Weightlessness and Rocket Propulsion

1 Risks of Long Term Space Flight and How to Migate Them

2 How do we measure radiaon?

In SI units, the amount of radiaon absorbed is measured in Grays (Gy):

1 Gy = 1 joule of energy absorbed per kilogram of material (Another unit you see is “rad”, where 1 rad = 0.01 Gy)

For human dosage, we typically talk about mGy.

Radiaon deposits energy in unique ways into biological systems.

milliSieverts (mSv) describes the biological equivalent radiaon dosage. (an older unit called the “rem”)

This number has the same units as Grays, but it does not measure the same thing. Sieverts take into account the type of radiaon and the biological damage it can cause.

3 How do we measure radiaon?

Let’s look at some typical numbers: An average six month stay aboard the ISS: Dose will be higher for astronauts • 80 mSv (solar max) parcipang in space walks. (Note: • 160 mSv (solar min) NASA avoids spacewalks during • 1 mSv is approximately equivalent to 3 high risk periods) chest X-rays (Note: the radiaon is different) • NASA limits exposure to 200 mSv/yr • NCRP limits flight crews to 500 mSv/yr

Typical radiaon dose on Earth is 2 mSv/yr due to background radiaon. hp://www.epa.gov/rpdweb00/understand/calculate.html

This will vary with altude, proximity to nuclear power, local geology, and other 4 factors. What is NASA doing to protect astronauts?

NASA follows the protecon pracces that are recommended by the US Naonal Academy of Space Science Board and the US Council on Radiaon Protecon and Measurement.

Astronauts wear dosimeters: • Keeps track of radiaon dosage • Understanding where the space staon has beer shielding • Serves as a warning device for increased solar radiaon

Healthy diet and exercise including anoxidants.

Researching amount of radiaon and biological effects to help protect future astronauts. 5 Outside the Earth’s Magnetosphere

The ISS is sll protected by the Earth’s magnetosphere.

The Mars Radiaon Environment Experiment (MARIE) was placed on board Mars Odyssey to study the amount of radiaon on the journey from Earth to Mars.

Mars has no large scale intrinsic magnec field to protect astronauts on the surface or in orbit.

Mars also has a thinner atmosphere, which stops a smaller amount of the Sun’s electromagnec radiaon and high energy parcles.

MARIE connues to study the amount of radiaon in while orbing Mars.

6 Shielding: Materials Aluminum is a poor material for radiaon shield, but we build spacecra out of it.

Lead produces many secondaries.

Hydrogen is a possibility, but leaking and cryogenic tanking are major difficules.

On Lunar and Maran surfaces we can burrow beneath the surface.

Polyethylene (C2H4)n is much, much beer. • Light elements have fewer neutrons. • C decays to helium nucleus instead of neutrons. • NASA sciensts have developed a sff version (RXF1) 7 Shielding: Electrostac Uses large electric fields to repel posively charged parcles.

Larges voltages are required. 1. Potenal arcing issues. 2. How do we generate these large voltages? 3. How much power is required?

Can this system stop X-rays and gamma rays?

Currently deemed unfeasible due to arcing issues.

8 Shielding: Magnec

Magnec shielding could work in a similar way to Earth’s magnec field deflects the solar wind.

The problem is that amounts of power are required to generate the necessary magnec fields to divert cosmic rays.

The magnec field also needs to be generated by large coils.

Because of these technical challenges it is not being funded.

9 Shielding: Plasmas Magnetosphere concepts like M2P2 could be used to protect astronauts from harmful space radiaon.

The plasma concepts work to overcome the limits of magnec systems, which require large power supplies and coils.

Currently being invesgated in the UK

10 Weightlessness We oen say astronauts in space are “weightless.” More accurately, we call this “freefall.”

We’re being sloppy.

Gravity is sll there, but the floor and the astronaut are both falling at the same rate, so the astronaut never hits the floor.

As the astronaut falls, he also moves forward; the Earth surface is curving away at the same rate. They are in orbit.

Acceleraon due to gravity is about 3% less than at the Earth’s surface. We’ll call LEO a “microgravity environment.”

11 Microgravity Influence: Blood Circulaon Puffy Face Syndrome: • On Earth, the heart must work to get blood to the head, but it does not have to work had to get blood to the legs. • In space, less blood flows to the legs. • Oen accompanied by nasal congeson, headaches, swollen/ red eyes, dizziness, nausea

12 Microgravity Influence: Blood Circulaon Puffy Face Syndrome: • Heart pumps more blood with each beat. • Kidneys respond by producing more urine. • Pituitary gland produces more an-diurec hormones, which makes an astronaut less thirsty. • This combo helps rid the head and chest of excess fluids. • Adapng typically takes 2 or 3 days. • On the ground, this return to normal quickly. 13 Microgravity Influence: Blood Circulaon Kidneys also decrease producon of erythropoien, which is a hormone that smulates red blood cell producons in the bones.

Since the volume of blood fluid has decreased, the number of red blood cells also decreases to keep the rao the same as it was on Earth.

Upon return to Earth, the red blood cell count returns to normal.

14 Microgravity Influence: Muscle Structure • Muscle Atrophy: connuously loose muscle mass and tone due to less use (especially in the leg).

• Body adopts a more fetal posion.

• Slow-twitch endurance muscle fibers decrease, while fast-twitch muscle fibers increase.

• Astronauts try to migate the effects by taking medicaon and exercising frequently.

• This could pose issues for long duraon Muscle cells before (A) and aer (B) a 17 day period space flights. in space: slowed protein producon, irregularies, increased fragility, and increased fat stores (L). 15 Microgravity Influence: Bone Structure • Osteoblasts are bone cells that lay down new minerals. In space they slow down. (No one knows why.)

• Osteoclasts are bone cells that chew up the mineral layers. They don’t slow down in space.

• This imbalance increases calcium in the blood, which can lead to kidney stones.

• Bone degradaon: bones can loose up to 10% of mass and become weaker at a rate of 1 % per month due to lack of minerals.

• Aer a 2-3 month mission, astronauts will regain normal bone density within 2-3 years.

• Drugs are being studied to migate the problem. 16 Microgravity Influence: Bones and Muscle Structure

Astronauts oen experience back pain while in microgravity.

Pain might be associated with relaxaon of muscles and ligaments in the spine.

17 Microgravity Influence: Balance and Orientaon

The brain ordinarily receives a variety informaon on its environment: 1. Eyes (visual) 2. Muscles and tendons (propriocepve apparatus) 3. Sensors which detect fluid movement in the inner ear (vesbular apparatus)

The brain is used to terrestrial informaon.

It has trouble processing the informaon from a microgravity environment.

Crew of STS-90 18 Spacewalk Issues

While in their spacesuits, astronauts breathe low pressure, pure oxygen.

Prior to using the suit, astronauts must purge their body of nitrogen by slowly transioning between the air in the shule/ISS and the pure oxygen of the spacesuit.

Aer a spacewalk, pressure can be restored quickly.

If the nitrogen purge is not done, astronauts are subject to decompression sickness.

Decompression sickness is damage to ssues caused by nitrogen bubbles in the cells and blood stream.

19 Some Soluons NASA Uses

We already menoned that NASA astronauts exercise (up to 2 hours/day) and take medicaon to help with muscle atrophy.

To help with the loss of fluids, astronauts use a device called lower body negave pressure.

This device uses vacuum cleaner like sucon to help bring fluid to your legs. Astronauts might spend 30 minutes/day in this device.

Much of the research on the ISS is devoted to understanding how long term space flight effects the human body, and what can be done to migate these effects.

20 Propulsion Methods

21 Conservaon of Momentum Our rocket has three parts 1. Payload (science, human, life support, communicaons…) 2. Structure (rocket, pumps, tanks, structural support…) 3. Fuel (expelled for propulsion)

We want a way to calculate the change in speed of the spacecra due to firing our rocket.

Momentum: Mass mes velocity (p = mv) Momentum is conserved: inial momentum equals final momentum

22 Conservaon of Momentum inial momentum = final momentum

(rocket is at rest) 0 = mcraftvcraft + m fuel (− v fuel )

So this shows the basic tradeoff mcraftvcraft = m fuelv fuel for rockets.

To go faster you must: m fuelv fuel 1. Maximize fuel speed vcraft = 2. Maximize fuel mass 3. Minimize craft mass mcraft 23 Conservation of Momentum (Rocket Style):

Actually it’s worse than it appears.

m fuelv fuel vcraft = mcraft

Rockets don’t work by throwing all of their fuel off at once. To get into orbit would kill an astronaut!

So, each part of the fuel ejected is pushing against the payload and the remaining fuel!!!!

24 An Example of a High G launcher is a Railgun • A railgun uses electromagnec forces to accelerate a payload • You have already seen an example of one – ring launcher • One example: 1.6 km barrel launches projecles are 7.5 km/s (that’s almost 2000 g’s).

25 Rocket Equaon To correctly compute the change in speed, we need to use calculus. Here’s the answer wrien in two ways:

" Δv % " m fuel % $ ' m fuel # ce & Δv = ce ln$1+ ' = e −1 # mrocket & mrocket

mfuel = fuel mass mrocket = payload + structure mass ce = exhaust speed Δv = change in spacecra’s speed

26 Rocket Equaon " Δv % $ ' mfuel = fuel mass m fuel c = e# e & −1 mrocket = payload + structure mass ce = exhaust speed mrocket Δv = change in spacecra’s speed m fuel 160 mrocket 140 120 100 80 60 40 20 Δv 0 c 0 1 2 3 4 5 6 e 27 Rocket Equaon " Δv % $ ' mfuel = fuel mass m fuel c = e# e & −1 mrocket = payload + structure mass ce = exhaust speed mrocket Δv = change in spacecra’s speed m fuel 1000 mrocket 100

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1 Δv 0.1 c 0 1 2 3 4 5 6 e 28 Rocket Equaon: Example How much fuel do we require to send a 1000 kg rocket to the Moon (Δv = 16 km/s) using a chemical rocket (ce = 4 km/s)?

" Δv % m $ ' fuel = e# ce & −1 mrocket " Δv % ) $ ' , m = m +e# ce & −1. fuel rocket + . * -

29 Rocket Equaon: Example How much fuel do we require to send a 1000 kg rocket to the Moon (Δv = 16 km/s) using a chemical rocket (ce = 4 km/s)?

" Δv % ) $ ' , m = m +e# ce & −1. fuel rocket + . * -

30 Rocket Equaon: Example How much fuel do we require to send a 1000 kg rocket to the Moon (Δv = 16 km/s) using a chemical rocket (ce = 4 km/s)? " Δv % ) $ ' , m = m +e# ce & −1. fuel rocket + . * -

31 Staging We can use staging as a trick to help improve the situaon.

Once the fuel in the first stage is used, we can drop that stage. Now we no longer need to carry that structure into space.

EX: If 10% of your inial mass is structures and Δv/ce = 2: 1 stage: payload fracon = 3.8%

2 stages: payload fracon = 7.5% 32 Types of Fuel: There are three types of physical process that we use for fuel in space propulsion.

1. Chemical Reactions: This amounts to triggering an energy releasing chemical reaction in a controlled (or uncontrolled) way. (By far the most common method). 2. Plasma Reactions: These thrusters use electric fields to accelerate ions. (Not used for launches, but more common now in trajectory corrections). 3. Nuclear Reactions: Nuclear propulsion relies on fission power to generate massive amounts of energy that propel exhaust at huge speeds. (Hard to control, but can also be an impulse drive).

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