View source for GPIS 10: I Was Just Wondering

== General Space Flight Questions ==

'''Why do most rockets have more than one stage?'''

This is due to a combination of necessity and efficiency. Most of the mass of any current technology rocket is fuel, and there also must be some structure and mechanism to make use of the fuel (fuel tanks, pipes, pumps, etc.) in addition to the rocket engines and the payload. As fuel is used up, weight is reduced, but it can be reduced even more by getting rid of any structures and mechanisms that are no longer needed. One way to do this is with stages – when the fuel for the first stage is exhausted, the entire stage is ejected, and the second stage engines can now be used more efficiently since they do not have to accelerate the unneeded mass of the discarded first stage.

There are variations on this, such as the Space Shuttle’s use of two solid rocket boosters (SRB) that are “strapped on” and burn to provide a lot of additional thrust for the first 2 minutes of flight, along with the Space Shuttle Main Engines (SSME) that burn liquid fuel and oxidizer from the external tank. At 2 minutes, the SRB fuel is exhausted and the SRB canisters are dropped (parachutes allow them to be recovered at sea for refurbishment and reuse). The SSME’s continue to burn, accelerating the now lighter Orbiter vehicle and external tank to orbit. This is sometimes called a “zero stage” method.

There are disadvantages in terms of complexity and costs of multi-stage rockets, but until advanced materials, propellants, and engines are developed that allow “single stage to orbit,” variations of the multi- stage approach will continue to be used. This could include aircraft-type first stages such as the hypothetical “TX” shown in chapter 8. See http://en.wikipedia.org/wiki/Staging_%28rocketry%29 .

'''How does a real spacecraft know how fast it’s going and where it is?'''

There are several ways that are used to keep track of the position, speed, and acceleration of spacecraft. One method is called an inertial navigation system (INS). In an INS, gyroscopes are used to keep track of the (x, y, z) directions in 3D space relative (say) to the stars, even as the spacecraft moves and rotates. Special sensors called accelerometers can continuously monitor any accelerations in each direction (from gravity, engines, thrusters, air drag while in the atmosphere, etc.). Since acceleration is defined as a change in velocity (speed and direction), and velocity is defined as a change in position, computers can use this constantly updating information to keep track of the acceleration, velocity, and position in real time.

For more details, see http://en.wikipedia.org/wiki/Inertial_navigation_system . By the way, Orbiter does much the same thing as this, but for Orbiter, the calculations are done many times a second to actually determine your simulated acceleration, velocity, and position as you fire thrusters or engines (thrust forces), fly through the atmosphere generating lift (aerodynamic forces), or get closer or farther from large bodies such as the Sun, planets, and moons (gravitational forces).

In addition to the INS, spacecraft near the Earth (or even quite far from Earth, even near Mars or Jupiter) are tracked by Earth-based radar systems. These systems send out high-power radio waves which reflect from the spacecraft. The reflected signal may be very faint, but they can still measure its direction, how long it took to get back, and any change in its frequency. From this information, they can accurately calculate the spacecraft’s position and velocity. They can also do similar things with a radio signal that can be sent by the spacecraft (from a transponder or communication radio).

'''What is “Delta-V” and why is it important?'''

Delta-V (ΔV) is an abbreviation for “delta velocity.” The Greek character delta (Δ) is used in mathematics to represent the change of some quantity, so delta-V means “change in velocity.” Since velocity is directly related to energy (specifically kinetic energy), and a certain amount of energy is required to move from one state (a particular position and velocity) to another, delta-V is very useful in the planning of space missions. This is because each type of flight (e.g., Earth surface to low Earth orbit [LEO], LEO to the Moon, LEO to Mars, etc.) has a characteristic delta-V associated with it. Although the delta-V can vary somewhat depending on the dates and on the exact route taken, there is always a minimum delta-V for any direct flight, and if you know the available delta-V from your launch vehicle and spacecraft, this establishes whether a particular mission is possible. Here are typical delta-V values for some flights.

{| class="wikitable" style="margin: auto;"
!Starting point
!End point
!Delta-V (meters/second)
|-
|Earth surface
|Low Earth Orbit (LEO)
|9,300-10,000
|-
|LEO
|Low Lunar Orbit (LLO)
|4,200 *
|-
|LEO
|Lunar Surface
|6,300
|-
|LLO
|Lunar Surface
|2,000
|-
|LEO
|Trans-Mars Injection (TMI)
|3,600
|-
|TMI
|Mars Orbit (300 km circ.)
|2,400
|-
|Mars Surface
|Low Mars Orbit (LMO)
|4,100
|-
|LEO
|Ceres (central asteroid belt)
|8,600
|}
 * includes delta-V for both Earth orbit eject and lunar orbit entry braking

'''What is “available delta-V?”''' 

This depends on the thrust of the engines, the amount of fuel carried, and the payload to be carried. For a given amount of thrust and fuel, you can give a small payload more delta- V than a large payload. This is why many satellites can be launched to orbit or even to the Moon or Mars with relatively small rocket boosters – unpiloted satellites can be made pretty small these days. There are equations for all this (the basic one is often called “the rocket equation,” see references below) and they can be worked “backwards,” meaning you start with the payload you want, say “100 metric tons on Mars” and from that you can figure out how much thrust and fuel you need to achieve the delta-V to reach Mars (probably concluding you don’t have a big enough booster to achieve that mission yet). Or you can say “how much mass can we get to Mars with available boosters?”

For reference purposes, the Space Shuttle Main Engines (SSME) and the Solid Rocket Boosters (SRB) provide very close to the delta-V needed to reach a circular low Earth orbit (LEO) of about 400 km altitude (about 9700 m/s – note that this is more than the speed you will have in the Orbit MFD, around 7800 m/s, because it includes the delta-V needed to overcome atmospheric drag). Once it is in orbit, the 
OMS engines (Orbital Maneuvering System) can provide the Shuttle with about 1300 m/s of additional delta-V for orbit changes (rendezvous with ISS, reentry burn, etc.). The (hypothetical and futuristic) Delta Glider, in contrast, has a very high main engine exhaust velocity (40,000 m/s – this seems to suggest a gas- core nuclear fission engine, and there’s a hint of this in the yellow radiation warning symbols above the main engines, but the details of engines are not modeled anyway, only their performance). With this exhaust velocity and with only 58% of its mass as fuel, its available delta-V is about 36,000 m/s.

You can’t fool Mother Nature when it comes to energy, but there are some “tricks.” The basic trick is multiple stages – throw away mass you don’t need (e.g., the empty first stage), giving a lower remaining mass that you can accelerate to a higher velocity. Another “trick” is gravitational assist – instead of going directly to the target planet, you calculate a path to pass close to another planet (without orbiting or hitting it) in a way that uses its gravity to speed up the craft and send it in the right direction for the target planet (sometimes called a “slingshot maneuver”). Venus can be used this way for Mars flights (for example).

For more on delta-V, see http://en.wikipedia.org/wiki/Delta-V and http://en.wikipedia.org/wiki/Delta- v_budget and the various links on those pages. There is also a very good web page on delta-V and rocketry here: http://www.pma.caltech.edu/~chirata/deltav.html (with a useful graph that relates fuel fraction, exhaust velocity, and delta-V, which I used in the Delta Glider estimates above).

Another great resource for interplanetary and lunar flight planning is the personal web site of Dr. Donald Rapp, http://www.mars-lunar.net/ and in particular for this question, his paper on the rocket equation and specific impulse (this three-page, 107 kB PDF file also includes a more extensive table of delta-V values for various flights): http://www.mars-lunar.net/Arch.Elements/1.Rocket.Equation.pdf. There are many other PDF files on trajectories and other technical planning issues, primarily for Moon and Mars missions. Dr. Rapp has extensive practical experience in space mission planning at JPL.

'''What about the risks of human space flight?'''

Every human activity has some risk – you could even get hurt climbing out of bed. But some things are clearly more risky than others, and many people think that flying in general, and space flight in particular, is especially risky. It is true that spacecraft use explosive fuels, fly very fast and high, and go where there is not enough air for humans to survive unprotected. These things can be dangerous if not handled carefully and correctly, but this is why space flight requires special hardware and software systems, extensive testing of the systems and procedures, and very intense training and practice for all the people involved. Even with these efforts, problems can occur, and in a few cases, the problems can be bad enough to kill people, as we know from the losses of Space Shuttles Challenger (1986) and Columbia (2003).

But consider that early airplanes were also very risky, and that when airline travel started in the 1930’s, there were many crashes. Some people wanted the speed and convenience (and adventure, at that time) of flying – they were willing to take the risk even though the systems and procedures still had many problems. A lot of advanced technology was developed for World War II (things like radar and jet engines that allow planes to fly very high, above most of the bad weather), and since then (with many more developments), airline flying has become so safe that almost everyone trusts it. Of course there are still airplane crashes, but they are so unusual that each one is considered major news (unlike car crashes, which kill many people every day and are only local news if that).

Piloted space flight is still in its early stages – it is experimental flight, not routine flight. NASA and other space agencies work hard to make it as safe as possible, but it is not perfectly safe, and astronauts understand this. They choose to do it because they feel it is important (and for the adventure and other 
reasons too). Many people accept risky jobs such as high rise building construction because they need the work, and perhaps also enjoy it as a challenge. Other people enjoy risky hobbies such as mountain climbing. As space flight becomes more common, it will also become safer. There will also be more benefits from space flight over time, some of them not fully known yet. Many people (including me) believe that the known and potential benefits justify the risks of space flight. But there definitely are risks. For information on space disasters, see http://en.wikipedia.org/wiki/List_of_space_disasters