Runaway Starship Ramps

Question

This question is partially a spinoff of some points that came up in another recent question at Worldbuilding about interstellar space travel.

Runaway Truck Ramps

I live in Colorado, and on the downward slopes of many of our high grade mountain roads are runaway truck ramps. The idea is that if you are driving a semi and your brakes fail and you are speeding up to unreasonable speeds as you hurl down the mountain, you can divert to the runaway truck ramp, lose some speed going uphill or along a lane with high friction soft sand, and if you are still not at a stop at the end, you can crash into a long line of trash cans full of water that are placed there.

The Canonical Hard Science Interstellar Voyage

Now, back to the starships. The generally accepted design plan of an interstellar starship that complies with the laws of physics is to have a ship that is mostly fuel and engines accelerate at 1 G to cruising speed, cruise for a while, and then turn around and decelerate at 1 G so that you are moving at some very low manageable speed when you reach your destination.

A previous post in this forum considered a few ideas for handling the deceleration phase of space travel, but had mostly only superficial answers, none of what really honed in on the issues presented in this question.

The target cruising speed at the boundaries of what seems like a medium term future engineering limit is generally on the order of 0.1c (i.e. 18,600 miles per second), although if you need more cargo and living space, and less fuel, you can always trim the intended cruising speed, and the precise cruising speed isn't really material to this question.

Another part of the conventionally accepted design plan is that in the time period starting when you are going at all fast and ending around the time you reach your destination, you don't want to hit anything that could puncture or destroy your ship, and usually, we just assume that this gets figured out somehow (because it is a rather complicated issue and requires a lot of hard to acquire information about how much stuff is in interstellar space that needs to be known to considerable precision).

In this case you can assume that our starship is built out of the strongest construction materials known to exist, or credibly hypothesized, that could be manufactured on the scale of a starship.

This is all good and well.

What Could Go Wrong?

But, Mr. Murphy being omnipresent, things don't always go according to plan.

Maybe you had to plough through interstellar gas on the way to your cruising speed only to realize with horror that as a result you had used more than half of your fuel by the time you hit your cruising speed.

Or, maybe you forgot to consider extra gravitational sling shot boosts you received in the acceleration phase, so you kept accelerating until you used up half of your fuel and have reached a cruise speed that is faster than you have enough fuel to slow down from.

Or, perhaps some micro-meteoroids have made some tiny holes in your fuel tanks and twenty years into your voyage when it is time to start slowing down again, you discover to your horror that 7% of the fuel you should have left to slow down with has seeped out of those holes over the course of the last twenty years.

Or, maybe when you ordered 50,000,000 tons of fuel tanks from the idiot marketing guy in Chicago, he told the engineers that you needed 50,000,000 short tons (907.185 kg each) rather than 50,000,000 metric tons (1000 kg each) and nobody caught the problem until the acceleration phase of the trip was over and your were cruising along at cruise speed. (Not as unrealistic as it seems. A real life NASA disaster was caused by an English system v. metric system screw up.)

Come up with your own scenarios. The bottom line is that shit happens and you don't have enough fuel to slow all of the way down when you need to.

Human Factors To Consider

Since a hard science interstellar ship is a one way trip and you need to rebuild civilization when you arrive, your ship will have 100,000+ passengers, men and women, and every age from babies to old people at the time you need to start slowing down.

Now, while it would be optimally comfortable to decelerate at 9.8 Newtons (a Newton is one meter per second squared for one kilogram) which is also known as 1 G, nobody is going to suffer long term ill effects is you do so at 15 Newtons/kg for a little while now and then.

But, at 50 Newtons/kg, people start passing out and frail people may suffer bodily injury. At 100 Newtons/kg your strongest people while properly cushioned in specialized chairs are struggling to stay conscious and avoid injury. At 1000 Newtons/kg, almost all of your passengers are dead, and the survivors will be seriously injured with no one to take care of them and won't have enough of a critical mass to build a society that can meet its basic needs when you arrive.

So, you can't slow down too fast and you have a pretty narrow window of acceptable solutions.

Runaway Starship Ramps

But, all is not lost.

Recall our runaway truck ramps. While there is no complete substitute for fuel when it comes to speeding up (although gravitational boosts can help a little), the universe is full of stuff that will slow you down if you crash into it - interstellar gas, gas giant and terrestrial planet and moon atmospheres, space dust, meteoroids big and small, asteroids, oceans, and solid ground on terrestrial planets, for example, via lithobraking.

Approaches into the gravitational fields of large objects like stars and planets at optimal angles can also slow you down just as other approaches can speed you up.

So, the trick is to consider what you can crash into at various speeds to reduce your starship's speed while not destroying the starship or causing deceleration of more than 50 Newtons/kg at any one moment if you can help it.

I can also imagine some additional strategies that could help.

My intuition is that the more diffuse and less rigid the object you are impacting is, the more gentle the impact will be and the more likely it is that your ship will not be destroyed on impact. So, you may want to use lasers or missiles to break up objects in the path of your ship to soften the blow.

Also, the top priority in this situation is to save the people and essential cargo on board. Towards the end of the trip, some portion of the ship can be sacrificed to make ablative barriers to protect the ship that are destroyed on impact with space debris.

Budget Space Travel: A Planned Crash Scenario

Finally, I have initially posed this question as a solution to an unplanned problem.

But, suppose that you have well established human colonies with advanced technology at your destination. To what extent could you limit your fuel requirements by intentionally having people on the destination set up an optimal "crash ramp" in deep space for your ship and what would that crash ramp look like.

My Questions

So, my questions are these:

Certainly, at some point, if you have enough fuel to go slow enough, the runaway spaceship ramp strategy for starships can work.

But, how slow do you have to get? What kind of crash targets and gravitational braking approaches are best at which parts of the controlled crash? Is it realistic to think that "runaway starship ramps" could provide any meaningful share of the needed deceleration without killing most of the passengers?

What crash targets and gravitational braking approaches make most sense in an unplanned crash scenario?

What would you do differently if you could have technologically advanced colonists at the destination build a crash ramp in deep space to your specifications in the vicinity of the destination?

Answer 1

Assume a colony starship Centauri Pilgrim is carrying one hundred thousand plus colonists, travelling at 0.1 c to Alpha Centauri. It undergoes a catastrophic propellent containment failure losing most of its fuel to decelerate. The ship can only decelerate by one percent of lightspeed, to 0.09 c (this is a velocity of 27,000 km/s).

Using gravitational fields to slow the ship will ineffective (next to useless actually). Lithobraking is insane; basically this would mean decelerating through 27,00 km/s in a minute fraction of a second. It also requires 'magic' deceleration technology and 'magic' force-fields to survive the collateral effects of fragments of the lithobraked planet.

The sensible way to decelerate the Centauri Pilgrim would be an interception strategy to send repair vehicles to fix any damage and tanker ships to replace the lost fuel. But this isn't exactly an interstellar runway ramp.

There is an even better way to decelerate our runaway Centauri Pilgrim and that involves particle beam propulsion technology. Essentially electromagnetic launchers or mass-drivers to dispatch a stream of smart pellets to transfer momentum to the incoming ship. The pellets can be blasted with powerful laser pulses and the resulting plasma is reflected from a magnetic deflector mounted on the bow of our runaway vessel.

An alternative to rockets is to push spacecraft with a reflected beam. The advantage is that it leaves most of the propulsion system mass at rest. Use of mass beams, as opposed to photons, allows great efficiency by adjusting the beam velocity so the reflected mass is left near zero velocity relative to the source. There is no intrinsic limit to the proper frame map velocity that can be achieved. To make a propulsion system, subsystems need to be developed to acquire propulsive energy, accelerate the mass into a collimated beam, insure that the mass reaches the spacecraft and reflect the mass. A number of approaches to these requirements have been proposed and are summarized here. Generally no new scientific discoveries or breakthroughs are needed. These concepts are supported by ongoing progress in robotics, in nanometre scale technologies and in those technologies needed to use of space resources for the automated manufacture of space-based solar power facilities. For mass beams specifically, work in particle sizing, acceleration, delivery and momentum transfer is needed. For human interstellar flight, a notional schedule to provide a mass beam propulsion system within a century is provided.

Gerry David Nordley is a major proponent of particle beam propulsion. Taking his proposal to send a probe to Alpha Centauri at a large fraction of lightspeed (0.8667 c) with an acceleration of 3 g over 122 sidereal days.

If a particle beam system is used to decelerate the runaway ship. let's use a PB system with exactly the same characteristics as Nordley's Alpha Centauri probe. It has to decelerate a ship from 27.000 km/s to relative rest with respect to Alpha Centauri.

The probe has 86.67/9 squared times the kinetic energy of the Centauri Pilgrim or 92.74 times if its velocity was pure Newtonian. At 0.8667 c, it has Lorentz factor of two. So multiply the value for kinetic energy by, say, nine. This gives an approximate relativistic kinetic energy of 834.6321 for the probe. Since the Nordley PB system accelerates the probe at an acceleration of 3 g, then we can multiply by another 3 to account for an acceleration of 1 g to decelerate the Centauri Pilgrim.

If a PB system is deployed in the Alpha Centauri to decelerate a runaway ship moving at 27,000 km/s, and with the same characteristics as Nordley Alpha Centauri probe, then it will be capable of decelerating a vessel with a mass of 2,503,896.3 tons at 1 g in a period of 31.89 days.

An interstellar vessel with a mass of 2.5 million tons can more than readily accommodate 100,000+ colonists. Using the known parameters of particle beam propulsion system and then calculating what mass vehicle it could decelerate confirms this system would be more than adequate to safely bringing a runaway colony ship to stop at its destination. The vessel turns out to have more than enough capacity to accommodate the OP's proposed complement of colonists.

REFERENCES:

Gerry David Nordley, Particle Beam Propulsion and Two-Way EML Propulsion

G. D. Nordley et al., Mass Beam Propulsion: An Overview (2015), JBIS, 68, pp.153-166