answer 5

It is the possible re-entry path resulting from the application of all deceleration, heatin and accuracy requirements.

Question 6

What would happen to a spacecraft that doesn’t follow the entry corridor?

Answer 6

It would either undergo too little drag and bounce off the atmosphere or undergo too much drag and burn-up/break.

Question 7

Can the entry corridor be modified mid-flight?

Answer 7

Yes, while the max acceleration and heating are usually fixed by design, Vre-entry and the flight path angle y can be modified to adjust the entry corridor’s width.

Question 8

What happens if the entry velocity and flight path angle are both increased?

Answer 8

The maximum deceleration and heating rate increase.

Question 9

What can you do to increase the size of the re-entry corridor?

Answer 9

Decrease the re-entry velocity and flight path angle.

Question 10

What are the two approaches to solve re-entry issues?

Answer 10

Trajectory Design

• Entry velocity
• Entry flight-path angle

Vehicle Design

• Vehicle size and shape
• Thermal protection system

Question 11

What is the ballistic coefficient of a spacecraft?

Answer 11

It is a measure of its ability to overcome air resistance in flight. It describes a spacecraft’s mass, drag coefficient and cross-section area.

Question 12

What are the effects of changing the ballistic coefficient of a spacecraft?

Answer 12

Low BC: Light, blunt behicle - slows quickly

High BC: Heavy, streamlined vehicle - slows slowly :)

Question 13

List three types of thermal protection systems.

Answer 13

• Heat sinks
• Ablation
• Radiative cooling

Question 14

How do Heat Sinks work?

Answer 14

Extra material spread out to store the heat, simple but heavy - it can reduce payload.

Question 15

How does Ablation work?

Answer 15

The shield material melts/evaporates taking away the heat. But the spacecraft must be completely refurbished after each flight.

Question 16

How does radiative cooling work?

Answer 16

A high emissivity material with a high melting point can emit almost as much energy as it absorbs. This is used in conjunction with an insulator to protect the spacecraft.

Question 17

What advantages are offered by lifting entry?

Answer 17

Lifting entry takes advantage of the creation of lift at certain angles of attack. It allows the entry corridor to stretch and improve accuracy.

Question 18

What is aerobraking and how is it used for interplanetary missions?

Answer 18

Aerobraking makes use of drag and lift to slow down a spacecraft. Drag can perform the equivalent of a ΔVretro. Aerobreaking is almost 10 times more efficient than propulsive deceleration.

Drag will reduce the speed of a spacecraft approaching a planet until capturing it into an elliptical orbit with the perigee located in the atmosphere. Circular parking orbit is then achieved by small ΔV.