Dear Maria:

No, there is no such thing as permissible suicide, but we need to define "suicide."

The soldier jumping on a grenade is not suicide any more than Jesus committed suicide on the Cross. Jesus went voluntarily to the Cross, but his intention was not to kill himself. Remember in the Garden Jesus sweat blood over the agony that was about to fall upon him. He ask the Father if that cup could be passed from him. But, Jesus had a mission to lay down his life for love of the world to redeem mankind.

In similar manner, the soldier is not intending to kill himself, rather he is intending to save the lives of his comrades. His own death in the process is a unintended consequence of his act to save his friends.

No greater love has any man that he who lays down his life for a friend, Jesus said. This is no suicide. Suicide is not about love, but about despair.

Theologically these sorts of incidents refer to the doctrine of "double-effect".  St. Thomas Aquinas observed, for example in the consideration of self-defense, that:

?Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. ? Accordingly, the act of self-defense may have two effects: one, the saving of one's life; the other, the slaying of the aggressor.?

?Therefore, this act, since one's intention is to save one's own life, is not unlawful, seeing that it is natural to everything to keep itself in being as far as possible.? However, Aquinas observes, the permissibility of self-defense is not unconditional: ?And yet, though proceeding from a good intention, an act may be rendered unlawful if it be out of proportion to the end. Wherefore, if a man in self-defense uses more than necessary violence, it will be unlawful, whereas, if he repel force with moderation, his defense will be lawful.?

The New Catholic Encyclopedia provides four conditions for the application of the principle of double effect:

1. The act itself must be morally good or at least indifferent.
2. The agent may not positively will the bad effect but may permit it. If he could attain the good effect without the bad effect he should do so. The bad effect is sometimes said to be indirectly voluntary.
3. The good effect must flow from the action at least as immediately (in the order of causality, though not necessarily in the order of time) as the bad effect. In other words the good effect must be produced directly by the action, not by the bad effect. Otherwise the agent would be using a bad means to a good end, which is never allowed.
4. The good effect must be sufficiently desirable to compensate for the allowing of the bad effect? (p. 1021).

In simpler terms:

1. the act itself must be a moral good, or at least morally indifferrent
2. the actor must intend the good effect and not the evil effect. The evil effect is merely permitted as a side-effect of the good effect 
3. the good effect must be produced directly by the morally good act and not by means of the bad effect
4. there must be a proportionately grave reason for permitting the evil effect

Applying this doctrine to the case of the soldier jumping on a gernade to save his friends...

1. Is the act of saving the lives of the soldier's friends a moral good? Yes, saving his friends lives is a moral good.

2. Would the soldier have used another means to save his friends lives if another means was available? Was his intention to a good intention? Yes.

3. Did the soldier's actions of self-sacrifice immediately cause the effect of saving the lives of his friends? DId the man's good actions save the lives of his friends (rather than his death itself saving the lives of his friends)? Yes.

4. Was the saving of the lives of the man's friends sufficiently desirable to compensate for his own death? Was there a proportionately grave reason for the man to permit himself to most likely die for his friends? Yes.

Thus, a person with the intention to save the lives of others, even if that act results in his own death, even if he knows that his own death will likely result, is permissible.

God Bless,
Bro. Ignatius Mary