Orbiter-Forum Delta-V to Mars outside the launch windows?
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 Math & Physics Mathematical and physical problems of space flight and astronomy.

 05-23-2016, 02:31 AM #16 RGClark Mathematician OK, one more stab at the Mars moons proposal. We have discussed before the surprising speed that New Horizons was able to pass the orbit of Mars due to its high departure speed: Math needed for 5-week flight from Earth to Mars. http://orbiter-forum.com/showthread....3&postcount=78 The transit time of New Horizons to pass Mars' orbit was 78 days for a LEO departure delta-v of 8.4 km/s. But as discussed in that post, the Mars capture delta-v would be close to 12 km/s, prohibitive when added to the departure delta-v. So instead I'm thinking of other ways of being captured at Mars such as aerocapture: https://en.wikipedia.org/wiki/Aerocapture This allows the spacecraft to be captured into Mars orbit with minimal propellant burn. So using a limiting LEO departure delta-v of 8.4 km/s as with New Horizons and assuming the departure takes place in the time frame of the last quarter of this year to the first quarter of next year, what would be the travel time then? Bob Clark
 05-23-2016, 05:13 AM #17 Keithth G Orbinaut This is doable. But I'm travelling without computer for a couple of weeks. Will get back to this upon my return to HK. Last edited by Keithth G; 03-05-2017 at 11:47 PM.
 05-23-2016, 02:19 PM #18 RGClark Mathematician Thanks. I'll try also calculating it using the circular orbit, coplanar approximation discussed in post #9. Bob Clark
 05-23-2016, 04:03 PM #19 Shifty Orbinaut Quote: Originally Posted by Keithth G  Here, I have assumed a simple mission design in which the spacecraft escapes a 200 x 200 km Low Earth Orbit (LEO); makes a direct transfer to Mars; and then inserts into a 200 x 200 km Low Mars Orbit (LMO). Don't most Martian missions inject into a highly elliptical orbit and then use successive aerobraking passes (sometimes over months) to pull the orbit in? Quote: Originally Posted by RGClark   So instead I'm thinking of other ways of being captured at Mars such as aerocapture: I'm fairly certain that aerocapture has never actually been used. It might not be worth it because of the extra weight required for heat shielding and stress mitigation. OTOH, MSL, I believe didn't bother with establishing orbit at all. It came in from the transfer orbit directly to atmospheric entry. Last edited by Shifty; 05-23-2016 at 04:13 PM.
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 05-23-2016, 05:14 PM #20 RGClark Mathematician Quote: Originally Posted by Shifty  Don't most Martian missions inject into a highly elliptical orbit and then use successive aerobraking passes (sometimes over months) to pull the orbit in? I'm fairly certain that aerocapture has never actually been used. It might not be worth it because of the extra weight required for heat shielding and stress mitigation. OTOH, MSL, I believe didn't bother with establishing orbit at all. It came in from the transfer orbit directly to atmospheric entry. Yes, aerobraking has been used when the spacecraft has already been put in capture orbit at Mars. Aerobraking then reduces the size of the orbit. It may also have been used to circularize. I'm not sure about that. Aerocapture is a more difficult proposition. It would require the spacecraft to plunge deep into Mars atmosphere, skimming the tree tops so to speak. That has not been tried yet. Since this is only a test flight anyway this may be a good chance to test it. Bob Clark
 05-23-2016, 05:23 PM #21 Keithth G Orbinaut According to Wikipedia atleast, the MSL entered the Martian atmosphere at 5.8 km/s and experienced a peak deceleration of 15g. Last edited by Keithth G; 03-05-2017 at 11:46 PM.
 06-06-2016, 04:22 AM #22 Keithth G Orbinaut I've now had a chance to run the problem through PyKEP: For possible transfer times of less than 500 days, the best day to leave is 11 October, 2016; the transfer time to Mars is 273 days; the departure dV is 8.4 km/s; and (at a 50 km periapsis altitude at Mars) the periapsis velocity would be 10.2 km/s. Last edited by Keithth G; 03-05-2017 at 11:44 PM.
 04-09-2017, 04:57 PM #23 RGClark Mathematician Quote: Originally Posted by RGClark  The launch window to Mars comes every two years where the delta-v to get there by a Hohmann trajectory is minimal. This year it was in March, when Exomars was launched. But you can still launch to Mars at different times. It just requires more delta-v. SpaceX wants to decide on an initial mission for the Falcon Heavy, scheduled to launch at the end of this year: SpaceX undecided on payload for first Falcon Heavy flight. May 3, 2016 Stephen Clark http://spaceflightnow.com/2016/05/03...-heavy-flight/ One possibility would be a lander mission to Phobos or Deimos. I calculated from a delta-v chart that it would be doable carrying a Dragon during the minimal delta-v window using an existing solid rocket upper stage for Earth escape. But that minimal delta-v launch window already passed in March. How do you calculate the delta-v requirements during other times such as the end of the year? The idea behind this post was that SpaceX was going to launch a Falcon Heavy flight to Mars outside the optimal launch window. However, now the plan will be to launch in 2018 which presumably will be within the optimal launch window. So my proposed mission is still possible for this first FH launch, to do a fast flight to Mars, ca. 35 day duration, to demonstrate its feasibility for a fast manned mission. The delta-v needed for such a fast flight to Mars in 2018, a particularly close opposition, was discussed here: Math needed for 5-week flight from Earth to Mars. http://orbiter-forum.com/showthread....6&postcount=17 It's about 10 km/s. The updated specifications for the Falcon Heavy with the upgraded Merlin engines are on the SpaceX Falcon Heavy page. I estimate we could get 2 to 3 metric tons as payload to Mars for the fast trip depending on whether we used for the in-space stage the small cryogenic Ariane 5 upper stage leaving from Trans Mars Insertion, or the larger Centaur leaving from geosynchronous transfer orbit. We still have the problem of slowing down when the use such fast flight speeds which result in fast arrival speeds at Mars. Some preliminary calculations suggest it might work by plunging deep into the Martian atmosphere, skimming the tree-tops so to speak. Bob Clark Attached Thumbnails   Last edited by RGClark; 04-12-2017 at 01:33 PM.
 04-22-2017, 04:56 PM #25 RGClark Mathematician Quote: Originally Posted by BrianJ  If you just want the dV values (rather than the fun of figuring out how to calculate it) then Piper's "Trajectory Planner" is very useful: Trajectory Planner I'm trying out the "Trajectory Planner" now. I assume the "Departure deltaV" means the dV from just the position of the Earth's solar orbit, i.e., no consideration of a spacecrafts orbit around Earth, and "Arrival deltaV" means the dV to match Mars in it's position in solar orbit, not what it takes to put it in orbit around Mars. So how do you find the delta-V needed to make the Mars transfer injection assuming the spacecraft is already in LEO? Bob Clark
 04-23-2017, 04:03 AM #26 dgatsoulis Orbinaut Quote: Originally Posted by RGClark  I'm trying out the "Trajectory Planner" now. I assume the "Departure deltaV" means the dV from just the position of the Earth's solar orbit, i.e., no consideration of a spacecrafts orbit around Earth, and "Arrival deltaV" means the dV to match Mars in it's position in solar orbit, not what it takes to put it in orbit around Mars. So how do you find the delta-V needed to make the Mars transfer injection assuming the spacecraft is already in LEO? Bob Clark where is the hyperbolic excess velocity (departure deltaV from trajectory planner). is the local escape velocity, aka the escape velocity for the parking orbit altitude. where is the gravitational constant, is the planet's mass, is the planet's radius and is the altitude of the parking orbit. is the parking orbit velocity. Same applies for arrival. If you want to simply calculate the periapsis velocity and not the orbit insertion/injection dV, then don't use the term. Source: ORBITAL MECHANICS Last edited by dgatsoulis; 04-23-2017 at 04:20 AM.
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 04-23-2017, 01:20 PM #27 RGClark Mathematician Quote: Originally Posted by dgatsoulis   where is the hyperbolic excess velocity (departure deltaV from trajectory planner). is the local escape velocity, aka the escape velocity for the parking orbit altitude. where is the gravitational constant, is the planet's mass, is the planet's radius and is the altitude of the parking orbit. is the parking orbit velocity. Same applies for arrival. If you want to simply calculate the periapsis velocity and not the orbit insertion/injection dV, then don't use the term. Source: ORBITAL MECHANICS Thanks for that. I was getting different numbers than in Keithth G's table in post #14. I suspected that was the answer. Bob Clark

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