Understanding the most practical type of near-future propulsion system for interplanetary travel

[Madrocketman]

Hello friends,

This is Madrocketman (formerly known on these forums as The Doodling Astronaut)

For purposes I cannot release publicly yet (it's a project,) I have been researching types of interplanetary engines. I've been researching on what would be the most practical and most likely for a near future civilization a few centuries from now would use.

I've been thinking it would be some type of Fusion or Fission reactor engine. But I have also been interested if large Methane engines are possibly the most practical. But I turn to the forums to ask and possibly determine what the most practical interplanetary-civilization type of engine would be.

So any suggestions on what type of engines would fit this position the best?

Happy launches,

-MRM

[kerbiloid]

Start with classics.

http://www.projectrho.com/public_html/rocket/engineintro.php

(The navigation is actually simple and linear, just the site menu at the bottom. Cuz gravity.)

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[Spacescifi]

1 hour ago, Madrocketman said:

Hello friends,

This is Madrocketman (formerly known on these forums as The Doodling Astronaut)

For purposes I cannot release publicly yet (it's a project,) I have been researching types of interplanetary engines. I've been researching on what would be the most practical and most likely for a near future civilization a few centuries from now would use.

I've been thinking it would be some type of Fusion or Fission reactor engine. But I have also been interested if large Methane engines are possibly the most practical. But I turn to the forums to ask and possibly determine what the most practical interplanetary-civilization type of engine would be.

So any suggestions on what type of engines would fit this position the best?

Happy launches,

-MRM

In my opinion it will be something something involving pulsed fusion and magnetic fields.

Nothing about either is forbidden by known physics, the current barriers are often a lack of the power needed to pull off scifi feats.... or prohibitive mass required to generate it (you are not launching it it's too big).

Having excess power on tap always helps.

Edited July 17, 2022 by Spacescifi

[JoeSchmuckatelli]

My delving into this suggest the Nuclear Salt Water Rocket has potential. 

I basically started with @Spacescifi's interest in Orion (which I don't like b/c it's always seemed a crude / desperation solution ala Footfall) and tried to 'step down' to something sub-explosive with a more constant and controllable thrust (mind you, for space travel, only - not take off /landing).

Check out Scott Manley's vid

Scott Manley had a video on that as well: 

 

 

 

 

 

  

 

 

 

 

  

 

 

 

  

 

 

 

 I think the title screen sums up his opinion of it.  But it is clear that the answer wanted isn't "can we make a nuclear reactor more like a bomb (supercritical)", but really "can we increase a nuclear reaction at a level that remains stable but at a higher rate of energy production".  I believe this does this, but maintaining that stability is questionable.

 

ment&comment=4155085

Edited July 17, 2022 by JoeSchmuckatelli

[farmerben]

If by most practical and likely we mean the most common and not the best.  For launches off the moon or Mars an electromagnetic launch ramp makes good sense.  The Earth's atmosphere mitigates the advantages here.   Launching from the top of Olympus Mons with 30,000 m/s initial velocity is totally doable.  

Weaker, though cooler and simpler rockets can make the Mars-Moon voyage.  Ethanol and hydrogen peroxide only gives 225 sec of ISP, but it's pretty cool in other respects.  And very simple and cheap assuming the next few centuries bring us revolutionary improvement in bio-ethanol production.  

 

[Nuke]

i think npp is the only near term option for manned interstellar travel at present. just be sure the crew is well trained in shock absorber maintenance, otherwise splat. i think we could do it in as little as 50 years of lead time. provided you get everyone on board, including reallocating 100% of the world's nuclear arsenals as rocket fuel.  and you would need to get everyone to pony up their nuclear secrets for the best optimization of the pulse unit design. 

Edited July 18, 2022 by Nuke

[sevenperforce]

2 hours ago, Nuke said:

i think npp is the only near term option for manned interstellar travel at present.

I think the OP asks about interplanetary travel not interstellar travel.

To the OP's question: I second @kerbiloid; Project Rho is excellent and you should definitely spend some time bingeing that site, as I have.

However, here is the answer to your question. There are basically three different paths we could take to regular interplanetary travel. I will call them Brute Force, Outpost, and Nuclear Option.

Brute Force

Earth's gravity well is punishing, but it does give us a nice boost from the Oberth effect. So if we're going interplanetary, the most efficient path (from an orbital mechanics perspective) is to use LEO propellant depots, fill up our interplanetary vehicles, and go from there. You can do this if you have rapidly reusable rockets; otherwise it's prohibitively expensive. Your rockets will need to be methalox or hydrolox. Hydrolox has a slightly better ISRU potential but it is not as storable as methalox so that's a tradeoff. An advantage is that you can use your engines both in-space and on the surfaces of other worlds.

Outpost

With enough investment and ISRU, you can stage interplanetary missions from the moon. There's tons and tons of water on the moon you can use for this and you can construct vehicles entirely in a vacuum with no worry about aerodynamics. Your rockets will need to be hydrolox.

Nuclear Option

Stage your interplanetary vehicles in LEO, but use near-future tech like Z-pinch fusion or VASMIR to propel your vehicle. Unlike the other options, there's no way to go to the surface and back using an engine designed solely for vacuum operations, so you will need extra dV to enter your destination orbit. However, your specific impulse will be RIDICULOUS so that's not as big of a problem, and you can even do continual-burn trajectories so that you have the lowest transit times between planets. This is the approach that is most likely to make manned missions to the outer planets feasible.  

[Superluminal Gremlin]

VASIMR is the most probable option. The only problem right now is the thrust. Its pitiful.

And does anyone have any good data on optional fuels for the VASIMR. I know the use argon, but wouldnt hydrogen be more efficient and easier to gather? 

[jimmymcgoochie]

Something like VASIMR or a similar electromagnet-based ion/plasma thruster is a good bet: high ISP gives lots of delta-V, they can use the relatively plentiful (on Earth at least) argon for propellant and low thrust doesn’t matter over interplanetary distances, hence the use of ion thrusters on a number of spacecraft including BepiColombo and Dawn. Using a nuclear reactor for power is a given for a crewed ship using such a propulsion system- a fission reactor now, but possibly fusion in the future which would probably benefit from any improvements in electromagnetic technologies gained by developing the propulsion system, and/or vice-versa.

Nuclear thermal propulsion is the higher thrust option, trading an order of magnitude of ISP for several orders of magnitude of thrust, however that also introduces the problem of hydrogen boil-off which would need to be actively managed, as well as needing very large fuel tanks to hold all the hydrogen needed for an interplanetary voyage. An NTR with a secondary loop to generate power when not producing thrust and possibly a lox afterburner would IMO be the best option if such a mission was to be launched within a decade from now, since NTRs have been tested since the 70s and liquid hydrogen infrastructure is relatively widespread in rocketry. A fusion thermal rocket would boost the ISP considerably, though this would probably require very large quantities of deuterium rather than ordinary hydrogen and so would be much more expensive.

Hydrolox chemical rockets are another option, trading even more ISP for lower mass, smaller fuel tanks and greater versatility, plus there’s extra oxygen available for the crew and to pressurise the ship. The delta-V margins are going to get very thin indeed and will probably involve a lot of staging and fuel tanks or boosters being scattered in its’ wake, but a mission to Mars or Venus is possible on chemical propulsion (or so my experiences with RO and RP-1 would suggest) and going further afield could be done if ISRU refuelling became practical. Methalox would have even slimmer margins and would be harder to refuel anywhere other than Mars, but also has less of a problem with boiloff.

[farmerben]

The  moon-mars transit only requires about 5000 m/s of dV.  That is about half of Earth to LEO.  A single stage chemical rocket can make the transit.  A mass driver (electromagnetic track) on the Moon could easily handle half the dV requirement.

Zubrin argues it is a two year mission with a free return trajectory in case of abort.   Extra propulsion should be used to increase payload, rather than cut travel time.   

[wumpus]

On 7/17/2022 at 10:40 AM, JoeSchmuckatelli said:

My delving into this suggest the Nuclear Salt Water Rocket has potential. 

I basically started with @Spacescifi's interest in Orion (which I don't like b/c it's always seemed a crude / desperation solution ala Footfall) and tried to 'step down' to something sub-explosive with a more constant and controllable thrust (mind you, for space travel, only - not take off /landing).

...

 I think the title screen sums up his opinion of it.  But it is clear that the answer wanted isn't "can we make a nuclear reactor more like a bomb (supercritical)", but really "can we increase a nuclear reaction at a level that remains stable but at a higher rate of energy production".  I believe this does this, but maintaining that stability is questionable.

After further pondering, I think that last bit misses the point.  The problem with nuclear propulsion isn't  that the nukes don't get hot enough (i.e. stability levels), but that they don't get hot enough (for higher Isp) because much hotter and you simply can't construct reactors that won't melt.  So for conventional NTR the melting point of your reactor is the limit, and then you pass hydrogen through to cool the reactor and heat the hydrogen, and then your Isp is based on the temperature of the hydrogen leaving the reactor.

From memory (no time to watch the video), salt water reactors aren't so picky about what is "the reactor" and "the rocket exhaust".   This helps bump up the temperature (and thus the Isp) a lot.  Presumably enough to justify ejecting matter that isn't hydrogen.  Or perhaps it is a different containment mechanism, but the issue isn't the temperature of the reactor, but how you heat up the ejection mass without melting down the rocket.

[JoeSchmuckatelli]

4 minutes ago, wumpus said:

From memory (no time to watch the video), salt water reactors aren't so picky about what is "the reactor" and "the rocket exhaust".   This helps bump up the temperature (and thus the Isp) a lot.  Presumably enough to justify ejecting matter that isn't hydrogen.  Or perhaps it is a different containment mechanism, but the issue isn't the temperature of the reactor, but how you heat up the ejection mass without melting down the rocket.

Your memory is correct.  It's not a 'reactor' design, as much as shooting several controlled streams of diluted fissionables fast enough so that they combine and flash the mediator (saltwater) in either the combustion chamber or the bell  - but also fast enough that the reaction can't climb upstream and into the ship itself.

This differs from the reactor design where you're using effectively the reactor coolant as your propulsive gas.

[kerbiloid]

53 minutes ago, JoeSchmuckatelli said:

shooting several controlled streams of diluted fissionables fast enough so that they combine and flash the mediator (saltwater) in either the combustion chamber or the bell  - but also fast enough that the reaction can't climb upstream and into the ship itself.

These people are saying that Orion is weird...

Maybe it's weird, but at least it's not liquid.

 

Edited July 19, 2022 by kerbiloid