Thrust in Space

[rickler]

Just a quick question, what exactly is a rocket in the vacuum of space pushing against?

 

[huey_pilot]

Yeah, I have also wondered about this too.

 

[arobie]

Newtons 3rd(?) law:<br /><br />For every action there is an equal and opposite reaction.<br /><br />When the rocket shoots fuel out of its back, it goes forward in reaction.

 

[CalliArcale]

To elaborate a bit, this is an effect you can see on Earth too. Any motor that works this way is called a <i>reaction motor</i>. All rockets (including bottlerockets) are reaction motors. So are jet engines -- the only real difference between a jet engine and a rocket is that the jet gets its oxygen from the atmosphere. There are even natural reaction motors -- all squids and some other cephalopods get around by squirting water out through a nozzle in their mantle. The water goes out the back, so the squid goes forward.<br /><br />Sit on a skateboard and fire a rifle. You will be propelled backwards, even though you didn't touch the ground at all. As the bullet is pushed out of the gun, the gun is pushed in the opposite direction. You could even propel yourself just by throwing baseballs, although you'd have to throw them pretty hard to overcome friction with the ground (even on well-greased wheels, you're not really in a frictionless situation). As you push off on the baseball, it is thrown away from you, and you are pushed away from it.<br /><br />Imagine a rocket engine as throwing a huge number of baseballs very fast. The principle is the same. As a chemical reaction causes the particles to be hurled out the back of the nozzle, the rocket is pushed forward very slightly. The rocket weighs a heck of a lot more than the particles, so a LOT of particles have to be thrown VERY fast in order to acheive the desired effect. The Space Shuttle consumes the entire contents of its External Tank in only eight minutes to speed up to 17,500 MPH. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[vogon13]

New York Times had problem with this concept, too. Were rather nasty to Goddard. Believe retraction was printed during the sixties. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[heyo]

Sweet.<br /><br />I'll be getting around from now on by throwing baseballs behind me. So... um... nobody follow me...<br /><br />Seriously, I remember reading that back when space travel was stil theoretical, some people scoffed at the notion, saying that there was nothing in the vacuum for rocket exhaust to push against so you'd never go anywhere. Crow anyone?<br /><br /><img src="/images/icons/laugh.gif" /><br /><br />Heyo

 

[dbzffmore]

Just a curiosity type question. The limit of how fast a rocket can get in space is how much fuel it can carry with it correct? Smaller amounts of fuel or replenishable amounts of fuel but longer burn times would in the end yield a better resut. Like the ion engine? Also, If you could continuosly accelerate during the whole trip, would that not allow you to mimic gravity, opposite the direction of acceleration?

 

[MeteorWayne]

Welcome to SDC!<br />According to Einstein, gravity and acceleration are the same thing. So the answer is yes. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[CalliArcale]

To answer your questions in reverse order (because the second question is easier):<br /><br /><b>Also, If you could continuosly accelerate during the whole trip, would that not allow you to mimic gravity, opposite the direction of acceleration?</b><br /><br />Yes! In fact, what you perceive as gravity is actually your body accelerating towards the Earth at about 9.8 meters per second squared. This is detected by your semicircular canals (there are three in each of your ears), which are basically just accelerometers. As a consequence, you cannot tell the difference between acceleration due to gravity and acceleration due to anything else.<br /><br /><b>The limit of how fast a rocket can get in space is how much fuel it can carry with it correct? Smaller amounts of fuel or replenishable amounts of fuel but longer burn times would in the end yield a better resut. Like the ion engine? Also, If you could continuosly accelerate during the whole trip, would that not allow you to mimic gravity, opposite the direction of acceleration?</b><br /><br />The limit of how fast a rocket can go in space is a combination of the available propellant (not just fuel; if it's a chemical rocket, you've got both fuel and oxidizer and both count as propellant), the specific impulse (efficiency) of the engine, and the mass of the vehicle (including remaining propellant, which leads to some interesting math). Rocket scientists express this as "delta-V" -- change in velocity. Some people spell it "delta vee"; both spellings are acceptable. Delta-v is what really matters for a lot of applications, because it tells you how much you can actually get out of the engine. The Space Shuttle, for instance, accelerates up to 18,500 MPH. That seems like a lot of delta-v, but it's not really. You mention ion engines. These do give you much more delta-v for the propellant mass. I wasn't able to get a figure for the whole Deep Space 1 mission, but that mission (intended in part to test an advanced solar- <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[R1]

I really wish we could get the ion propulsion program going faster.<br />I recall reading a while back that if we send out a next generation Voyager<br />using ion drives it could reach and even pass the existing old generation Voyagers.<br /> <div class="Discussion_UserSignature"> </div>

 

[lukman]

<blockquote><font class="small">In reply to:</font><hr /><p> using ion drives it could reach and even pass the existing old generation Voyagers. <p><hr /></p></p></blockquote><br /><br />Ion drive is a very slow accelerating engine,, weak thrust, photon thrust velocity, but promisingly near light speed is possible. NASA planing to use them for probe, probably 20 years from now. <div class="Discussion_UserSignature"> </div>

 

[Swampcat]

<font color="yellow">" NASA planing to use them for probe, probably 20 years from now."</font><br /><br />Ion drives have been used on several "probes" by NASA (Deep Space 1), ESA (SMART-1) and JAXA (Hayabusa) and, IIRC, other spacecraft use ion thrusters for stationkeeping. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[lukman]

<blockquote><font class="small">In reply to:</font><hr /><p> Ion drives have been used on several "probes" by NASA (Deep Space 1), ESA (SMART-1) and JAXA (Hayabusa) and, IIRC, other spacecraft use ion thrusters for stationkeeping. <p><hr /></p></p></blockquote><br /><br />I thought they are only testing prototype, only carry few basic observing function? <div class="Discussion_UserSignature"> </div>

 

[spacechump]

They may all be considered engineering proof-of-concept spacecraft but all of them did very real science. And because of them the Dawn mission can do what it needs to do to get to its intended targets without carrying excess fuel.<br /><br />As for earth orbiting satellites a good deal have been using Ion propulsion for orbit stabilization for years.

 

[ai_sci]

Hi Calli, <br /><br />Great answer, your depth of knowledge is always a delight. <br /><br />Just a minor point: the semicircular canals of the inner ear are actually rotational acceleration detectors; they detect changes in acceleration/deceleration in roll, pitch, and yaw for the most part.<br /><br /> However, there is another inner ear set of acceleration detectors that detect <b><i>linear</i> </b>acceleration (such as the gravity vector) called the otolith organs. These sensors are crucial in detecting which way is sensed as "down", "up", and "tilt", vise rotation, and are the source of a number of sensory illusions that occur in a wide variety of acceleration environments not normally experienced by humans who do not fly, dive, or drive extremely fast. Or, in the case of microgravity, the lack of a constant acceleration environment.<br /><br />It is difficult to experimentally tease apart the individual functions of each of these acceleration organs in isolation while under a constant 1g environment on earth. It is even more difficult to model the combination of acceleration effects of these organs working in concert, for abnormal acceleration environments, such as in modern jet fighter aircraft. This is why vestibular research is an important part of the biomedical research being conducted on the ISS. Microgravity produces a "natural experiment" wherein the different vestibular organs' functions can be somewhat isolated. <br /><br />*Caveat* Microgravity is not an environment the otolith organs are evolved for at all. Lots of interesting problems associated with that issue.<br /><br />"Abnormal" environments are defined as any acceleration environments that are greater than sitting, standing, walking, running, turning, tumbling, or climbing.<br /><br />For those interested, here is a good source for a basic description of the human vestibular system:<br /><br />http://vestibular.wustl.edu/vestibu <div class="Discussion_UserSignature"> </div>

 

[ai_sci]

stevehw33,<br /><br />I am not trying to be critical, but if you are attempting to educate, you need to get your biology correct. About half of your post has incorrect statements. I do not have much time to spend here these days, so I will not attempt to dissect your post in detail. I also do not want to cherry-pick arguments, so I will only address your first point at this time. I should in all fairness add that I have worked in this field. All I ask is that you do not post "information" that you do not appear to have a good handle on, and to not post as an authority on current data about which you may be unfamiliar.<br /><br /><font color="yellow">stevehw33: Very interesting points about the Vestibular system (balance system) which is rather complex. It's basically the end organ, semi-circular canals, which are connected to the brain stem by the 7th cranial nerve, the auditory nerve, which carries the information from both the cochlea (essentially a sound frequency and amplitute analyzer) and the SC canals.</font><br /><br />Actually, I think you should take a look at the <b>8th</b> cranial nerve for that. Here is one source:<br /><br />8th Cranial Nerve<br /><br />Take a look at the link I posted in my first post here as well, and if you still have some questions, I will try to address them this weekend.<br /><br />Regards,<br /><br />AI Sci<br /><br /> <br /> <div class="Discussion_UserSignature"> </div>