Mars round trip time

RobertDyck · 2020-07-18 19:21:07

Many people have raised the idea of advanced propulsion reducing trip time. But operating a settlement ship, the critical figure is not one-way trip time, it's round trip time. That determines how many trips with paying customers the ship can make for the expected life of the ship. So I'm asking you guys.

I don't want to limit this, but just as one example. If we have open cycle gas core nuclear thermal rocket with liquid hydrogen propellant, able to produce relatively high thrust but more importantly high specific impulse. That engine could just stay on until propellant runs out, accumulating speed. If we could reduce one-way trip time from Earth to Mars down to 4 months, could aerocapture still work? Remember that means the ship arrives at Mars very fast. Can we slow sufficiently to enter Mars orbit?

But more to the point, how quickly could we return? Assume refilling propellant tank with in-situ propellant. We're dealing with planetary alignment.

SpaceNut · 2020-07-18 19:33:02

https://mars.nasa.gov/odyssey/mission/t … robraking/
https://mars.nasa.gov/mro/mission/timel … robraking/
https://en.wikipedia.org/wiki/Aerobraking
https://ntrs.nasa.gov/archive/nasa/casi … 001048.pdf
https://trs.jpl.nasa.gov/bitstream/hand … sequence=1
https://www.colorado.edu/event/ippw2018 … -uillinois

https://en.wikipedia.org/wiki/Aerocapture
https://engineering.purdue.edu/people/j … lation.pdf

GW Johnson · 2020-07-19 10:20:58

When I did the reverse-engineering estimates that I posted on "exrocketman" for Starship to Mars, I got the fast trip at 4.26 months one way, and an estimated entry interface speed of 7.4 km/s, remarkably close to the fastest Mars entry speed Spacex quotes: 7.5 km/s.

The difference between that and entry from an 8.5 month Hohmann trajectory is in both speed and direction. From Hohmann, the entry interface speed is about 5.6 km/s, parallel to Mars's orbit path abut the sun. The fast trajectory direction is at a strong angle to Mars's path about the sun.

The planet is round, so the direction doesn't matter in a broad sense. You must do a final course correction so that your entry path grazes the planet more-or-less tangentially, above the surface, but not too high in the atmosphere. The key is shallow angle to local horizontal: usually 1 to 2 degrees. Too steep and you both burn and crush, then smack the surface. Too shallow and you bounce off into space above escape speed. Both are fatal.

When I rough-estimate entry trajectories with my crude mid-1950's technique at something around 300-400 kg/sq.m ballistic coefficient and entry speeds from 5 to 7.5 km/s, I get end-of-hypersonics at local Mach 3 (on Mars about 0.7-0.8 km/s velocity), at about 5 km altitude. Spacex has a real trajectory simulation program that gives just about that same answer for its Starship landing on Mars, complete with early downlift to stop the bouncing-off, and later uplift to stop the trajectory drooping.

Now that's for a one-pass direct entry. I dunno about multipass aerobraking, and I have no tools or experience with which to examine that scenario. But my gut tells me you just hit a tad shallower than 1-2 degrees below local horizontal, seriously risking only bounce-off.

What that really says is you need a downward force during your first pass, whether it be a lift or a thrust, to stop the bounce-off. And you need the shallow angle so as not to smack the surface, but to stay hypersonic back out of the atmosphere. The key is to be well below escape, and not very far above circular orbit speed, when you exit that first pass.

What you want is an elliptic orbit with a period measured in hours or days, not weeks or months. One whose periapsis is in the Martian atmosphere for energy subtraction on every pass. The periapsis won't change, but the apoapsis will reduce, after every pass into the atmosphere. It will eventually try to circularize: that's your start of final entry.

GW

RobertDyck · 2020-07-19 11:24:08

This can be simplified. The goal of aerocapture is to enter Mars orbit. That orbit can be very high and very elliptical. Apoapsis can be very high, and for Mars can stay that way. For Earth, you want to drop into circular LEO, to rendezvous with a space station. But for Mars, the ship can be barely in orbit. That actually reduces energy required to depart Mars orbit. Since a shuttle will rise from Mars surface to carry passengers down, an orbital period of weeks would be acceptable. As long as the ship is in the part of it's orbit headed toward apoapsis when it's time to depart for Earth. Don't know what that's called. Ascending?

RobertDyck · 2020-07-19 11:28:15

Two great replies about aerocapture. Thanks! Now the other part, assuming we can transit that quickly (eg GW Johnson's 4.26 months) then can we return to Earth quickly? How do the planets align? How long do we have in Mars orbit before having to depart? For a settler transport ship, you want enough time to debark passengers and cargo, embark returning passengers and supplies, and as little extra time as possible.

RobertDyck · 2020-07-19 14:58:02

Interesting... Found a NASA web page to search for precalculated trajectories. Found a bunch for Mars; range 1.18 to 1.27 years round trip. Stay time consistently 30 days.
https://trajbrowser.arc.nasa.gov/traj_b … ad_results

I took a screen shot. Let's see if this works...
109519280_10158858379632718_4475568233570984247_o.jpg?_nc_cat=103&_nc_sid=8024bb&_nc_ohc=7MDkgGILaQUAX_wMiGk&_nc_ht=scontent.fyyc3-1.fna&_nc_tp=7&oh=1c60f5bf87c4052da7e8dae8da4e8fa2&oe=5F3C05E1

RobertDyck · 2020-07-19 15:02:36

Ooh! That worked. I also searched for one-way trajectories. Something faster.
https://trajbrowser.arc.nasa.gov/traj_b … ad_results
111038914_10158858392857718_6709574435757659236_n.jpg?_nc_cat=100&_nc_sid=8024bb&_nc_ohc=Slc-YRGP_BwAX-I2uev&_nc_ht=scontent.fyyc2-1.fna&oh=f552632433a2f17823d90d75d24e05b0&oe=5F3B04C9

SpaceNut · 2020-07-19 15:11:42

What is the point of a fast transition when you spend the what it would normally take going around in circles until you can land on the planet since the speed has been slowly decreasing....

RobertDyck · 2020-07-19 15:19:47

Then there's this. So still only one trip every 2 years.
https://trajbrowser.arc.nasa.gov/traj_b … ad_results

110236690_10158858431357718_1666300077661848585_o.jpg?_nc_cat=105&_nc_sid=8024bb&_nc_ohc=WIZdKNmEfh4AX9_8VHa&_nc_ht=scontent.fyyc2-1.fna&_nc_tp=7&oh=dbbfcee904a25e7c434188820dbae5b9&oe=5F3B2E27

GW Johnson · 2020-07-19 15:20:26

The results from anybody with proper codes would be better than mine, but I did a by-hand orbit thing for a fast trajectory to Mars. The answer about the return is yes, you get off the trajectory at Mars after a 4.26 month one-way trip (for a 2 year period orbit). You wait a few months (about 15.48) at Mars doing your thing(s) there, then you get back on that very same trajectory to go home in the same one-way 4.26 months. Total mission time is the 2 year period of the transfer orbit.

This is posted over at my "exrocketman" site as "2020 Starship/Superheavy Estimates for Mars", dated 6-21-20. Specifically, the orbital transfer for the 2-year-period "abort" orbit and the Hohmann transfer are shown (sketched not to scale) in Figure 1 of that article, for average planetary distances from the sun. The data for the fast trip are given in Figure 6 of that article.

You can see that I corrected an error in the rocket vehicle performance spreadsheets on 7-2-20. That did NOT affect the delta-vees from the orbit analyses. Scope of the article included both the fast trip and the Hohmann transfer orbits, and also stopping in LMO versus direct entry and direct escape. It looked at Starship's capability to go there and do any of those things, and to return after refueling on Mars, as we understand that design as of spring 2020.

So, that mission is different from the ones y'all found, which have 30-day stay times at Mars. But 4.26 months is a 128 day one-way trip to (and from) Mars. I thought that was actually a pretty practical mission plan. The apohelion of that 2 year orbit is near the inner edge of the asteroid belt, by the way. Its perihelion I set to Earth's average distance from the sun. That's how you make it an abort orbit, that and selecting a period that is an integer multiple of 1 year.

I'm no orbital mechanics expert, but in doing what I did with nothing but the basics, I thought the result was rather good.

GW

Last edited by GW Johnson (2020-07-19 15:23:28)

RobertDyck · 2020-07-19 15:22:30

SpaceNut wrote:

What is the point of a fast transition when you spend the what it would normally take going around in circles until you can land on the planet since the speed has been slowly decreasing....

The point is for a commercial settler transport. To try to increase the number of trips per given time. To increase revenue. Again this isn't for exploration or science, this is transporting settlers for profit. But appears physics if fighting against me.

RobertDyck · 2020-07-19 15:27:40

GW Johnson wrote:

I'm no orbital mechanics expert, but in doing what I did with nothing but the basics, I thought the result was rather good.

images?q=tbn%3AANd9GcQAy9cFNXJ2z41cJhHADGDO_d4JWedHvsNRRg&usqp=CAU

SpaceNut · 2020-07-19 15:34:23

We need to do what we do with aircraft carriers and airplanes with the drag line on the deck to break the craft rapidly once we catch it for the fast transit for mars but what is the deck and how would we do the speed reduction for the line that we would catch.

Maybe even a drag chute that is long past the ship for a second aerobrake but with solid or semi rigid unit that opens something like the Adapt shield...cable dragged inside of a payload like shroud until it feels the air pressure and not seeing the heat of entry....

GW Johnson · 2020-07-19 15:39:54

There's a velocity change from your interplanetary trajectory, accelerated by gravity as you near Mars. It changes to the periapsis velocity for your elliptic capture orbit. That change in velocity with respect to Mars is a change in vehicle kinetic energy wrt Mars. That change in kinetic energy must equal your pathwise integral of drag force, as you transit through the Martian atmosphere.

I have no way to model and calculate such things, but somebody with a proper trajectory code that includes orbital mechanics and hypersonic drag could do it. But you do have to investigate the min and max density profile cases, and remember: Martian upper atmosphere density varies erratically by factors of 2, according to the Justus & Braun EDL tome.

You will find for your elliptic capture orbit that its periapsis velocity is very nearly Mars escape velocity. I checked a similar orbit between 300 km and the distance of Phobos, and that is what I found. Trips to and from the surface to something in an orbit like that will be more energetically expensive than trips from low Mars orbit. Like 5 km/s kinematic delta-vee versus 3.6 km/s, one way.

To stay in such an elliptic orbit, you need to adjust its periapsis altitude, by adding energy to it in an apoapsis burn, which is not very expensive because the velocity there is low. If you pull that periapsis altitude up out of the Martian atmosphere, then it is stable. If not, your apoapsis altitude is lower after every draggy periapsis pass through the atmosphere.

Circularizing at low Martian orbit from your elliptic capture is expensive, because your periapsis velocity is very nearly Mars escape 5 km/s, while circular orbit speed is only 3.6 km/s. You have to pull your periapsis up out of the atmosphere with an apoapsis burn, before you can circularize with a periapsis burn.

Direct landing is easier, if you don't care where you land very much. Just lower your periapsis to graze the surface with an inexpensive apoapsis burn. But, if you care where you land, it is better just to time it from a low circular orbit. Which means you circularize first. Complicated, but "you gets what you pays for".

According to Justus & Braun, the entry interface altitude at Mars is taken to be 135 km. They say this is 140 km for Earth.

GW

Last edited by GW Johnson (2020-07-19 15:44:48)

RobertDyck · 2020-07-19 16:00:38

I keep remembering when members of the Mars Society wanted to send a small probe to Mars for real. We put together quite a team. One individual just graduated with a Ph.D. in aerospace engineering in Australia, specializing in aeroshells. He wanted to design the aeroshell for our mission. Of course I said yes. Rather than whine about the demise of that project, that individual was Paul Wooster. He got a job with SpaceX, Priniciple Mars Development Engineer. So he's not available. That project was 1999-2000. That's 20 years now! My, how time flies.

Calliban · 2020-07-19 16:05:36

The high capital cost of an interplanetary ship and the relatively few trips it can make within its operating lifetime (compared to lunar missions) is one of the reasons that interplanetary cycler asteroids were proposed. You find an asteroid whose orbit just clips the orbits of Earth and Mars at perihelion and aphelion, or you alter its orbit accordingly. You hollow out the asteroid and use its materials for radiation shielded habitation spaces and life support during the trip. You can also use rock just as reaction mass for the relatively small transfer vehicles between Earth and Asteroid and asteroid to Mars. If the asteroid yields plenty of valuable minerals, it ends up becoming a worthwhile destination in itself.

I am sure that I am not the only one to have noticed that the Japanese space agency has shown a lot of interest in volatile rich NEOs which appear to have about the right orbital characteristics to function as Earth-Mars cyclers. A coincidence? I doubt it. Those Japs are clever people who clearly have a lot of foresight.

Last edited by Calliban (2020-07-19 16:11:57)

louis · 2020-07-19 18:03:54

Well done Robert - never been able to find that sort of page before! Thanks! !

RobertDyck wrote:

Interesting... Found a NASA web page to search for precalculated trajectories. Found a bunch for Mars; range 1.18 to 1.27 years round trip. Stay time consistently 30 days.
https://trajbrowser.arc.nasa.gov/traj_b … ad_results
I took a screen shot. Let's see if this works...
https://scontent.fyyc3-1.fna.fbcdn.net/ … e=5F3C05E1

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Mars round trip time

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