# Why don't spacecraft burn up on the way up? **URL:** \ **Category:** Factual Questions\ **Created:** [April 7, 2009, 3:35pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257 "2009-04-07T15:35:35Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![wrdwzrd](https://avatars.discourse-cdn.com/v4/letter/w/e0b2c6/32.png) [@wrdwzrd](https://boards.straightdope.com/u/wrdwzrd)\ **Post date:** [April 7, 2009, 3:35pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/1 "2009-04-07T15:35:35Z")
We’re all familiar with the role of heat shields to protect astronauts/cosmonauts as they reenter the earth’s atmosphere from their extraterrestrial adventures. But if their spacecraft will burn up from atmospheric friction on the way down, what prevents the non-shielded craft from burning up on the way up as it accelerates to escape velocity? I’m guessing that the craft doesn’t reach maximum speed until beyond the thick part of the atmosphere that would cause the most friction, but it seems to me that a speeding rocket is going plenty fast by the time it gets up even to suborbital heights. Which leads to a second, related question: what kind of temperatures are reached on a spacecraft’s outer surface before it hightails out of the atmospheric danger zone? ---
**Author:** ![TruCelt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trucelt/32/523_2.png) [@TruCelt](https://boards.straightdope.com/u/TruCelt)\ **Post date:** [April 7, 2009, 3:47pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/2 "2009-04-07T15:47:55Z")
Brilliant question. I’ll be watching for the answer myself. . . ---
**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\ **Post date:** [April 7, 2009, 3:57pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/3 "2009-04-07T15:57:19Z")
We just did this question not too long ago. … and if I didn’t have to wait an hour between searches I’d keep looking for the thread for you. ---
**Author:** ![Waffle\_Decider](https://avatars.discourse-cdn.com/v4/letter/w/77aa72/32.png) [@Waffle\_Decider](https://boards.straightdope.com/u/Waffle_Decider)\ **Post date:** [April 7, 2009, 3:58pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/4 "2009-04-07T15:58:46Z")
I’m not a rocket scientist, but I did take some aero engineering courses in the university before I switched major. I believe your guess is correct. When launching a spacecraft, the goal is to try to get out of the atmosphere as soon as practical before accelerating to orbital speed. Any drag is a waste of energy, and at our current level of technology, we need every little bit of energy we can muster. The spacecraft hasn’t really gone very fast yet when it first gets up there. Now I’ll let the real experts step in and fill in the details… ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [April 7, 2009, 4:01pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/5 "2009-04-07T16:01:43Z")
They do burn on the way up. At the tail end. During launch, energy is added to the spacecraft by burning fuel. During reentry, this energy is dissipated by the heat shields. ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [April 7, 2009, 4:03pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/6 "2009-04-07T16:03:48Z")
[Relevant thread](http://www.bautforum.com/archive/index.php/t-3733.html) from (former?) doper **The Bad Astronomer** ’s site. Particularly what “Thumper” has to say: > [@](#): > > …the shuttle and anything that wants a stable orbit for any length of time start in orbits above 150 miles or so. At these altitudes, the shuttle needs to be going 17,000-18,000 mph. When it launches it accelerates to that speed and maintains that speed until re-entry. > > Friction isn’t a major problem because it is going relatively slow as it goes through the thick portions of the asmosphere and picks up speed as it gains altitude and the atmosphere thins. (The main engines of the shuttle actually don’t run at full power until about a minute after launch to reduce aerodynamic loading at first.) The trajectory and the shapes and orientation of the spacecraft minimize drag and thus friction. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 7, 2009, 4:39pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/7 "2009-04-07T16:39:59Z")
There _is_ significant heating during ascent. This [page from the _Spacecraft Thermal Control Handbook_](http://books.google.com/books?id=-GYGlwG8PkUC&pg=PA63&lpg=PA63&dq=temperature+ascent+shuttle&source=bl&ots=Ljae9Z-geL&sig=DH601yHjHFW-odB2evk1Z7Luaxs&hl=en&ei=0n_bSaq0D4qyMZjb4K0I&sa=X&oi=book_result&ct=result&resnum=1) has some info - scroll down to p65 and you’ll see a graph showing the fairing temperature of an Atlas rocket. It goes up to about 200[sup]o[/sup]C. But of course this is nothing compared to the heat of reentry, for the reasons already mentioned by others. Also keep in mind, the heating during reentry is _intentional_. The intent is to slow down the spacecraft by converting its kinetic energy into heat. ---
**Author:** ![davekhps](https://avatars.discourse-cdn.com/v4/letter/d/a5b964/32.png) [@davekhps](https://boards.straightdope.com/u/davekhps)\ **Post date:** [April 7, 2009, 5:59pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/8 "2009-04-07T17:59:21Z")
> [@jjimm](#): > > [Relevant thread](http://www.bautforum.com/archive/index.php/t-3733.html) from (former?) doper **The Bad Astronomer** ’s site. Particularly what “Thumper” has to say: To add, we’re all very familiar with this in reality: if you’ve ever watched a Shuttle launch, you’ve heard the words “Go with throttle up.” [Here’s a NASA explanation](http://www.nasa.gov/mission_pages/shuttle/launch/sts-115/qa-leinbach.html): > [@](#): > > Megan McKenna from Beecher, IL: After the shuttle launches, how fast is the vehicle going when the command is given to “throttle up,” and how long (in seconds) does it take to reach 17,000 mph? > > Leinbach: Another good question. The “go” at throttle up command is interesting because as we ascend through the atmosphere in the early stages of ascent, we go through a regime that’s called the maximum dynamic pressure. The maximum dynamic pressure is a combination of the speed of the vehicle and the density of the atmosphere we are flying through. So, shortly after T zero, shortly after lift off, we throttle the main engines back down to around 64% rated power to keep that dynamic pressure on the vehicle to a minimum. If we didn’t throttle down, the loads on the external tank and the solid rocket boosters and the orbiter would be too high because we’d be flying faster through this regime in the atmosphere called the maximum dynamic pressure. Once we get through that area, then it’s safe to throttle back up and go for the maximum acceleration of the vehicle. That occurs when the vehicle is about 35,000 feet high. At that point in time, the vehicle is going 1,636 miles per hour when we are “go” for throttle up. Then the engines stay at the maximum power rated level all the way through ascent. We do throttle them back down slightly as we get really close to orbit to maintain no more than three G’s on the astronauts and on the orbiter itself, but that is late in the ascent - maybe around eight minutes or so during the eight and half minute flight. So, the throttle up is to bring the main engines back up to speed, the full rated speed, once we get though that maximum dynamic pressure. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 7, 2009, 7:15pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/9 "2009-04-07T19:15:04Z")
And in case anyone’s wondering, they _could_ come down the same way they go up, using a big rocket motor to slow down and ultimately landing gently tail-down back on the launchpad. But the fuel needed to do so, and the fuel needed to launch that fuel, would be hideously expensive, so nobody does that. ---
**Author:** ![rbroome](https://avatars.discourse-cdn.com/v4/letter/r/838e76/32.png) [@rbroome](https://boards.straightdope.com/u/rbroome)\ **Post date:** [April 8, 2009, 1:54am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/10 "2009-04-08T01:54:05Z")
lack of air pressure by the time it gets really going. OTOH, I have been told by the designers that the nose of the external fuel tank of the shuttle has 1 inch of insulation-and the outer surface reaches 800F while the inner surface remains at -180 (liquid oxygen which is what is in the upper part of the fuel tank). ---
**Author:** ![Munch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/munch/32/5281_2.png) [@Munch](https://boards.straightdope.com/u/Munch)\ **Post date:** [April 8, 2009, 2:13am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/11 "2009-04-08T02:13:43Z")
> [@UncleRojelio](#): > > We just did this question not too long ago. > > … and if I didn’t have to wait an hour between searches I’d keep looking for the thread for you. Just curious what your search term was that resulted in not finding the thread in question. I hadn’t seen it come up before, but went with “friction atmosphere” as those were both extremely likely terms to be included, and rare enough in other threads to exclude any noise. I came up with about 8 threads, one of which was this: [why can shuttles go straight up but not straight down?](http://boards.straightdope.com/sdmb/showthread.php?t=496597) ---
**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\ **Post date:** [April 8, 2009, 2:21am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/12 "2009-04-08T02:21:00Z")
> [@Munch](#): > > Just curious what your search term was that resulted in not finding the thread in question. Now I can’t remember but I guess if I had remembered that I had actually posting in that thread it may have been easier for me to find. Sucks getting old. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [April 8, 2009, 2:33am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/13 "2009-04-08T02:33:54Z")
> [@rbroome](#): > > the nose of the external fuel tank of the shuttle has 1 inch of insulation-and the outer surface reaches 800F while the inner surface remains at -180 Do I get an income tax deduction if I put some of this stuff on my house? ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [April 8, 2009, 8:14am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/14 "2009-04-08T08:14:56Z")
I once read an article about the SR-71 spy plane, the fastest plane in the world (Mach 3). A civilian was being given a demonstration flight in a two-seater and at one point the pilot told him to touch the canopy. When he did he instantly pulled it back because even through his pressure suit glove the canopy was blazingly hot! ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 8, 2009, 8:36am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/15 "2009-04-08T08:36:27Z")
> [@carnivorousplant](#): > > Do I get an income tax deduction if I put some of this stuff on my house? It sounds like amazing insulation but remember that it only has to slow down the transfer of heat. A few minutes after launch the shuttle is out of the atmosphere and the fuel is gone. ---
**Author:** ![Svejk\_1](https://avatars.discourse-cdn.com/v4/letter/s/ea666f/32.png) [@Svejk\_1](https://boards.straightdope.com/u/Svejk_1)\ **Post date:** [April 8, 2009, 8:43am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/16 "2009-04-08T08:43:37Z")
Isn’t this the explained by the same reason that if you jump into a pool belly-first it, the water can hurt like hell, but if you somehow jumped out of the water, belly-first, at the same velocity, the air wouldn’t hurt at all? ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [April 8, 2009, 8:49am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/17 "2009-04-08T08:49:56Z")
There must be something I’m not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn’t it the same craft, with the same shielding, that makes the return trip? 😕 ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 8, 2009, 9:51am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/18 "2009-04-08T09:51:47Z")
> [@Svejk\_1](#): > > Isn’t this the explained by the same reason that if you jump into a pool belly-first it, the water can hurt like hell, but if you somehow jumped out of the water, belly-first, at the same velocity, the air wouldn’t hurt at all? No. In your example in one case you are hitting the water boundary at speed and in the other you are not. You are in the water already. This bears no relationship to the shuttle’s experience. There is no sharp atmosphere/vacuum boundary. The heat on the way down is simply a factor of speed through atmosphere. If it were doing the same speed in the atmosphere on the way up, it would heat up in precisely the same way. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 8, 2009, 9:58am UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/19 "2009-04-08T09:58:10Z")
> [@Reply](#): > > There must be something I’m not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn’t it the same craft, with the same shielding, that makes the return trip? > 😕 Because the boosters, and the fuel tank, and the top side of the shuttle, all of which are fully exposed to atmospheric friction on the way up are not shielded. The heavy shielding used on the way down is only on the underside of the shuttle, which has to remain in a very careful attitude on the way down to avoid exposing upper surfaces. ---
**Author:** ![Munch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/munch/32/5281_2.png) [@Munch](https://boards.straightdope.com/u/Munch)\ **Post date:** [April 8, 2009, 1:41pm UTC](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257/20 "2009-04-08T13:41:39Z")
> [@Reply](#): > > There must be something I’m not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn’t it the same craft, with the same shielding, that makes the return trip? > 😕 [Space shuttle at launch](http://www.flatrock.org.nz/topics/odds_and_oddities/assets/space_shuttle_sonic_boom.jpg) [Space shuttle at re-entry (artist rendered)](http://farm1.static.flickr.com/55/186004657_28a7b34955.jpg?v=0) [Next page](https://boards.straightdope.com/t/why-dont-spacecraft-burn-up-on-the-way-up/492257.md?page=2)