ISS Reboost frequency and amount?

[disownedsky]

<p>I'm trying to sanity check something, and am wondering if the information on ISS reboost history and future planning is publically available somewhere. I am particularly keen to see what the plans are for Solar Cycle 24, which is just starting now. My understanding (which is subject to correction) is that Progress has been doing the reboost maneuvers at each visit, and they are generally fairly small: on the order of 2-3 meters/second.</p>

 

[mrmorris]

Well -- that's really difficult information to locate. I had to go to all the way to Google and put into the search field the folllowing terms: <b>ISS reboost history</b> <br /><br />The data you want is in the third link from the top<br /><br />Now that gives the reboost frequency, and the changes in terms of altitude. Your post indicates you're looking for it in terms of meters per second. To be able to aonvert the two -- you'd need a calculator that gives you obital velocities for a given altitude. Going <b>all</b> the way back to Google again (this is tiring), I put in the serch terms: <b>orbital velocity altitude</b><br /><br />The second returned link is a NASA supplied tool to give velocities and periods for a given altitude. Between the calculator and the graph on the first link, you should be able to satisfy your curiosity.

 

[disownedsky]

<p>I saw that, but didn't scroll all the way to the bottom since it was an old status report. One doesn't expect status reports to recount much history.</p><br /><p>Anyway, it looks like reboosts have gotten smaller as the ISS grows, which makes sense, but I wonder how comfortable NASA is operating at 340 km - that's about 1 scale height below 400 km. I'd also like to see the history for the last year and plans for the next 3-5 years.</p>

 

[gunsandrockets]

Not that it's much help, but I have a vague recollection that ISS maneuvers consume 7 tonnes of propellant per year.

 

[chode]

The requirements for reboost are not a function of the size or mass of the ISS, but are almost completely determined by the density of the atmosphere at orbital altitude. We are indeed entering the next solar cycle, in which increases in solar activity will occur, and so we should expect much more frequent and sizeable boosts of the ISS over the next 6 years or so, compared to the last 6 years or so. Since the Shuttle should be a frequent visitor over the next few years, I would expect that it would be used to boost the ISS orbit as required. The prediction of future solar activity is quite uncertain, so the magnitude of the re-boosts are likewise uncertain for more than a few months in advance.

 

[gunsandrockets]

"Since the Shuttle should be a frequent visitor over the next few years, I would expect that it would be used to boost the ISS orbit as required."<br /><br />It would surprise me to ever see the Shuttle used in such a role. The Shuttle OMS has enough to do just moving the bulk of the Shuttle around. I doubt there is enough capacity to spare for ISS reboosting.

 

[mrmorris]

<font color="yellow">"The requirements for reboost are not a function of the size or mass of the ISS..."</font><br /><br />If you're talking in terms of <b>frequency</b>, this is (with caveats) true. However, the post you were responding to was talking about the reboosts getting <b>smaller</b>. The degree to which Progress or the orbiters are able to increase the altitude of the ISS is <font color="orange">directly</font>related to the mass of the ISS.

 

[mrmorris]

<font color="yellow">"The Shuttle OMS has enough to do just moving the bulk of the Shuttle around. I doubt there is enough capacity to spare for ISS reboosting. "</font><br /><br />Say what? Google "Shuttle ISS reboost". The shuttle reboosts the ISS pretty much every visit.

 

[MeteorWayne]

I know you're looking for long therm info, but here is a record for the last year, updated regularly <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>

 

[mrmorris]

<font color="yellow">"The OMS engines are not used ..."</font><br /><br />Which bring up an interesting (to me) question. Do the OMS and RCS engines use a shared propellent/oxidizer source?

 

[radarredux]

Eyeballing the two graphs (graph 1 and graph 2), it looks like ISS, at about 337 km, is about at its lowest altitude in about 6-7 years.

 

[disownedsky]

<p>Thank you - my thoughts exactly.</p>

 

[disownedsky]

<p>That is helpful. Anyone know the approximate beta-angle averaged area/mass ratio of ISS now?</p>

 

[disownedsky]

<p>I noticed that too. You would think that near the bottom of the solar cycle, you'd want to be bias the altitude high. In 2009 -2010, the rate of orbit decay will be several times faster. The Progress reboosts look like they're about half the magnitude of the STS reboosts, but were, of course, much less frequent.<br /><p>Is ISS going to get into trouble should there be problems that limit the number of Shuttle visits? At what altitude will the aerodynamic torques get excessive?</p></p>

 

[erioladastra]

"You would think that near the bottom of the solar cycle, you'd want to be bias the altitude high"<br /><br />You have to do reboost at some point to prevent reentry - but you also need to phase the altitude with future vehicles which is a complicated equation (and one that is always changing). So you have to take into acount the upcoming shuttle, and how much, if any, reboost it will do etc. You don't just reboost to the highest attainable level and leave it there (heck, if we did that, no one would be able to reach it!).

 

[gunsandrockets]

Are these numbers accurate? If so then 232 kg of propellant amounts to less than 3% of the Shuttle's supply and 1626 kg is less than 20%. I may have been wrong about Shuttle reboosting of the ISS but I was right that the Shuttle has little propellant to spare.<br /><br />"Progress M holds 1100 kg; Progress M1 holds 1950 kg. ESA ATV holds 4000 kg. The cancelled U.S. Interim Control Module holds 5000 kg of fuel. A Shuttle Orbiter ISS generic reboost has 232 kg of fuel available. An Orbiter Max reboost mission has 1626 kg of reboost fuel available. Zarya FGB holds 5500 kg and the Zvezda Service module holds 860 kg, however, it is generally preferred to keep the main thrusters on Zarya and Zvezda in reserve, as they have finite lifespans."<br /><br />http://en.wikipedia.org/wiki/ISS_Propulsion_Module<br />

 

[gunsandrockets]

"Say what? Google "Shuttle ISS reboost". The shuttle reboosts the ISS pretty much every visit."<br /><br /><br />Guess what I found googling 'shuttle reboost ISS'...<br /><br />http://www.space.com/spacenews/archive03/issarch_021803.html<br /><br />excerpt...<br /><br />"The Russian Progress spacecraft are the primary means of re-boosting the international space station, NASA officials said. The space shuttle, used for re-boost in the early stage of station construction, is largely ineffective now that the station has grown so large. The Russian Zvezda Service Module also provides built-in re-boost capability."<br /><br />I guess a reboost using 232 kg of propellant vs a combined mass of over 300 tonnes doesn't amount to much! 232 kg is so little, and the 80+ tonne Shuttle is such an anchor.<br />

 

[gunsandrockets]

I guess you didn't bother to notice in my earlier reply to you that I admitted my error about Shuttle reboosting of the ISS.<br /><br />But the fact remains the Shuttle today isn't an important factor for ISS reboosting and hasn't been for a long time. The Shuttle is too massive, has too little propellant to spare and the ISS has grown too large for the Shuttle to be important in reboosting. 232 kg of propellant to push 300+ tonnes of mass doesn't amount to much. That's what, 2.3 m/s? 5 mph?

 

[thinice]

ISS reboosts do not change its velocity.

 

[rfoshaug]

<blockquote><font class="small">In reply to:</font><hr /><p>ISS reboosts do not change its velocity.<p><hr /></p></p></blockquote><br /><br />Yes they do. To raise the orbit of a spacecraft in circular Earth orbit, you need to increase its velocity.<br /><br />Obital mechanics 101:<br /><br />Let's say that ISS is at an altitude of 300 km above Earth's surface. If you increase velocity (you accelerate prograde, which is along the direction of travel) at a point in your orbit (let's call this Point A), this extra speed will cause you to arrive at a higher altitude and slower speed at the point that is on the exact opposide side of Earth (Point B) than you would if you hadn't done the speed change.<br /><br />If you accelerated by just the right amout of speed this will cause you to arrive at point B at, say, 350 km altitude. If you don't do anything more, you will stay in that orbit, with a low point of 300 km and a high point of 350 km. At point A you move faster than Earth's gravity can pull you in (and faster than what you did when you were in a circular 300 km orbit), so you slip away. As your altitude increases and you get closer to point B, your speed goes down. At point B you don't have enough speed to stay at that altitude, so you start falling back in towards Earth.<br /><br />So at point B, you need to do another prograde burn to increase speed again, so that you raise the altitude in point A to 350 km. You then have a circular orbit that is higher than the one you had.<br /><br />Funny thing is that you end up in a higher orbit, which has <i>less forward speed</i> than you had to begin with, despite the fact that you did 2 prograde speed increases. But your <i>total orbital energy</i> has increased since you're now at a higher altitude.<br /><br /><br />This leads to some counter-intuitive effects when you're in orbit. Imagine that you're in the same orbit as ISS, but you're 200 km behind it in its orbit (which at orbital speeds mean that ISS arrives at any point about 25 seconds be <div class="Discussion_UserSignature"> <p><font color="#ff9900">----------------------------------</font></p><p><font color="#ff9900">My minds have many opinions</font></p> </div>

 

[mrmorris]

<font color="yellow">"Basically you speed up to slow down and slow down to speed up."</font><br /><br />For anyone who has ever wondered (and not taken the time to Google), the detailed explanation on this follows:<br /><br /><i>"The difference in gravitational attraction with change in distance (orbital altitude) is where the speed-up/slow-down paradox comes in. You must travel faster in a lower orbit than a higher one to stay in orbit. If you want to go to a higher orbit, you must fire your rocket engines to accelerate. The acceleration causes your spacecraft to climb higher above Earth. As you climb higher, your velocity diminishes until you are traveling at the right velocity for the higher orbit. It is a slower velocity than you were traveling before the firing of the engines. In other words, you sped up so that you could slow down in a higher orbit.<br /><br />The reverse is true if you want to go to a lower orbit. To descend, you fire rocket engines in the opposite direction you are traveling. This causes your spacecraft to slow. Earth's gravity pulls your spacecraft downward, and, as you fall, your speed increases until you are at the right speed for the new altitude. Your speed is greater in the lower orbit. Thus, you slowed down to speed up."</i><br /><br />Put in different terms -- higher orbits have a lower <i>kinetic</i> energy but higher <i>potential</i> energy than lower orbits. The increase in potential energy more than offsets the loss of kinetic energy (i.e. the thruster firing *does* add energy to the craft despite the reduction in velocity).