Quote from: TakeOff on 01/26/2015 09:48 amI wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling.Are you perhaps thinking of nuclear-electric propulsion? A nuclear-thermal rocket cools itself by dumping heat into its exhaust.QuoteThe gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.For a weak gravitational field like the sun's, the angle (in radians) by which a ray of electromagnetic radiation is deflected as it passes the sun is 4GM/(c2b), where G is the universal gravitational constant, M is the mass of the sun, c is the speed of light and b is the impact parameter, the distance of closest approach of the ray. This is independent of frequency, so everything will focus at the same point.Quote from: Tass on 12/10/2011 02:25 pmQuoteBut with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.Why bother with a rotating tether? Just go to the focus and look back at the sun with a coronograph (i.e., a telescope having a little dot in its center to block out the light of the sun itself). Image the ring of light around the sun and then process the image to remove the massive distortion.It seems to me that the real limiting factor with the gravitational-lens telescope is that you can look only at one very small part of the sky, namely the part that's opposite the sun.
I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling.
The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.
QuoteBut with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.
But with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.
a couple of additional points.(1) If the thing was pointed in the direction of alpha centauri you would have three targets for the price of one. possibly more depending on whether any other interesting but more distant stars were in the general direction. But proxima, AC A and AC b are within spitting distance of each other. You could definitely hit those three without worrying about traversing along the 550 au orbital.
A good start might be to dive in close to the Sun and there blast off a high thrust engine to make maximum use of the Oberth effect in apohelium. Solar Probe Plus will have a top speed of 200 km/s. The dive would require the orbital speed of Earth to be canceled, which is done either by a flyby of Jupiter or like Solar Probe Plus, by several flybys of Venus.I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling. It would only need to work for hours or days. Could nuclear weapons be used to give a shove when the Oberth bonus is at its maximum?For the multi-decadal journey from there on, nuclear electric ion propulsion would be great. The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.It is a mission which seems doable this century, and it is an interstellar mission of sorts. It could be used to reconnaitre a star before a truly interstellar mission is sent there, and also to communicate with such an interstellar probe using the same phenomenal magnification.Quote from: Tass on 12/10/2011 02:25 pmBut with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.(Old thread, but unfortunately interest seems to be too low to start another one on the same topic)
Quote from: TakeOff on 01/26/2015 09:48 amA good start might be to dive in close to the Sun and there blast off a high thrust engine to make maximum use of the Oberth effect in apohelium. Solar Probe Plus will have a top speed of 200 km/s. The dive would require the orbital speed of Earth to be canceled, which is done either by a flyby of Jupiter or like Solar Probe Plus, by several flybys of Venus.I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling. It would only need to work for hours or days. Could nuclear weapons be used to give a shove when the Oberth bonus is at its maximum?For the multi-decadal journey from there on, nuclear electric ion propulsion would be great. The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.It is a mission which seems doable this century, and it is an interstellar mission of sorts. It could be used to reconnaitre a star before a truly interstellar mission is sent there, and also to communicate with such an interstellar probe using the same phenomenal magnification.Quote from: Tass on 12/10/2011 02:25 pmBut with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.(Old thread, but unfortunately interest seems to be too low to start another one on the same topic)Well, since you want to dive close to the sun in the first place, why not a Nuclear Fusion drive? you're going to wind up preheating the fuel to a fairly high temp, so pumping the fuel into the fusion chamber should not be an issue, simply open a valve.The heat should be high enough that pushing it over the edge to fusion shouldn't require much energy, and with magnetic constrictionthe fuel will be under such high compression, it'll be close to fusion in the first place!
If some of the near term prospects for faster transport work out this will not be a problem. I consider NEP VASIMR short term. i consider the Slough Fusion rocket near term. Even M2P2. if these do not happen it will only be because of political will and funding. It will not be a technology show stopper that prevents any of these from happening.
what's your concept of "near term"? And what is the latest news about Slough's Fusion rocket?
Quote from: Proponent on 01/26/2015 10:49 amQuote from: TakeOff on 01/26/2015 09:48 amI wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling.Are you perhaps thinking of nuclear-electric propulsion? A nuclear-thermal rocket cools itself by dumping heat into its exhaust.QuoteThe gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.For a weak gravitational field like the sun's, the angle (in radians) by which a ray of electromagnetic radiation is deflected as it passes the sun is 4GM/(c2b), where G is the universal gravitational constant, M is the mass of the sun, c is the speed of light and b is the impact parameter, the distance of closest approach of the ray. This is independent of frequency, so everything will focus at the same point.Quote from: Tass on 12/10/2011 02:25 pmQuoteBut with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.Why bother with a rotating tether? Just go to the focus and look back at the sun with a coronograph (i.e., a telescope having a little dot in its center to block out the light of the sun itself). Image the ring of light around the sun and then process the image to remove the massive distortion.It seems to me that the real limiting factor with the gravitational-lens telescope is that you can look only at one very small part of the sky, namely the part that's opposite the sun.Great answer! (Especially since you bring good news).But I fear, I don't know I just fear, that there is a limit to the resolution achievable in telescopes, because space itself contains gas which scatters light. If the light is scattered before it hits the telescope, there's no telescope technology which can fix that. There might be a limit of resolution beyond which better telescopes don't help.
WRT the VLA idea just using the gravity lensing effect multiplies the power by millions and millions of times. I do not see how the added complexity and points of failure for no certain benefit would be justifiable.