I previously asked how best to use wormholes for spaceflight, focusing mainly on propulsion in space. I'm now wondering if there's a good way to get off Earth with this ship.

The ship needs to be composed mostly of metal, for setting-related reasons, and also needs to be as simple as possible. So on the one hand, something based on a scaled-down version of SpaceX's Starship would cover the metal requirement, but the Rapter Engines are not available. There is also only one portal available.

The primary question is how much thrust can be generated, preferably with the lowest tech possible. Mas pulled through the portal can be artificially accelerated to some degree, if necessary, but I'd like to minimize this.

My estimate for the mass of the ship is between 25000 and 55000kg. The far side of the portal is near a star, with at least one gas-giant (probably a Hot Jupiter, to minimize the energy required to move the portal between them if necessary). There can also be various storage stations near the far side (I imagine they'd orbit the gas-giant, but the important thing is that they're secure and accessible when needed).

An answer that allows the engine to be as simple as possible, in terms of materials and mechanics, is preferred.

I used the atmospheres of Jupiter and Neptune as references, and attempted some calculating of what an ideal engine would be able to output by just pulling gas from the Hot Jupiter's storms, and am certain that I missed many, many details (I came up with something around 1300kn, which seems implausibly high, if convenient).

The important distinction between this and the previous question is the focus on launching from Earth. It would be nice if this could be done without committing any war-crimes in the process. If this can't be more than a glorified jet when in Earth's atmosphere and/or gravity well, that would be a problem, but also a valid answer.

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    $\begingroup$ to escape from earth using kenetic energy, rocket technology and traditional physics a speed greater than 8 miles/second is needed so 1300knots does not even come close. However if we are using non traditional physics like portals then all bets are off. If sufficient high pressure gas is available then the space ship can launch at relatively low speed propelled by millions of tons of high pressure gas from where ever. But note we are talking perpetual motion here... $\endgroup$
    – Slarty
    Oct 8, 2019 at 15:36
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    $\begingroup$ @Slarty with portals, who even needs thrust? Put it on a rail car and ROLL it through the portal. You could push it with a steam engine. or Elephants. $\endgroup$ Oct 8, 2019 at 15:46
  • $\begingroup$ @Morris The Cat LOL, yes, althoughI got the impression that the portal was to be used for gathering propellant rather than transportation. $\endgroup$
    – Slarty
    Oct 8, 2019 at 16:24
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    $\begingroup$ @Slarty apparently you're correct and I completely misunderstood the question, lol. $\endgroup$ Oct 8, 2019 at 17:01
  • $\begingroup$ I also get the sense that you don't want to go through the portal but you never actually say that. Maybe specify that the portal is unfit for transportation, maybe because it is too small in diameter? Also, what type of control do you have over the portal? Can it open/close, move quickly, change size? $\endgroup$ Oct 9, 2019 at 12:26

3 Answers 3


You don't need thrust, just horsepower.

Perhaps even literally.

The easiest solution here is a purely mechanical system for 'launching' the ship through the portal. You can use whatever fits your setting to move around on the other side, but unless you're using a reactionless drive there's no point wasting any fuel on THIS side so just put your ship at the bottom of a big hole (think missile silo) and the portal at the top, and just use a Big Ass Elevator to lift the ship through the portal. With the use of counterweights you could build easily build something like this with 19th century technology.

Hell, depending on how the portal works, you could ALSO just use something like NASA's Crawler-Transporter to carry your spaceship over the portal and just DROP it through, although at that point you'd have somewhere between 20 to 30 meters per second of velocity on the other side depending on how tall your ship is which might be a benefit or a drawback depending on how the portal is oriented on the other side.

The main problem you want to account for one way or the other is making sure that anything that goes through winds up in a stable orbit. If the Portal is itself orbiting the star like a physical object would, then you're fine. If the Portal is just SITTING with a fixed position relative to the star then it's more problematic, since your ship is immediately going to start falling directly towards the star. Given that (just for an example) Earth is moving at 30ish KILOMETERS per second in its orbit around the sun, accellerating a 20-ish ton spacecraft to that velocity BEFORE you go through the portal is tricky, and doing it afterwards requires a LOT of thrust, so I'd recommend you just stick with "My portal is orbiting the star" and call it good.

EDIT: I missed the part where the ship isn't supposed to travel THROUGH the portal, the portal is just providing energy.

  • $\begingroup$ FWIW, the previous question on this topic suggested that this wasn't an option. Too bad the author didn't mention that this time... $\endgroup$ Oct 8, 2019 at 16:33
  • $\begingroup$ @StarfishPrime I thought the previous question just specified that the portal could be kept open indefinitely? Did I miss something? EDIT: OOOOOHHH, I see.... the ship isn't travelling THROUGH the portal, it's just an energy source... ah. $\endgroup$ Oct 8, 2019 at 16:48
  • $\begingroup$ You'd need 30 km/s of additional velocity to come out of a stationary portal with an earth-like, nearly circular orbit, but you could get away with far less delta v to enter in a highly elliptical but stable orbit that just misses the star. $\endgroup$ Oct 8, 2019 at 17:00
  • $\begingroup$ @NuclearWang username checks out. $\endgroup$ Oct 8, 2019 at 17:51

Method 1: Ambient pressure.

Stick the portal in a high-pressure area, like deep in a gas giant, with one side sealed. The atmosphere will rush through at high speeds. Now, the portal shouldn't actually interact with the atmosphere, so there will be no force, so no impulse or work done.

In order to get work out of this, you'll need the rocket to interact with the atmosphere. Have the water make a U-turn, giving you between one and two times the momentum of the water.

To keep the force generated by redirecting the water along the spaceship's centerline, have the water split into multiple different pipes that lead to nozzles. If the spaceship is clyindrical, the back will be ringed by tiny nozzles, each one taking a small amount of the water from the portal.

Method 2: Infinite SRBs

Solid rocket motors are the simplest kind of engine. To use, the portal faces an open hole on the back of the ship. The ground crew drops a SRB in the portal, and it falls into the hole before latching into place against locks in the inside of the hole.

If the crew burns the entire SRB, then they can unlatch it and let the new SRB push it out the back of the rocket, out of the way.

If they want to cut thrust immediately, they point the other end of the portal to somewhere besides the ground station, and unlatch the SRB. The SRB, still burning, flies through the portal and out into wherever.


Option 1: glorified jet using pressurized materials from far side of portal.

This question deals with similar things. pressure differences and stargates

If the far side of your portal contains mass at greater pressure than the near side, and your portal setup allows it, that mass will come thru the portal energetically. You could do this terrestrially - put the intake portal at depth in the ocean and use the pressurized water to lift your ship. The water can fall back into the ocean. Everyone is happy. If you want to get stupid you can put your intake portal deep in a high mass planet, or a star. Maybe save tricks like that for once you are away from your planet.

A problem is that I don't think the exiting materials push on the frame of the portal as they leave. You would need a capture space within the space craft where high pressure materials from the entrance side are accumulated and then released so as to push against the ship and lift it.

Option 2. Lift with ambient air pressure on your side of the portal.
Position portal above ship. If your portal is above your ship and the exit side is in space, air will rush past the ship through the portal. The consequent low pressure in front of your ship (between it and the portal) and high pressure behind your ship will lift your ship into the low pressure zone. The low pressure zone will stay in front of your ship because the portal is generating it. Lift produced decreases with altitude. You vent a fair bit of atmosphere to space this way so make sure you have plenty. Consideration for aerodynamics will maximize the lift you can make this way and produce an unusual looking ship.

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    $\begingroup$ If you route the portal's output through a u-turn, you'll transfer twice the momentum rate (mass flow times velocity) to your ship's structure, minus losses. A fire hose nozzle feeding from a couple miles deep in the ocean would be enough to lift the Starship Mk. 1, I think. $\endgroup$
    – Zeiss Ikon
    Oct 8, 2019 at 15:42
  • $\begingroup$ The U-shaped path is more or less what I was thinking. Would the portal be positioned at one end, and a nozzle at the other? My main concern in that case would be keeping it balanced. $\endgroup$
    – CAE Jones
    Oct 8, 2019 at 16:15

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