# Can you refuel chemical rockets to increase the speed?

Chemical rockets have max delta v ( max.speed ) about 10 km/s.

Can you refuel them in deep space to increase the speed?

For example Nasa Deep space habitat (DSH). Once it is assembled in orbit with full tanks it can achieve max.speed 10 km/s which makes about 6 months trip to Mars.

But can you at same time send other rocket with same speed and trajectory with all payload being fuel. After DSH run from fuel, connect them both, refuel DSH and throw away rocket like expendable stage.

Than repeat this with other rockets which were already refueled 2,3,4 times.

This way you can theoretically increase max.speed (delta v) above 10 km/s and shorten 6 months trip to mars to just 3,2,1 months.

Am I wrong with this? If yes then why? It is physically impossible or just too expensive and did NASA consider this for Mars mission?

• Yes you could, but instead they would probably just add staging to the gateway constructed in orbit – user20636 Sep 20 '18 at 9:14
• What is a " 6 mounts trip "? Do you mean months? If you arrive much faster at Mars, decceleration into orbit will be more difficult and expensive. Extra fuel may be necessary for orbit maneuver. – Uwe Sep 20 '18 at 11:35
• To reuse a chemical rocket you need not only fresh fuel and oxidizer. The tanks for gases used to pressurize the tanks need refilling too. The batteries used to power the electronics need to be recharged or replaced. – Uwe Sep 20 '18 at 20:59

You work against the very same old problem of Tyranny of Rocket Equation, except you distribute the rocket - many smaller instead of one huge, sending the many pieces that are to meet up deeper in space. Yes, you can increase the speed that way. And the increase will be small, or the cost will be prohibitive. After all, you must accelerate the rockets that deliver fuel to the speed of the "final rocket" - and with what fuel?

If, like in normal rockets, 90% launch mass is fuel, 5% is payload (5% to structural overhead) and you want the rocket to be refueled to the launch state, doubling its delta-V - then you have to send 18 refueling rockets with payload of fuel. Want to triple the delta-V? Each of these 18 supply rockets needs to be refueled in orbit before it can catch up with the 'core'. One core rocket. One orbital refueling for 2x delta-V - 18 rockets, plus one refueling 'on the fly' - another 18. And 18 per each of these. All in all your delta-V increases 3x. Your number of launches - 361x.

But yes, a moderate, middle-ground approach makes sense. BFR is planned for orbital refueling. It launches on a booster that is way insufficient to reach orbit, then flies to orbit under own power, depleting most of its own fuel supply, then receives fuel from other BFR launches which use the payload/passenger space as extra fuel tank (6 fueling launches, if memory serves me correctly) and then it's ready for departure to Mars.

• The payload of fuel needs a tank to hold it, therefore more than 18 refueling rockets are needed. A docking adapter is needed too as well as pumps and hoses for fuel transfer. All rockets should be at the same place within a short time interval for fuel transfer, all rockets should be launched within a short time from multiple launch pads. An additional stage for the rocket needs no precise rendezvous and no fuel transfer. Only all lower stages have to be much bigger to carry the additional stage. The whole rocket would be gigantic in size. – Uwe Sep 20 '18 at 21:14

The exact scenario you describe doesn't make much sense. If you are starting in space, there is no reason not to simply bolt the tanker onto the main spaceship and actually use it as a first stage. You burn all the fuel from the tanker and then drop it before starting to use the fuel from the main spaceship. Since the waste mass of the tanker ends up moving more slowly that way, more energy is available to accelerate the payload. Since you are in space you can accelerate fairly gently, so the "bolting together" isn't too hard.

It's different if you are launching from Earth because if the huge stresses associated with getting off the ground and out of the atmosphere. The whole rocket needs to be designed to cope with that, and designing a bigger one might be much harder than doing multiple launches (BFR/BFS style).

• Are you saying that by simply assembling multistage rocket ( 3,4,5 stages ) you can increase speed above 10 km/s and shorten trip to mars. Than why NASA never used it with their deep space habitat plans. DSH is always portrait as single stage to mars and they say 6 months is something which is given. It is because increase in speed will be small and assembling/refuelling all those expendable stages will be to expansive. – John Bambi Sep 20 '18 at 18:33
• Yes, but you have to add a LOT of really huge extra stages, in order to shorten the trip to Mars by a fairly small amount. The standard six month trip makes very efficient use of the way Earth and Mars are moving and the effect of the suns gravity. If you try and get a shorter trip, you lose some of that. – Steve Linton Sep 20 '18 at 20:09
• Just to put some numbers on that. A Falcon 9 sitting on the launchpad is about 550 tons. It can deliver about 23 tons to Low Earth orbit, a delta-V of about 9.4 km/s. So, to give that payload a delta-V of twice that, 19 km/s would mean lifting a fully fueled falcon 9 into orbit. Scaling up, that would need a rocket with a total mass of about $550/23\times 550 = 13000$ tons. (since the falcon has two stages, this would be a four stage system in total). To get to 28.5 km/s (three times the original) would mean about 300 000 tons -- a six stage rocket the mass of a supertanker. – Steve Linton Sep 20 '18 at 21:43
• In fact the New Horizons probe was launched with a total delta-V over 16 km/s. To do this needed a large core rocket, 5 strap-on solid boosters, a highly efficient centaur second stage and a special third stage. The total mass on the launch pad was about 500 tons and the payload mass launched to that velocity was about 0.5 tons. – Steve Linton Sep 20 '18 at 21:51
• If your numbers are correct, I don't understand why SpaceX ( their CEO E.Musk ) promised that their Big falcon rocket, gross mass 4,300 tons, with two stages and payload 100 tons (after refueling in orbit) can shorten trip to mars to 3 months. Is this physically even possible with this rocket ? – John Bambi Sep 21 '18 at 1:04

Yes you can but not with only chemical rockets. The solution is to send your deepspace refuel tanks using a high isp electric engine, which will require a very long time to reach their destination (unless powered by a nuclear reactor), but since they unmanned, cosmic/artificial radiation is not an issue.

• Sounds like you are proposing to send a "slow" tanker, and then sometime much later, send a "fast" ship that will rendezvous with the tanker. But how will that rendezvous be possible if they are travelling at different speeds? – user27176 Sep 20 '18 at 13:11
• On the other hand, If you were planning a round trip, perhaps the rendezvous would take place at the destination, and the "fast" ship would use the fuel from the tanker for the return trip. – user27176 Sep 20 '18 at 13:14
• Having low acceleration does not mean it can't reach the same relative speed as the manned ship, just that it has to have a carefully timed head start. – Avi Cherry Sep 20 '18 at 17:51

Sounds like you're thinking of delta-v kind of the wrong way. If you're thinking in car analogies, delta-v is more a measure of distance than a measure of speed. It's the change of speed you need to get from A to B. See delta-v budget on Wikipedia.

So if you want to have a shorter trip to Mars (in terms of months), you actually have to make a trip with more delta-v. Because your travel-velocity is greater in comparison to your starting-velocity. (You'll also have to break even more towards the end of the trip, which would add even more delta-v to your journey, but you might be able to use the Martian atmosphere to aerobreak.)

(Play some Kerbal Space Program and you'll get the hang of it.)

The limiting factor in traveling to Mars with rockets is not that they have too little acceleration per se (as you seem to imply in your question). The problem is that they run out of fuel way too quickly. But yes, the more fuel you take with you, the less acceleration you have. The way this roughly works in practice, is that you point your rocket the right way, fire it for a couple of minutes, shut off the engines, then wait a couple of months, then turn the rocket around 180° and fire the engines again for a couple of minutes to break. Hopefully, you still have enough fuel by then to make your way back to Earth ;)

So to answer your question: no, this wouldn't work. Going back to car analogies: yes, a car can accelerate faster with an almost empty tank (since the total weight of the car including fuel is less). But sending a fuel-truck after the car, and refuelling the car mid-acceleration, won't save you any gas in total.

Yes. Although as other commenters have already pointed out, if you launch the refuel tanker on a rocket, you won't gain anything.

The answer to those concerns is simple: launch the refuel tanker from a rail gun. Because the refuel tank only contains fuel, it can be built to survive much higher g-forces than a conventional rocket.

Launching refuel containers by railgun is actually more practical than launching the rocket itself by railgun: if you're launching the rocket by railgun, you need a much longer railgun to reduce g-forces to the point that they're safe for the crew and the payload. By launching the refuel tanks separately, by railgun, you use significantly less fuel overall, thereby allowing greater velocities for the rocket itself.

No one has mentioned the deceleration issue. If you could, hypothetically, increase delta-V - or your change in velocity, to whatever you wanted, then you still face the problem of slowing enough to enter the orbit. You would have to increase the amount of fuel you'd have to burn in the opposite direction to slow down.

I'm no rocket scientist. But I've read enough science fiction to play one on TV. I don't know that Delta-V is the right way to think about this anyway. Delta-V is dependent on the Gravity one is experiencing (its delta V, provided by positive Acceleration, in magnitude sufficient to counteract the negative acceleration of gravity, commonly give as -9.81 m/s^2). Delta-V represents the kinetic energy of the rocket, ($$KE = \frac{1}{2}mv^2$$) and momentum ($$p=mv$$), which must be sufficient to escape a gravitational well.

At the same time, delta-V does not exactly represent the theoretical change in velocity of the actual rocket. It's really a way of taking specific impulse and putting it in terms of the body a specific impulse is trying to effect. There is no specific delta-v that works to achieve this. Rockets are typically given strength in terms of specific impulse, which is more useful than this "delta-V" measure, which I've never seen used in this way before, and I think it might be inappropriate.

A rocket can achieve a given delta-v in a specific set of circumstances- but another in another set- the two most important determinants would be the gravity and of course payload (or better yet total rocket mass). Delta-V is just the change in velocity the rocket can achieve for a given gravity and mass.

$$\Delta v = \int_{t_0}^{t_1} \frac{|T(t)|}{m(t)} dt,$$

where $$T(t)$$ is the time-dependent thrust (assumed to be in the same direction as the velocity) and $$m(t)$$ is the time-dependent mass of the rocket as it uses up propellants.

Source

SpaceX proposes to do something like this to go to Mars with the BFR. It isn't so much about increased speed (as the much smaller Falcon Heavy demonstrated that it could throw a payload all the way to Mars), as it is about mission mass and the size of an available launcher. Note that the Viking program placed two landers on Mars, each launched by a far smaller rocket than the BFR. A human journey to Mars would require an enormous mass in supplies and equipment, far more than could be accomplished by a single BFR launch. So, SpaceX proposes to build a tanker; one BFR launch would only have to lift the required Mars-bound payload to Earth orbit (the BFS), where it would rendezvous with a tanker launched on a second BFR. The BFS would refuel in orbit off the tanker to provide the necessary propellant to complete its mission.

One of the early (discarded) Apollo concepts also involved Earth orbit rendezvous as a way to circumvent the payload limit of the available launcher. A drawback of this approach is the need to achieve two successful launches with as little time as possible between them, co-ordinated such that the vehicles can easily rendezvous. Project Gemini showed how difficult this can be.

Apart from the challenges of co-ordinating the launches, SpaceX would be breaking new ground with an on-orbit refueling, but then they seem to be in the business of breaking new ground.

As to the delta-V question itself... cutting the trip time in half isn't a meaningful benefit. Launch or return windows open only so often, so once you put yourself onto a Mars transfer trajectory, you're committed to something far longer than the 3 or 6 month travel time to Mars. Also, all that extra delta-V will have to be removed when you get to Mars, and if something goes wrong, even temporarily, you are stuck on a much longer trip before you see Earth again. The 6 month transfer means that failing Mars capture should get you back to Earth ("free return") without excessive delay.

• Transit exposes the passengers to much more cosmic and solar radiation and a much higher threat from solar flares than the surface of Mars, as well as a microgravity environment, not to mention taking up a noteworthy amount of each person's life. Halving transit time is a quite significant benefit. A situation where they can not perform the landing on Mars but can return to Earth is extremely unlikely, the overwhelmingly more likely result is a new crater on Mars. – Christopher James Huff Sep 22 '18 at 23:00