Another Busted Concept

guest blogger john hare

With the information from Bob Steinke, I was able to prove to myself that the retro station concept wouldn’t work for Mars return even with two Lunar passes and one Earth pass. It may still be of interest to some NEO return missions that are really tight on the mass budget.

busted retro

The lines in black represent the flight path of the attempt to save the concept with a double Lunar flyby using the Earth to focus the second pass. The red represents what is possible with a minor retro burn at perigee. A 400 m/s burn can capture a Mars return vehicle to a highly eccentric orbit. The burn to capture is not shown in the same capture plane to leave room in the cartoon, not because it is in the correct orientation.

The question is, can a 400m/s capability be given to the craft without excessive mass ratio penalty for the there and back?  The required propellant will mass a substantial fraction of a heatshield that would return the craft to the Earth’s surface. It will also have to be accelerated from Mars or other point of interest at the cost of more propellant. Since some or all of that propellant probably comes from Earth, the initial mass requirements get quite interesting.

My next thought is a very small refueling tanker or tug  in Earth orbit that accelerates to dock with the return vehicle in time to make a capture burn into HEEO. That will take a 4,000 m/s tanker/tug  burn, rendezvous, and 400 m/s braking burn in roughly 45 minutes. That would require a  rendezvous and docking time so far unheard of in real space operations.

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johnhare

johnhare

I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.
johnhare

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johnhare

About johnhare

I do construction for a living and aerospace as an occasional hobby. I am an inventor and a bit of an entrepreneur. I've been self employed since the 1980s and working in concrete since the 1970s. When I grow up, I want to work with rockets and spacecraft. I did a stupid rocket trick a few decades back and decided not to try another hot fire without adult supervision. Haven't located much of that as we are all big kids when working with our passions.
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8 Responses to Another Busted Concept

  1. A_M_Swallow says:

    If a 0.4 km/s burn puts the spacecraft into Earth orbit does this mean that a correctly timed 0.4 km/s burn will send the spacecraft back to Mars? This sounds useful to a reusable Mars Transfer Vehicle. After the first trip only the passengers, consumables and replacement parts need lifting.

    A delta-V of 4.0 + 0.4 = 4.4 km/s uses considerably less propellant than the approx 6 km/s needed for a direct LEO to Mars trip. Expendable transfer vehicles are expensive.

  2. john hare says:

    A Hohman orbit doesn’t seem to require 6 km/s for a direct LEO to Mars injection. It would need to wait in Earth orbit until the next window, but yes if I got this one right. Rockets are so much easier than orbital mechanics.

    I know the concept is not original to me, but this massive critical heatshield and use once mindset is just wrong. I think that like a ship that is reworked after it’s shakedown cruise, these interplanetary ships will become more comfortable and functional with each trip until they become obsolete with better ships coming available.

  3. A_M_Swallow says:

    Not many electric propulsion thrusters can perform a Hohmann transfer, they need to spiral which has a much higher delta-v.

  4. john hare says:

    If you have a vehicle in a HEEO that requires a ~400m/s perigee burn for Mars Hohmann injection, then an impulsive chemical burn at perigee might be more propellant efficient than an electric thruster manuever. If the propellant mass fraction is in the 10-15% range for both cases, then system costs and trip times become more relatively important.

  5. A_M_Swallow says:

    Although a SEP tug can push your chemical vehicle from LEO to HEEO.

    Launch window size and time between windows can also be important.

    So “horses for courses” and the entire network needs planning.

  6. john hare says:

    Planning is the key. I was thinking of the impulse burn just for the vehicles in HEEO that needed just a small sharp push at the right time to put them on a Mars Hohmann trajectory. If NEOs are the goal, we still need your electric propulsion.

  7. Why not combine the concepts? Use the moon to slow it down some and put it in an earth-skimming trajectory (albeit, too fast to orbit); then at perigee, burn to put it in a HEEO, using less propellant than you would’ve if you hadn’t gone around the moon first. Or if you want to get complicated, maybe you could use an even smaller perigee burn to send it back for another moon-skimmer, to slow it down still further.

    I’m thinking that a problem with this whole class of idea is that it gives you very narrow launch windows from Mars…

    Chris

  8. john hare says:

    When I checked it, the perigee burn is very similar either way. I thought I had found a way of eliminating propulsion requirements completely for Earth capture, which was incorrect.

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