TWR for landing in Vacuum

[lugge]

Hi folks,

OK, following scenario:

I'm working on a Munar-Hopper using the Dawn engine (never used it before :-)).

I also want to do the math by myself, only using KER for testing purpose.

I can do the math for the TWR:

f_thrustDawn = 2000N

a_gravMun = 1.63 m/S²

m_dawn = 250kg

Thus, TWR = f / (a * m) = 4.9

(Yes, I know, this is only the mass of the engine itself, without xenon, electricity and probe core, but this shall only be an example)

Thus, my craft will be able to lift off the Mun.

Everthing OK till now?

What I don't know, is the TWR sufficient to slow down my craft from orbital velocity to about 1-2m/s before hitting the ground?

(This is also something KER doesn't show, as far as I know).

I guess I have to do some math with orbital velocity, orbital height...

Maybe using the formula d = 0.5 * a * t² + v_0 * t + d_0 ??

Where a is calculated somehow with engine thrust and g_mun?

Ok, to make it short, I have no idea :-)

However, this is a common problem, I think.

One likes to know if a engine can slow down a craft before hitting the ground.

Any suggestions on this?

Regards,

lugge

[Speadge]

Hi folks,

One likes to know if a engine can slow down a craft before hitting the ground.

Any suggestions on this?

Regards,

lugge

Hi,

from my understanding: as Long as your SURFACE-TWR is higher than 1, you will always decelerate at full burn.

I recommend KER or MechJeb for beeing sure about it without calculating, cause it depends on the gravity of the Body you are influenced by.

The Higher the gravity the higher your weight and the lower your TWR.

But, given that you have enough time, you can decelerate from every Speed with a TWR > 1.0 if you have at least that amount of dV.

best one: try the "kamikaze-burn" view in KER. it should tell you on which height you have to start a full throttle with the actual active engine to come to vertical 0m/s.

Edited August 13, 2015 by Speadge

[mhoram]

The formula you use is the right one:

TWR = F / (a * m)

and I also arrive at the same TWR of 4.9 for your example "ship"

A sealevel-local-TWR > 1 is always enough to land a ship on the planet, if you have enough fuel.

You just need to adjust the flightpath in such a way that you have enough time to decelerate.

A few things to consider:

- engines use up fuel so you get lighter with time and the TWR at the time of landing is the TWR to be taken for the question "it is possible to land?"

- the gravity gets lighter the higher you are above the surface so only the surface gravity is essential to answer the question "is it possible to land?"

- the pre-landing orbit has no influence on the question "is it possible to land?"

- The higher the TWR the less fuel you need. (Link to a forum post by tavert detailing this with pre-v1.0 engines. This link also contains a nice video showing how to land with very-low-TWR ships) This affects how much fuel you need to have on board

- Picking a landingspot on higher altitudes (like on a mountaintop) will reduce the TWR-requirement a bit, because the local-TWR on the mountaintop is higher than at sealevel on that planet because of the lower local gravity.

edit:

As a side note TWR>1 is always enough because at this point your thrust is larger than the gravitational force of the planet on your ship, so you can decelerate.

Edited August 13, 2015 by mhoram

[Streetwind]

As long as your TWR is >1 relative to ther body you're orbiting, you can land on that body, because if all else fails you can still turn your engine 89 degrees down and hover while the minimal sideways component ever so slowly bleeds off speed.

However, as you can imagine from that description, the more you approach a TWR of 1, the more ardous, dangerous and less fuel efficient such a landing becomes.

I've used Mun landers that started at ~2.4 Mun-relative TWR (and obviously got lighter during the burn), and I have to say, I personally would not be comfortable with much less. I already had to be more defensive in my approach trajectory than I liked.

Though it wasn't nearly as spectacular as

to handle an extremely low TWR landing... Edited August 13, 2015 by Streetwind

[nekogod]

Hi,

from my understanding: as Long as your SURFACE-TWR is higher than 1, you will always decelerate at full burn.

I recommend KER or MechJeb for beeing sure about it without calculating, cause it depends on the gravity of the Body you are influenced by.

The Higher the gravity the higher your weight and the lower your TWR.

But, given that you have enough time, you can decelerate from every Speed with a TWR > 1.0 if you have at least that amount of dV.

That's true if you start at a vertical velocity of 0 as you'll be able to out thrust gravity all the way down.

However lets say you can accelerate at 2ms^2 and gravity is cancelling out 1ms^2 of that and you're going to impact the ground in 100s say accounting for deceleration then if your initial speed was anything over 100m/s you will not slow down in time.

Whether or not you can slow down in time is directly related to your height above the ground, your vertical speed and your horizontal speed.

Horizontal speed is important as it means the ground will be curving away from you as you fall, however it too will have to be zeroed to land.

As a side note TWR>1 is always enough because at this point your thrust is larger than the gravitational force of the planet on your ship, so you can decelerate.

As said above this is not always true. If I've got a ship with a huge TWR say 45 or something and I'm 5000m from the mun surface and I'm moving towards it at 1500m/s even pointed directly at the ground at 100% thrust I'm unlikely to be able to slow down in time. Initial height and vertical speed are critical.

Or rather yes any TWR > 1 means yes it's possible to land on that body, but it does not guarantee it's possible from the all given orbits/speeds

Edited August 13, 2015 by nekogod

[Sharpy]

I've made a munar rover with dawn and TWR of 1.05. Probably 1.01 would suffice, providing you can finish the maneuver before you run out of sunlight and batteries.

The worst problem was the panels angle to catch enough sunlight.

The trick is not to burn directly retrograde as you'll gain too much vertical speed too fast. Tilt from straight vertical orientation towards retrograde just enough to keep vertical speed at zero or dropping very slowly, that way you will be losing speed gradually. Monitor your vertical speed at all times, because as gravity drag increases with orbital speed dropping, you will need to return more towards vertical orientation.

- - - Updated - - -

As said above this is not always true. If I've got a ship with a huge TWR say 45 or something and I'm 5000m from the mun surface and I'm moving towards it at 1500m/s even pointed directly at the ground at 100% thrust I'm unlikely to be able to slow down in time. Initial height and vertical speed are critical.

Or rather yes any TWR > 1 means yes it's possible to land on that body, but it does not guarantee it's possible from the all given orbits/speeds

Providing you have enough fuel and power, it's possible from all orbits.

If you're moving towards the surface a 1500m/s you're not in the orbit. If you're moving 1500m/s and 5000m from the surface, probably not even a Rhino is enough. But if you begin the maneuver in orbit, as long as you are able to sustain TWR >1 you can land.

[Jouni]

I've used Mun landers that started at ~2.4 Mun-relative TWR (and obviously got lighter during the burn), and I have to say, I personally would not be comfortable with much less. I already had to be more defensive in my approach trajectory than I liked.

There are two aspects to lander TWR. If you only care about efficiency, initial TWR 1.5 is already good enough, while 2.0 is very good. If you fly manually and want to make your life easier, the absolute value of g·(TWR-1) is what you should really care about. During the critical phase of the landing, you're probably flying almost vertically, and the absolute acceleration is what makes you succeed or fail.

In low-speed situations, 3-4 m/s is already pretty good, which corresponds to TWR 2.85 to 3.45 for a Mun landing. In high-speed situations, you'll probably want to double that. In a Tylo touchdown, 6-8 m/s corresponds to TWR 1.75 to 2.0.

Edited August 13, 2015 by Jouni

[nekogod]

If you're moving towards the surface a 1500m/s you're not in the orbit.

Sure you are. It's just an elliptical orbit where you intersect the ground before completing it.

[Speadge]

Sure you are. It's just an elliptical orbit where you intersect the ground before completing it.

that's called "sub-orbital"

[Skorpychan]

It's possible to land with any TWR. But for landing SAFELY, you probably want at least 1.5. 2.0 is the lowest I'd go myself, because I like to have some extra thrust in reserve in case I'm landing on a slope or something. Or have to tip the lander back onto it's legs again.

[nekogod]

that's called "sub-orbital"

Sub orbital is defined as an elliptical orbit that intersects the ground. Or an orbit whose perigee distance is less than the radius of the orbited body

[Sharpy]

Sub orbital is defined as an elliptical orbit that intersects the ground. Or an orbit whose perigee distance is less than the radius of the orbited body

So if I jump ten centimeters up, I'm entering an orbit? That's an... extremely liberal definition.

[MaverickSawyer]

I never go below 2:1 for a lander.

[LN400]

My 2 cents for what it's worth for out of atmosphere landing. Once you know the initial velocity, the mass of the ship, the engine thrust and the body's gravitational pull, you can work out the maximum acceleration of your ship. The reason acceleration is important is because it tells you how many seconds you need to change your speed from x to y. Any TWR above 1 is capable of stopping the decent completely but only if you have time for it, else the ground will bleed off your remaining speed. So know your max acceleration and you can tell if there is time to come to a safe landing speed.

[Admac]

Yeah, Theorycrafing aside, take 1.5 + local TWR for vacuum landers. Also don't cheap out on reaction wheels either its a lot easier and safer if you can pivot quickly.

Landing with stock Xenon is a pretty challenging thing, I don't really see a way to do it witha comfortable twr. My prediction is you will have to do a painfully small margin lander like 1.1 or so.

Since landers are tiny and their mass efficiency is less important, I'd recommend making a very wide lander with a big reaction wheel and a TWR of 3-5. Easy and safe landings are worth an extra ton IMO.

Unless you are doing it for the challenge of making a challenging low TWR lander, then light it up!