Mars capture

[jhoblik]

1/Aerocapture device I think could look like balloon. Do we have idea what will be temperature of front part of device, it will be enough to have balloon from heat resistance fabric or we have to use special material like shuttle have it.<br /> 2/We need dv 2.5km/s. What is safe dv using Marsion atmosphere and for how much dv we have use rocket engine. Is it 0.5km/s or less or more.<br />

 

[spacester]

My understanding is that the most one can hope for from Aerobraking in a single pass is ~0.8 km/s. Aerocapture by definition is done in a single pass.<br /><br />The device you describe is called a 'Ballute' - nice idea, did you come up with it yourself? Last time I checked, the Russians were the world leaders in developing the idea, and NASA had done little to nothing in the area. That was a few years ago, so google 'ballute' and let us know what you find, eh?<br /><br />Is the 2.5 km/s a number from another thread? It's a number that represents an appx average. It varies a LOT depending on just what you want to do. In some cases, the number can get tantalizingly close to 0.8 km/s. Note that a failed aerocapture maneuver sends you either WAY too close to the Sun (prograde pass) or in a stranded orbit passing thru the Asteroid belt (retrograde pass). <div class="Discussion_UserSignature"> </div>

 

[jhoblik]

I know that Marsion atmosphere is still not completely predictable. My question is what is safe dv to leave to rocket engine. During aerocapture we don't like to landed, if we would like to stay in the orbit. How much deceleration has to be left to rocket engine after we past marsion atmosphere to achieve Mars orbit.<br />What will be temperature of are ballute during aerocapture if we need to decelerate through Marsion atmosphere by 2.5km/s. Could be handle by heat fabric.<br />

 

[spacester]

Um I'm going off of memory here prolly too much, but . . .<br /><br />When I say that 0.8 km/s is most we can hope for, that means if you try to do more, you're going to burn up all known materials using the best conceivable geometry. I am certainly NOT any kind of expert on this.<br /><br />So if the max possible is 0.8 and we need 2.5, well we need 2.5 - 0.8 = 1.7 km/s from the rocket engines. Just stating the obvious here.<br /><br />Note that the rough estimate of 2.5 km/s is assuming entering into a HEMO (Highly Elliptical Martian Orbit). The idea is that you try to *just barely* achieve a Martian orbit by allowing yourself to go to a very high apoapse after you dip into the atmosphere. As you say, the atmosphere is unpredictable, so it becomes a gambling game - if you don't get captured, you're screwed.<br /><br />Personally, I don't like Aerocapture or even Aerobraking for critical missions, and what missions aren't critical? I'm a fan of putting big tanks on your rocket and paying the price to fill them up and burning into orbit. This means not having to worry about the aero loading on your spaceship. Yes that fuel could be 'traded in' for more payload, but so be it in my book. <div class="Discussion_UserSignature"> </div>

 

[krrr]

If you look at this, you'll notice that the dV for capture into HEMO is actually (and typically) less than 1 km/s. (C3=0 means an HEMO with apoapsis at infinity.)<br /><br />Hence, one possible scheme would be to do these 1 km/s by conventional means and then use "soft" aerobraking (perhaps ~20 to 100 m/s per periapsis pass) to lower the orbit.

 

[spacester]

You're looking at the 0.9 at the top of the chart, to Low Energy Transfer Orbit from Mars C3=0?<br /><br />C3=0 is not the same as a HEMO, which has a low periapse, meaning it is down in the gravity well. C3=0 is a parabolic orbit, which means if you had just a tiny fraction less bound energy, you would be in a very high circular orbit.<br /><br />Lowering the orbit on each pass is a good option but you have to get down into the gravity well first to do it. <div class="Discussion_UserSignature"> </div>

 

[krrr]

No, C3=0 means an escape trajectory, actually from the surface of the planet, for which a low (circular) orbit is a good approximation. As you say, it's a parabolic trajectory which, if you lower C3 just a little, becomes a highly elliptical orbit.

 

[krrr]

Mars Express killed 804 m/s to get into a highly elliptical (14 days!) orbit.

 

[spacester]

<font color="yellow">No, C3=0 means an escape trajectory, actually from the surface of the planet, for which a low (circular) orbit is a good approximation. As you say, it's a parabolic trajectory which, if you lower C3 just a little, becomes a highly elliptical orbit. </font><br /><br />Well, we're both abusing the term C3, I was just trying to go with the flow and reduce my pedantic tendencies a bit. And I think you're slightly misquoting me.<br /><br />C3 is realted to Vhyp = 'excess hyperbolic velocity', relative to the planet you're trying to escape.<br /><br />Vhyp = sqrt(2*C3)<br /><br />If C3 is even slightly negative, you will eventually come back cuz you didn't escape. If you have positive C3, you escape. 'C3=0' is a rather abstract mathematical concept in the real solar system.<br /><br />But a low circular orbit is NOT a good approx for C3=0. It has a lot of bound energy, but by definition C3=0 means zero bound energy. Also, if you say 'highly elliptical' that implies a low periapse, which means you're down in the gravity well a lot deeper than C3 = slightly less than parabolic. <div class="Discussion_UserSignature"> </div>

 

[krrr]

Well these things are difficult and sometimes counter-intuitive. For instance the fact that a geosynchronous orbit is more difficult to reach (in terms of delta-v) than a low lunar orbit.<br /><br />In any case, it's a fact that insertion into a highly elliptical Mars orbit (from a Hohmann trajectory) usually takes less than 1 km/s. As another example, MGS used 977 m/s to reach a 48-hour orbit with a periapsis of 314 km.

 

[krrr]

Mars Express didn't use aerocapture/aerobraking.

 

[gunsandrockets]

"When I say that 0.8 km/s is most we can hope for [aerobraking], that means if you try to do more, you're going to burn up all known materials using the best conceivable geometry. I am certainly NOT any kind of expert on this."<br /><br /> Well the Apollo capsules and Zond capsules seemed to handle 11 km/s pretty well when returning to Earth from the moon. Or were you refering only to ballutes?<br /><br /><br /><br />"Personally, I don't like Aerocapture or even Aerobraking for critical missions, and what missions aren't critical? I'm a fan of putting big tanks on your rocket and paying the price to fill them up and burning into orbit. This means not having to worry about the aero loading on your spaceship. Yes that fuel could be 'traded in' for more payload, but so be it in my book. "<br /><br />Yowch! And a mighty big cost it is. Aerocapture/braking is God's gift to spaceflight. It makes it so much easier to get past the limitations of rocketry. It's what makes going to Mars easier. And without it we would still be trapped in Earth's gravity-well (except for never-return one-way missions) because rocketry isn't good enough by itself to do the job.

 

[gunsandrockets]

"What will be temperature of are ballute during aerocapture if we need to decelerate through Marsion atmosphere by 2.5km/s. Could be handle by heat fabric."<br /><br />I think it's doable.<br /><br /><br />Here is an interesting article on Inflatable Reentry and Descent Technology (IRDT). IRDT is a system that uses an inflatable cone-shaped heat-shield and parachute to return cargo from LEO.<br /><br />http://www.space.com/businesstechnology/technology/051012_irdt.html

 

[spacester]

<font color="yellow">Or were you refering only to ballutes? </font><br /><br />Yes. At Mars. For Aerocapture.<br /> <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

"Yes. [ballutes] At Mars. For Aerocapture."<br /><br />Well. Then. The IDART must be awfully shocking to you seeing as it's designed to cope with speeds about 10 times greater than what you assumed were possible.<br /><br />http://www.space.com/businesstechnology/technology/051012_irdt.html <br />

 

[spacester]

<font color="yellow">. . . what you assumed . . . </font><br />I assumed nothing. I related my limited understanding based on old research, in an effort to promote discussion and introduce the concept that the math indicates that there is a maximum value of deltaV we can hope for from ballutes. Do you have any constructive input as to what that value may be?<br /><br />Perhaps I need to find other phrases to indicate that I’m just trying to introduce some concepts for discussion that have not yet come up on the thread, and maybe start talking about some numbers, phrases other than<br /><font color="yellow">My understanding is that . . . .</font><br />and<br /><font color="yellow"> I am certainly NOT any kind of expert on this. </font><br />and<br /><font color="yellow">Last time I checked . . . </font><br />and<br /><font color="yellow">That was a few years ago . . . </font><br />and<br /><font color="yellow">I'm going off of memory here prolly too much</font><br /><br />Any suggestions on better qualifying phrases? I’d prefer to stick to English if that’s OK with you. Um, you do get the concept of qualifying phrases, yes?<br /><br />Let’s see, if I want to help further the discussion and not worry about having to waste my time defending statements from the attack-dog reader, statements that were already well qualified, I need to<br />1. Check with Shuttle_Guy that’s it’s OK to present material in a different way than found on Wikipedia . . . <br />2. Check with frodo that it’s OK to dream about things above and beyond current official plans<br />3. Wait for gunsandrockets to show up on the thread.<br />4. Present a compare and contrast between all known design concepts and how they apply in different ways to all conceivable applications<br /><br /><img src="/images/icons/rolleyes.gif" /><br /><br />If I didn’t know better, I’d think that my recent activities in discussing out-of-the-box concepts for space development have pre <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

"Let’s see, if I want to help further the discussion and not worry about having to waste my time defending statements from the attack-dog reader, statements that were already well qualified, I need to..."<br /><br />Touchy touchy touchy!<br /><br />Dude you take things way too personally. Just because someone contradicts you is no reason to take it as a personal attack, especially when the contradiction is correct. But if you give attitude expect to receive attitude in reaction. <br /><br />"the math indicates that there is a maximum value of deltaV we can hope for from ballutes. Do you have any constructive input as to what that value may be?"<br /><br />Uh, yes as a matter of fact.<br /><br />http://www.space.com/businesstechnology/technology/051012_irdt.html <br /><br /><br /><br />This is the link I have now posted three times, twice now in direct response to you. I guess you just didn't notice? <br />

 

[scottb50]

I could see some advantage to aerobraking, if you can also use the mass you would have to take for other uses. Say you use a heat shield to protect the arriving mission, and effect aerobraking, then use the same shield to take payloads through the high heat of re-entry before they return to orbit. Then it might be cost effective. If not I would still fall back on brute strength to enter orbit, though I would still use the heat shields to take payloads into the atmosphere. <div class="Discussion_UserSignature"> </div>